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Title: Telescopic Work for Starlight Evenings
Author: Denning, William F.
Language: English
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Transcriber’s Notes
Obvious typographical errors have been silently corrected. Variations
in hyphenation have been standardised but all other spelling and
punctuation remains unchanged.
Footnotes are placed at the end of chapter.
Italics are represented thus _italic_, and superscripts thus ^.
The periods of the satellites of Uranus have been added to the table as
specified in a subsequent note.
The layout of several tables has been modified to maintain clarity
within wdth restrictions.
[Illustration:
LONDON STEREOSCOPIC CO. PHOTOMEZZOTYPE.
STANMORE OBSERVATORY.
INSIDE VIEW.]
TELESCOPIC WORK
FOR
STARLIGHT EVENINGS.
BY
WILLIAM F. DENNING, F.R.A.S.
(FORMERLY PRESIDENT OF THE LIVERPOOL ASTRONOMICAL SOCIETY).
“To ask or search I blame thee not, for heaven
Is as the book of God before thee set,
Wherein to read his wondrous works.”
MILTON.
LONDON:
TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
1891.
[_All rights reserved._]
[Illustration: ALERE FLAMMAM.]
PRINTED BY TAYLOR AND FRANCIS,
RED LION COURT, FLEET STREET.
PREFACE.
It having been suggested by some kind friends that a series of articles
on “Telescopes and Telescopic Work,” which I wrote for the ‘Journal of
the Liverpool Astronomical Society’ in 1887-8, should be reprinted, I
have undertaken the revision and rearrangement of the papers alluded
to. Certain other contributions on “Large and Small Telescopes,”
“Planetary Observations,” and kindred subjects, which I furnished to
‘The Observatory’ and other scientific serials from time to time, have
also been included, and the material so much altered and extended that
it may be regarded as virtually new matter. The work has outgrown my
original intention, but it proved so engrossing that it was found
difficult to ensure greater brevity.
The combination of different papers has possibly had the effect of
rendering the book more popular in some parts than in others. This
is not altogether unintentional, for the aim has been to make the
work intelligible to general readers, while also containing facts
and figures useful to amateur astronomers. It is merely intended as a
contribution to popular astronomy, and asserts no rivalry with existing
works, many of which are essentially different in plan. If any excuse
were, however, needed for the issue of this volume it might be found
in the rapid progress of astronomy, which requires that new or revised
works should be published at short intervals in order to represent
existing knowledge.
The methods explained are approximate, and technical points have
been avoided with the view to engage the interest of beginners who
may find it the stepping-stone to more advanced works and to more
precise methods. The object will be realized if observers derive any
encouragement from its descriptions or value from its references,
and the author sincerely hopes that not a few of his readers will
experience the same degree of pleasure in observation as he has done
during many years.
No matter how humble the observer, or how paltry the telescope,
astronomy is capable of furnishing an endless store of delight to its
adherents. Its influences are elevating, and many of its features
possess the charms of novelty as well as mystery. Whoever contemplates
the heavens with the right spirit reaps both pleasure and profit, and
many amateurs find a welcome relaxation to the cares of business in
the companionship of their telescopes on “starlight evenings.”
The title chosen is not, perhaps, a comprehensive one, but it covers
most of the ground, and no apology need be offered for dealing with one
or two important objects not strictly within its scope.
For many of the illustrations I must express my indebtedness to the
Editors of the ‘Observatory’ to the Council of the R.A.S., to the
proprietors of ‘Nature,’ to Messrs. Browning, Calver, Cooke & Sons,
Elger, Gore, Horne Thornthwaite and Wood, Klein, and other friends.
The markings on Venus and Jupiter as represented on pages 150 and 180
have come out much darker than was intended, but these illustrations
may have some value as showing the position and form of the features
delineated. It is difficult to reproduce delicate planetary markings
in precisely the same characters as they are displayed in a good
telescope. The apparent orbits of the satellites of the planets,
delineated in figs. 41, 44, &c., are liable to changes depending on
their variable position relatively to the Earth, and the diagrams are
merely intended to give a good idea of these satellite systems.
W. F. D.
Bishopston, Bristol,
1891.
Plates I. and II. are views of the Observatory and Instruments recently
erected by Mr. Klein at Stanmore, Middlesex, lat. 51° 36′ 57″ N.,
long. 0° 18′ 22″ W. The height above sea-level is 262 feet. The
telescope is a 20-inch reflector by Calver, of 92 inches focus; the
tube is, however, 152 inches long so as to cut off all extraneous
rays. It is mounted equatoreally, and is provided with a finder of
6 inches aperture—one of Tulley’s famous instruments a century ago.
The large telescope is fixed on a pillar of masonry 37 feet high, and
weighing 115 tons. Mr. Klein proposes to devote the resources of his
establishment to astronomical photography, and it has been provided
with all the best appliances for this purpose. The observatory is
connected by telephone with Mr. Klein’s private residence, and the
timepieces and recording instruments are all electrically connected
with a centre of observation in his study.
CONTENTS.
CHAPTER I.
Page
THE TELESCOPE, ITS INTENTION AND THE DEVELOPMENT OF ITS POWERS 1
CHAPTER II.
RELATIVE MERITS OF LARGE AND SMALL TELESCOPES 20
CHAPTER III.
NOTES ON TELESCOPES AND THEIR ACCESSORIES 38
CHAPTER IV.
NOTES ON TELESCOPIC WORK 66
CHAPTER V.
THE SUN 87
CHAPTER VI.
THE MOON 113
CHAPTER VII.
MERCURY 137
CHAPTER VIII.
VENUS 145
CHAPTER IX.
MARS 155
CHAPTER X.
THE PLANETOIDS 167
CHAPTER XI.
JUPITER 170
CHAPTER XII.
SATURN 195
CHAPTER XIII.
URANUS AND NEPTUNE 215
CHAPTER XIV.
COMETS AND COMET-SEEKING 227
CHAPTER XV.
METEORS AND METEORIC OBSERVATIONS 260
CHAPTER XVI.
THE STARS 286
CHAPTER XVII.
NEBULÆ AND CLUSTERS OF STARS 324
NOTES AND ADDITIONS 347
INDEX 353
ILLUSTRATIONS.
PLATE I. Interior of Mr. Klein’s Observatory _Frontispiece_
II. View of Mr. Klein’s Grounds and Observatory _To face_ p. 82
FIG. PAGE
1. The Galilean Telescope 7
2. Royal Observatory, Greenwich, in Flamsteed’s time 8
3. Sir Isaac Newton 10
4. Gregorian Telescope 10
5. Cassegrainian Telescope 11
6. Newtonian Telescope 11
7. Common Refracting-Telescope 12
8. Le Mairean or Herschelian Telescope 13
9. 10-inch Reflecting-Telescope on a German Equatoreal, by Calver 17
10. Lord Rosse’s 6-foot Reflecting-Telescope 22
11. Refracting-Telescope, by Browning 32
12. “The Popular Reflector,” by Calver 40
13. 3-inch Refracting-Telescope, by Newton & Co. 41
14. Huygens’s Negative Eyepiece 46
15. Ramsden’s Positive Eyepiece 47
16. Berthon’s Dynamometer 50
17. Cooke and Sons’ Educational Telescope 52
18. Refracting-Telescope on a German Equatoreal 67
19. The Author’s Telescope: a 10-inch With-Browning Reflector 77
20. Sun-spot of June 19, 1889 95
21. Solar Eclipses visible in England, 1891 to 1922 98
22. Total Solar Eclipse of August 19, 1887 98
23. Belts of Sun-spots, visible Oct. 29, 1868 104
24. Shadows cast by Faculæ 109
25. Light-spots and streaks on Plato, 1879-82. (A. Stanley
Williams.) 126
26. Petavius and Wrottesley at Sunset. (T. Gwyn Elger.) 129
27. Birt, Birt A, and the Straight Wall. (T. Gwyn Elger.) 130
28. Aristarchus and Herodotus at Sunrise. (T. Gwyn Elger.) 132
29. Mercury as a Morning Star 143
30. Venus as an Evening Star 150
31. Mars, 1886, April 13, 9^h 50^m 157
32. Orbits of the Satellites of Mars 159
33. Jupiter, as drawn by Dawes and others 178
34. Jupiter, 1886, April 9, 10^h 12^m 180
35. Occultation of Jupiter, Aug. 7, 1889 186
36. Jupiter and Satellites seen in a small glass 187
37. Shadows of Jupiter’s Satellites II. and III. 192
38. Saturn as observed by Cassini in August 1676 198
39. Saturn, 1885, Dec. 23, 7^h 54^m 201
40. Saturn as observed by F. Terby, February 1887 203
41. Apparent orbits of the Five Inner Satellites of Saturn 212
42. Transit of the Shadow of Titan 213
43. Uranus and his belts 218
44. Apparent orbits of the Satellites of Uranus 221
45. Apparent orbit of the Satellite of Neptune 224
46. Mars, Saturn, and Regulus in same field, Sept. 20, 1889 226
47. Comet 1862 III. (Aug. 19, 1862) 237
48. Sawerthal’s Comet, 1888 I. (March 25, Brooks) 237
49. Brooks’s Double Comet, Sept. 17, 1889 239
50. Pons’s Comet (1812). Telescopic view, 1884, Jan. 6 242
51. Ditto. Ditto, 1884, Jan. 21 242
52. Radiation of Meteors. (Shower of early Perseids, 1878) 263
53. Double Meteor. Curved Meteor. Fireball 265
54. Meteorite found in Chili in 1866 265
55. Meteorite which fell at Orgueil in 1864 265
56. Fireball of Nov. 23, 1877, 8^h 24^m (J. Plant.) 269
57. Flight of Telescopic Meteors seen by W. R. Brooks 272
58. Meteor of Dec. 28, 1888, 6^h 17^m 277
59. Large Meteor and streak seen at Jask 278
60. The Constellation Orion 289
61. Diagram illustrating the Measurement of Angles of Position 291
62. Double Stars 301
63. Trapezium in Orion as seen with the 36-inch refractor 319
64. Nebulæ and a Star-cluster 336
65. Nebula within a semicircle of stars 342
TELESCOPIC WORK
FOR
STARLIGHT EVENINGS.
CHAPTER I.
_THE TELESCOPE, ITS INVENTION AND THE DEVELOPMENT OF ITS POWERS._
The instrument which has so vastly extended our knowledge of the
Universe, which has enabled us to acquire observations of remarkable
precision, and supplied the materials for many sublime speculations in
Astronomy, was invented early in the seventeenth century. Apart from
its special application as a means of exploring the heavens with a
capacity that is truly marvellous, it is a construction which has also
been utilized in certain other departments with signal success. It
provided mankind with a medium through which to penetrate far beyond
the reach of natural vision, and to grasp objects and phenomena which
had either eluded detection altogether or had only been seen in dim and
uncertain characters. It has also proved a very efficient instrument
for various minor purposes of instruction and recreation. The invention
of the telescope formed a new era in astronomy; and though, with a
few exceptions, men were slow at first in availing themselves of its
far-seeing resources, scepticism was soon swept aside and its value
became widely acknowledged.
But though the telescope was destined to effect work of the utmost
import, and to reach a very high degree of excellence in after
times, the result was achieved gradually. Step by step its powers
were enlarged and its qualities perfected, and thus the stream of
astronomical discovery has been enabled to flow on, stimulated by every
increase in its capacity.
There is some question as to whom may be justly credited with
the discovery of its principles of construction. Huygens, in his
‘Dioptrics,’ remarks:—“I should have no hesitation in placing above all
the rest of mankind the individual who, solely by his own reflections,
without the aid of any fortuitous circumstances, should have achieved
the invention of the telescope.” There is reason to conclude, however,
that its discovery resulted from accident rather than from theory. It
is commonly supposed that Galileo Galilei is entitled to precedence;
but there is strong evidence to show that he had been anticipated.
In any case it must be admitted that Galilei[1] had priority in
successfully utilizing its resources as a means of observational
discovery; for he it was who, first of all men, saw Jupiter’s
satellites, the crescent form of Venus, the mountains and craters on
the Moon, and announced them to an incredible world.
It has been supposed, and not without some basis of probability, that a
similar instrument to the telescope had been employed by the ancients;
for certain statements contained in old historical records would
suggest that the Greek philosophers had some means of extending their
knowledge further than that permitted by the naked eye. Democritus
remarked that the Galaxy or “Milky Way” was nothing but an assemblage
of minute stars; and it has been asked, How could he have derived this
information but by instrumental aid? It is very probable he gained the
knowledge by inferences having their source in close observation; for
anyone who attentively studies the face of the sky must be naturally
led to conclude that the appearance of the “Milky Way” is induced by
immense and irregular clusterings of small stars. In certain regions
of the heavens there are clear indications of this: the eye is enabled
to glimpse some of the individual star-points, and to observe how they
blend and associate with the denser aggregations which give rise to the
milky whiteness of the Galaxy.
Refracting lenses, or “burning-glasses,” were known at a very early
period. A lens, roughly figured into a convex shape and obviously
intended for magnifying objects, has been recovered from the ruins
of Herculaneum, buried in the ejections from Vesuvius in the year 79
A.D. Pliny and others refer to lenses that burnt by refraction, and
describe globules of glass or crystal which, when exposed in the sun,
transmit sufficient heat to ignite combustible material. The ancients
undoubtedly used tubes in the conduct of their observations, but no
lenses seem to have been employed with them, and their only utility
consisted in the fact of their shutting out the extraneous rays of
light. But spectacles were certainly known at an early period. Concave
emeralds are said to have been employed by Nero in witnessing the
combats of the gladiators, and they appear to have been the same in
effect as the spectacles worn by short-sighted people in our own times.
But the ancients supposed that the emerald possessed inherent qualities
specially helpful to vision, rather than that its utility resulted
simply from its concavity of figure. In the 13th century spectacles
were more generally worn, and the theory of their construction
understood.
It is remarkable that the telescope did not come into use until so
long afterwards. Vague references were made to such an instrument, or
rather suggestions as to the possibilities of its construction, which
show that, although the principle had perhaps been conceived, the idea
was not successfully put into practice. Roger Bacon, who flourished in
the 13th century, wrote in his ‘_Opus Majus_’:—“Greater things may
be performed by refracted light, for, from the foregoing principles,
follows easily that the greatest objects may be seen very small, the
remote very near, and _vice versâ_. For we can give transparent bodies
such form and position with respect to the eye and the object that the
rays are refracted and bent to where we like, so that we, under any
angle, see the objects near or far, and in that manner we can, at a
great distance, read the smallest letters, and we can count atoms and
sand-grains, on account of the greatness of the angle under which they
are seen.”
Fracastor, in a work published at Venice in 1538, states:—“If we look
through two eye-lenses, placed the one upon the other, everything
will appear larger and nearer.” He also says:—“There are made certain
eye-lenses of such a thickness that if the moon or any other celestial
body is viewed through them they appear to be so near that their
distance does not exceed that of the steeples of public buildings.”
In other writings will also be found intimations as to the important
action of lenses; and it is hardly accountable that a matter so
valuable in its bearings was allowed to remain without practical
issues. The progressive tendency and the faculty of invention must
indeed have been in an incipient stage, and contrasts strongly with the
singular avidity with which ideas are seized upon and realized in our
own day.
Many important discoveries have resulted from pure accident; and it
has been stated that the first _bonâ fide_ telescope had its origin in
the following incident:—The children of a spectacle-maker, Zachariah
Jansen, of Middleberg, in Zealand, were playing with some lenses, and
it chanced that they arranged two of them in such manner that, to their
astonishment, the weathercock of an adjoining church appeared much
enlarged and more distinct. Having mentioned the curious fact to their
father, he immediately turned it to account, and, by fixing two lenses
on a board, produced the first telescope!
This view of the case is, however, a very doubtful one, and the
invention may with far greater probability be attributed to Hans
Lippersheim in 1608. Galilei has little claim to be considered in this
relation; for he admitted that in 1609 the news reached him that a
Dutchman had devised an appliance capable of showing distant objects
with remarkable clearness. He thereupon set to work and experimented
with so much aptitude on the principles involved that he very soon
produced a telescope for himself. With this instrument he detected
the four satellites of Jupiter in 1610, and other successes shortly
followed. Being naturally gratified with the improvements he had
effected in its construction, and with the wonderful discoveries he had
made by its use, we can almost excuse the enthusiasm which prompted him
to attribute the invention to his own ingenuity. But while according
him the honour due to his sagacity in devoting this instrument to
such excellent work, we must not overlook the fact that his claim to
priority cannot be justified. Indeed, that Galilei had usurped the
title of inventor is mentioned in letters which passed between the
scientific men of that time. Fuccari, writing to Kepler, says:—“Galileo
wants to be considered the inventor of the telescope, though he, as
well as I and others, first saw the telescope which a certain Dutchman
first brought with him to Venice, and although he has only improved it
very little.”
In a critical article by Dr. Doberck[2], in which this letter is quoted
and the whole question reviewed with considerable care, it is stated
that Hans Lippersheim (also known as Jan Lapprey), who was born in
Wesel, but afterwards settled at Middleberg, in the Netherlands, as
a spectacle-maker, was really the first to make a telescope, and the
following facts are quoted in confirmation:—“He solicited the States,
as early as the 2nd October, 1608, for a patent for thirty years,
or an annual pension for life, for the instrument he had invented,
promising then only to construct such instruments for the Government.
After inviting the inventor to improve the instrument and alter it so
that they could look through it with both eyes at the same time, the
States determined, on the 4th October, that from every province one
deputy should be elected to try the apparatus and make terms with him
concerning the price. This committee declared on the 6th October that
it found the invention useful for the country, and had offered the
inventor 900 florins for the instrument. He had at first asked 3000
florins for three instruments of rock-crystal. He was then ordered
to deliver the instrument within a certain time, and the patent was
promised him on condition that he kept the invention secret. Lapprey
delivered the instrument in due time. He had arranged it for both
eyes, and it was found satisfactory; but they forced him, against the
agreement, to deliver two other telescopes for the same money, and
refused the patent because it was evident that already several others
had learned about the invention.”
The material from which the glasses were figured appears to have been
quartz; and efforts were made to keep the invention a profound secret,
as it was thought it would prove valuable for “strategetical purposes.”
The cost of these primitive binoculars was about £75 each.
It is singular that, after being allowed to rest so long, the idea of
telescopic construction should have been carried into effect by several
persons almost simultaneously, and that doubts and disputes arose as
to precedence. The probable explanation is that to one individual
only priority was really due, but that, owing to the delays, the
secret could not be altogether concealed from two or three others
who recognized the importance of the discovery and at once entered
into competition with the original inventor. Each of these fashioned
his instrument in a slightly different manner, though the principle
was similar in all; and having in a great measure to rely upon his
individual faculties in completing the task, he considered himself in
the light of an inventor and put forth claims accordingly. Not only
were attempts made to assume the position of inventor, but there arose
fraudulent claimants to some of the discoveries which the instrument
effected in the hands of Galilei. Simon Marius, himself one of the very
first to construct a telescope and apply it to the examination of the
heavenly bodies, asserted that he had seen the satellites of Jupiter
on December 29, 1609, a few days before Galilei, who first glimpsed
them on January 7, 1610. Humboldt, in his ‘Physical Description of the
Heavens,’ definitely ascribes the discovery of these moons to Marius;
but other authorities uniformly reject the statement, and accord to
Galilei the full credit.
It is stated that Galilei’s first instrument magnified only three
times, but he so far managed to amplify its resources that he was
ultimately enabled to apply a power of 30. The lenses consisted of a
double-convex object-glass, and a small double-concave eye-glass placed
in front of the focal image formed by the object-glass. The ordinary
opera-glass is constructed on a similar principle.
[Illustration: Fig. 1.
The Galilean Telescope.]
The discoveries which Galilei effected with this crude and defective
instrument caused a great sensation at the time. He made them known
through the medium of a publication which he issued under the title
of ‘_Nuncius Siderus_,’ or ‘The Messenger of the Stars.’ In that
superstitious age great ignorance prevailed, bigotry was dominant,
and erroneous views of the solar system were upheld and taught
by authority. We can therefore readily conceive that Galilei’s
discoveries, and the direct inferences he put upon them, being
held antagonistic to the ruling doctrines, would be received with
incredulity and opposition. His views were regarded as heretical. In
consequence of upholding the Copernican system he suffered persecution,
and had to resort to artifice in the publication of his works. But the
marvels revealed by his telescope, though discredited at first, could
not fail to meet with final acceptance, for undeniable testimony to
their reality was soon forthcoming. They were not, however, regarded
until long afterwards as affirming the views enunciated by their clever
author. Ultimately the new astronomy, based on the irrepressible
evidence of the telescope, and clad in all the habiliments of truth,
took the place of the old fallacious beliefs, to form an enduring
monument to Copernicus and Galilei, who spent their lives in advancing
its cause.
No special developments in the construction of the telescope appear
to have taken place until nearly half a century subsequent to its
invention. Kepler suggested an instrument formed of two convex
lenses, and Scheiner and Huygens made telescopes on this principle
in the middle of the 17th century. Huygens found great advantage
in the employment of a compound eyepiece consisting of two convex
lenses, which corrected the spherical aberration, and, besides being
achromatic, gave a much larger field than the single lens. This
eyepiece, known as the “Huygenian,” still finds favour with the makers
of telescopes.
[Illustration: Fig. 2.
Royal Observatory, Greenwich, in Flamsteed’s time[3].]
Huygens may be said to have inaugurated the era of _long_ telescopes.
He erected instruments of 12 and 23 feet, having an aperture of 2-1/3
inches and powers of 48, 50, and 92. He afterwards produced one
123 feet in focal length and 6 inches in aperture. Chief among his
discoveries were the largest satellite of Saturn (Titan) and the true
form of Saturn’s ring. Hevelius of Dantzic built an instrument 150 feet
long, which he fixed to a mast 90 feet in height, and regulated by
ropes and pulleys. Cassini, at the Observatory at Paris, had telescopes
by Campani of 86, 100, and 136 French feet in length; but the highest
powers he used on these instruments do not appear to have exceeded
150 times. He made such good use of them as to discover three of the
satellites of Saturn and the black division in the ring of that planet.
The largest object-glasses employed by Hevelius and Cassini were of
6, 7, and 8 inches diameter. This was during the latter half of the
17th century. In 1712 Bradley made observations of Venus, and obtained
measures of the planet’s diameter, with a telescope no less than 212
feet in focal length. The instruments alluded to were manipulated with
extreme difficulty, and observations had to be conducted in a manner
very trying to the observer. Tubes were sometimes dispensed with, the
object-glass being fixed to a pole and its position controlled by
various contrivances—the observer being so far off, however, that he
required the services of a good lantern in order to distinguish it!
The immoderate lengths of refracting-telescopes were necessary, as
partially avoiding the effects of chromatic aberration occasioned
by the different refrangibility of the seven coloured rays which
collectively make white light. In other words, the coloured rays having
various indices of refraction cannot be brought to a coincident focus
by transmission through a single lens. Thus the red rays have a longer
focus than the violet rays, and the immediate effect of the different
refractions becomes apparent in the telescopic images, which are
fringed with colour and not sharply defined. High magnifying powers
serve to intensify the obstacle alluded to, and thus the old observers
found it imperative to employ eye-glasses not beyond a certain degree
of convexity. The great focal lengths of their object-lenses enabled
moderate power to be obtained, though the eye-glass itself had a focus
of several inches and magnified very little.
Sir Isaac Newton made many experiments upon colours, and endeavoured
to obviate the difficulties of chromatic aberration, but erroneously
concluded that it was not feasible. He could devise no means to correct
that dispersion of colour which, in the telescopes of his day, so
greatly detracted from their effectiveness. His failure seems to have
had a prejudicial effect in delaying the solution of the difficulty,
which was not accomplished until many years afterwards.
[Illustration: Fig. 3.
Sir Isaac Newton[4].]
[Illustration: Fig. 4.
Gregorian Telescope.]
The idea of reflecting-telescopes received mention as early as 1639;
but it was not until 1663 that Gregory described the instrument,
formed of concave mirrors, which still bears his name. He was not,
however, proficient in mechanics, and after some futile attempts to
carry his theory into effect the exertion was relinquished. In 1673
Cassegrain revived the subject, and proposed a modification of the form
previously indicated by Gregory. Instead of the small concave mirror,
he substituted a convex mirror placed nearer the speculum; and this
arrangement, though it made the telescope shorter, had the disadvantage
of displaying objects in an inverted position. But the utility of these
instruments was not demonstrated in a practical form until 1674, when
Hooke, the clever mechanician, gave his attention to the subject and
constructed the first one that was made of the kind.
[Illustration: Fig. 5.
Cassegrainian Telescope.]
In the meantime (1672) Sir Isaac Newton had completed with his own
hands a reflecting-telescope of another pattern. In this the rays from
the large concave speculum were received by a small plane mirror fixed
centrally at the other end of the tube, and inclined at an angle of
45°; so that the image was directed at right angles through an opening
in the side, and there magnified by the eye-lens. But for a long period
little progress was effected in regard to reflecting-telescopes, owing
to the difficulty of procuring metal well adapted for the making of
specula.
[Illustration: Fig. 6.
Newtonian Telescope.]
In 1729 Mr. Chester Moor Hall applied himself to the study of
refracting-telescopes and discovered that, by a combination of
different glasses, the colouring of the images might be eliminated.
It is stated that Mr. Hall made several achromatic glasses in 1733. A
quarter of a century after this John Dollond independently arrived at
the same result, and took out a patent for achromatic telescopes. He
found, by experiments with prisms, that crown and flint glass operated
unequally in regard to the divergency of colours induced by refraction;
and, applying the principle further, he obtained a virtually colourless
telescope by assorting a convex crown lens with a concave flint lens
as the object-glass. Dollond also made many instruments having triple
object-lenses, and in these it was supposed that previous defects were
altogether obliterated. Two convex lenses of crown glass were combined
with a concave lens of flint glass placed between them.
Whether we regard Hall or Dollond as entitled to the most praise in
connection with this important advance, it is certain that it was
one the value of which could hardly be overestimated. It may be said
to have formed a new era in practical astronomy. Instruments only 4
or 5 feet long could now be made equally if not more effective than
those of 123 and 150 feet previously used by Huygens and Hevelius.
All the troubles incidental to these long unmanageable machines now
disappeared, and astronomers were at once provided with a handy little
telescope capable of the finest performances.
[Illustration: Fig. 7.
Common Refracting-Telescope.]
Reflecting-telescopes also underwent marked improvements in the
eighteenth century. Short, the optician, who died in 1768, was
deservedly celebrated for the excellent instruments he made of the
Gregorian form. Towards the latter part of the century William
Herschel, by indomitable perseverance, figured a considerable number
of specula. Some of these were mounted as Newtonians; others were
employed in the form known as the “Front view,” in which a second
mirror is dispensed with altogether, and the rays from the large
concave speculum are thrown to the side of the tube and direct to the
eyepiece. This construction is often mentioned as the “Herschelian,”
but the idea had long before been detailed by Le Maire. In 1728 he
presented a paper to the Académie des Sciences, giving his plans for
a new reflecting-telescope. He proposed to suppress the small flat
speculum in Newtonians, and “by giving the large concave speculum a
little inclination, he threw the image, formed in its focus, to one
side of the tube, where, an eye-glass magnifying it, the observer
viewed it, his back at the time being turned towards the object in the
heavens; thus the light lost in the Newtonian telescope by the second
reflexion was saved.”
[Illustration: Fig. 8.
The Le Mairean or Herschelian Telescope.]
After making several instruments of from 18 to 24 inches aperture,
Herschel began one of larger calibre, and it was finished on August
28, 1789. The occasion was rendered historical by the discovery of one
of the faintest interior satellites of Saturn, Enceladus. The large
telescope had a speculum 48 inches in diameter; the tube was made of
rolled or sheet iron, and it was 39 ft. 4 in. long and 4 ft. 10 in. in
diameter. It was by far the largest instrument the world had seen up
to that time; but it cannot be said to have realized the expectations
formed of its powers, for its defining properties were evidently
not on a par with its space-penetrating power. Many of Herschel’s
best observations were made with much smaller instruments. The large
telescope, which was mounted in Herschel’s garden at Slough, soon fell
into comparative disuse, and, regarding it as incapable of further
usefulness, Sir John Herschel sealed it up on January 1, 1840.
During the next half-century we hear of no attempts being made to
surpass the large instrument which formed one of the working-tools
of Herschel. Then, however, Lord Rosse entered the field, and in the
‘Philosophical Transactions’ for 1840 described a reflector of 3-feet
diameter which he had set up at his residence at Parsonstown, Ireland.
In 1845 the same nobleman, distinguished alike for his scientific
attainments as for his generosity and urbanity of disposition, erected
another telescope, the large speculum of which was 6 feet in diameter,
5½ inches in thickness, and its weight 3 tons. Lord Rosse subsequently
cast a duplicate speculum of 6 feet and weighing 4 tons. In point of
dimensions this instrument far exceeded that of Herschel, and it is
still in use, retaining its character as the largest, though certainly
not the best, telescope in existence. Its tube is made of 1-inch deal,
well bound together with iron hoops; it is 56 feet long and 7 feet in
diameter.
Mr. Lassell soon afterwards made large specula. He erected one of
2-feet aperture and 20-feet focus at his residence at Starfield, near
Liverpool, and in 1861 mounted one of 4-feet diameter and 37-feet
focus. This instrument was for some time usefully employed by him
at Malta. After Mr. Lassell’s return to England his great telescope
remained in a dismantled state for several years, and ultimately
the speculum was broken up and “consigned to the crucible of the
bell-founder.”
It is not a little remarkable that Herschel, Rosse, and Lassell
personally superintended and assisted in the construction of the
monster instruments with which their names are so honourably associated.
In or about the year 1867 a telescope of the Cassegrainian form, and
having a metallic speculum 4 feet in diameter and 28-feet focus, was
completed by Grubb of Dublin for the observatory at Melbourne. This
instrument, which cost something like £14,000, was found defective
at first, though the fault does not appear to have rested with the
optician.
Up to this period specula were formed of a metal in which copper and
tin were largely represented. But the days of metal specula were
numbered. Leon Foucault, in the year 1859, published a valuable memoir
in which he described the various ingenious methods he employed in
figuring surfaces of glass to the required curve. He furnished data for
determining accuracy of figure. Formerly opticians had considerable
trouble in deciding the quality of their newly-ground specula or
object-glasses. They found it expedient to mount them temporarily,
and then, by actual trial on difficult objects, to judge of their
efficiency. This involved labour and occasioned delay, especially in
the case of large instruments. Foucault showed that crucial tests might
be applied in the workshop, and that glasses could be turned out of
hand without any misgivings as to their perfection of figure.
Foucault’s early experiments in parabolizing glass led him to
important results. By depositing a thin coating of silver on his
specula he obtained a reflective power far surpassing that of metal.
Thereafter metal was not thought of as a suitable material for
reflecting-telescopes. Silver-on-glass mirrors immediately came into
great request. The latter undoubtedly possess a great superiority
over metal, especially as regards light-grasping power, the relative
capacity according to Sir J. Herschel being as ·824 to ·436. Glass
mirrors have also another advantage in being less heavy than those of
metal. It is true the silver film is not very durable, but it can be
renewed at any time with little trouble or expense.
With of Hereford, and after him Calver of Chelmsford, became noted
for the excellency of their glass mirrors. They were found nearly
comparable to refractors of the same aperture.
A tendency of the times was evidently in the direction of large
instruments. One of 47·2-inches aperture (for which a sum of 190,000
francs was paid) was completed by Martin in 1875 for the Paris
Observatory, but its employment since that year has not furnished a
very successful record. The largest instrument of the kind yet made has
a speculum 5 feet in diameter and 27½-feet focal length. It was placed
in position in September 1888, and was made by the owner, Mr. Common,
of Ealing, whose previous instrument was a 37-inch glass reflector by
Calver. The 5-foot telescope is undoubtedly of much greater capacity
than the colossal reflector of Lord Rosse, though it is not so large.
Mr. Calver has recently figured a 50-inch mirror for Sir H. Bessemer,
but the mounting is not completed; and he is expecting to make other
large reflectors, viz. one over 5 feet in diameter and another over
3 feet. The late Mr. Nasmyth also erected some fine instruments, and
adopted a combination of the Cassegrainian and Newtonian forms to
ensure greater convenience for the observer. Instead of permitting the
rays from the small convex mirror to return through the large mirror,
he diverted them through the side of the tube by means of a flat
mirror, as in Newtonians. But this construction is not to be commended,
because much light is lost and defects increased by the additional
mirror.
Smaller telescopes of the kind we have been referring to have become
extremely popular: and deservedly so. They are likely to maintain
their character in future years; for the Newtonian form of instrument,
besides being thoroughly effective in critical work, is moderate in
price and gives images absolutely achromatic. Moreover, it is used
with a facility and ease which an experienced observer knows how to
appreciate. Whatever may be the altitude of the objects under scrutiny,
he is enabled to retain a perfectly convenient and natural posture, and
may pursue his work during long intervals without any of the fatigue or
discomfort incidental to the use of certain other forms of instrument.
Returning now to refractors: many years elapsed after Dollond patented
his achromatic object-glass before it was found feasible to construct
these instruments of a size sufficient to grasp faint and delicate
objects. Opticians were thwarted in their efforts to obtain glass of
the requisite purity for lenses, unless in small disks very few inches
in diameter. It is related that Dollond met with a pot of uncommonly
pure flint glass in 1760, but even with this advantage of material he
admitted that, after numerous attempts, he could not provide really
excellent object-glasses of more than 3-3/4-inches diameter. It may
therefore be readily imagined that a refractor of 4½ or 5-inches
aperture was an instrument of great rarity and expense. Towards the
latter part of the 18th century Tulley’s price was £275 for a 5-inch
equatoreally mounted.
[Illustration: Fig. 9.
10-inch Reflecting-Telescope on a German Equatoreal, by Calver.]
In later years marked improvements were effected in the manufacture of
glass. A sign of this is apparent in the fact that, in 1829, Sir James
South was enabled to purchase a 12-inch lens. Four years before this
the Dorpat telescope, having an objective of 9½ inches, had created
quite a sensation. As time went on, still larger glasses were made. In
1862 Alvan Clark & Sons, of New York, U.S.A., finished an instrument
of 18½-inches aperture, at a cost of £3700; and in 1869 Cooke & Sons
mounted a 24·6-inch object-glass for the late Mr. Newall, of Gateshead.
The latter instrument was much larger than any other refractor hitherto
made, but it was not long to maintain supremacy. One of 25·8 inches
and 29-feet focus was finished in 1872 by Alvan Clark & Sons for the
Naval Observatory, Washington, at a cost of £9000. Another, of similar
size, was supplied by the same firm to Mr. McCormick, U.S.A. Several
important discoveries, including the satellites of Mars, were effected
with the great Washington telescope. A few years later a 27-inch was
completed by Grubb for the Vienna Observatory, and quite recently the
four largest refractors ever made have been placed in position and
are actively employed in various departments of work. These include a
29-inch by Martin for the Paris Observatory, a 30-inch by Henry Bros.
for Nice, a 30-inch by A. Clark & Sons for Pulkowa, and a 36-inch,
also by A. Clark & Sons, for the Lick Observatory on Mount Hamilton
in California. The latter has no rival in point of size, though
rumours are current that still larger lenses are in contemplation.
The tube of the 36-inch is 56 feet long and 3½ feet in diameter at
the ends, but the diameter is greater in the middle. It is placed
within a great dome 75 feet in diameter. The expense of the entire
apparatus is given as follows:—Cost of the dome, $56,850; of the visual
objective, $53,000; of the photographic objective, $13,000; of the
mounting, $42,000. Total, $164,850. This noble instrument—due to the
munificence of one individual, the late Mr. James Lick, of Chicago, who
bequeathed $700,000 for the purpose—may be regarded as the king of
refracting-telescopes. Placed on the summit of Mount Hamilton, where
the atmosphere is exceptionally favourable for celestial observations,
and utilized as its resources are by some of the best observers in
America, we may confidently expect it to largely augment our knowledge
of the heavenly bodies.
The great development in the powers of both refracting and
reflecting-telescopes, as a means of astronomical discovery,
exemplifies in a remarkable degree the ever-increasing resources and
refinements of mechanical art. In 1610 Galilei, from his window at
Padua, first viewed the moon and planets with his crude instrument
having a power of 3, and he achieved much during the remaining years he
lived, by increasing it tenfold, so that at last he could magnify an
object 30 times. Huygens laboured well in the same field; and others
who succeeded him formed links in the chain of progress which has
almost uninterruptedly run through all the years separating Galilei’s
time from our own. The primitive efforts of the Florentine philosopher
appear to have had their sequel in the magnificent telescope which has
lately been erected under the pure sky of Mount Hamilton. The capacity
of this instrument relatively to that of earlier ones may be judged
from the fact that a power of about 3300 times has lately been employed
with success in the measurement of a close and difficult double star.
Could Galilei but stand for a few moments at the eyepiece of this great
refractor, and contemplate the same objects which he saw, nearly three
centuries ago, through his imperfect little glasses at Padua, he would
be appalled at the splendid achievements of modern science.
FOOTNOTES:
[1] Galileo Galilei is very generally called by his christian name, but
I depart from this practice here.
[2] ‘Observatory,’ vol. ii. p. 364.
[3] Reproduced, by permission, from Cassell’s ‘New Popular Educator.’
[4] Reproduced, by permission, from Cassell’s ‘New Popular Educator.’
CHAPTER II.
_RELATIVE MERITS OF LARGE AND SMALL TELESCOPES._
The number of large telescopes having so greatly increased in recent
years, and there being every prospect that the demand for such
instruments will continue, it may be well to consider their advantages
as compared with those of much inferior size. Object-glasses and
specula will probably soon be made of a diameter not hitherto attained;
for it is palpably one of the ambitions of the age to surpass all
previous efforts in the way of telescopic construction. There are some
who doubt that such enormous instruments are really necessary, and
question whether the results obtained with them are sufficient return
for the great expense involved in their erection. Large instruments
require large observatories; and the latter must be at some distance
from a town, and in a locality where the atmosphere is favourable.
Nothing can be done with great aperture in the presence of smoke and
other vapours, which, as they cross the field, become ruinous to
definition. Moreover, a big instrument is not to be manipulated with
the same facility as a small one: and when anything goes wrong with
it, its rectification may be a serious matter, owing to the size.
Such telescopes need constant attention if they would be kept in
thorough working order. On the other hand, small instruments involve
little outlay, they are very portable, and require little space. They
may be employed in or out of doors, according to the inclination and
convenience of the observer. They are controlled with the greatest
ease, and seldom get out of adjustment. They are less susceptible to
atmospheric influences than larger instruments, and hence may be used
more frequently with success and at places by no means favourably
situated in this respect. Finally, their defining powers are of
such excellent character as to compensate in a measure for feeble
illumination.
In discussing this question it will be advisable to glance at the
performances of certain instruments of considerable size.
The introduction of really large glasses dates from a century ago,
when Sir W. Herschel mounted his reflector, 4 feet in aperture, at
Slough. He discovered two of the inner satellites of Saturn very soon
after it was completed; but apart from this the instrument seems to
have achieved little. Herschel remarked that on August 28, 1789, when
he brought the great instrument to the parallel of Saturn, he saw
the spots upon the planet better than he had ever seen them before.
The night was probably an exceptionally good one, for we do not find
this praise reiterated. Indeed, Herschel appears to have practically
discarded his large instrument for others of less size. He found that
with his small specula of 7-ft. focus and 6·3-in. aperture he had
“light sufficient to see the belts of Saturn completely well, and
that here the maximum of distinctness might be much easier obtained
than where large apertures are concerned.” Even in his sweeps for
nebulæ he employed a speculum of 20-ft. focus and 18½-in. aperture in
preference to his 4-ft. instrument, though on objects of this nature
light-grasping power is essentially necessary. The labour and loss of
time involved in controlling the large telescope probably led to its
being laid aside for more ready means, though Herschel was not the man
to spare trouble when an object was to be gained. His life was spent
in gleaning new facts from the sky; and had the 4-foot served his
purpose better than smaller instruments, no trifling obstacle would
have deterred him from its constant employment. But his aim was to
accomplish as much as possible in every available hour when the stars
were shining, and experience doubtless taught him to rely chiefly upon
his smaller appliances as being the most serviceable. The Le Mairean
form, or “Front view,” which Herschel adopted for the large instrument
may quite possibly have been in some degree responsible for its bad
definition.
[Illustration: Fig. 10.
Lord Rosse’s 6-foot Reflecting-Telescope.]
Lord Rosse’s 6-ft. reflector has now been used for nearly half a
century, and its results ought to furnish us with good evidence as
to the value of such instruments. It has done important work on the
nebulæ, especially in the re-observation of the objects in Sir J.
Herschel’s Catalogues of 1833 and 1864. To this instrument is due
the discovery of spiral nebulæ; and perhaps this achievement is its
best. But when we reflect on the length of its service, we are led
to wonder that so little has been accomplished. For thirty years the
satellites of Mars eluded its grasp, and then fell a prize to one
of the large American telescopes. The bright planets[5] have been
sometimes submitted to its powers, and careful drawings executed by
good observers; but they show no extent of detail beyond what may be
discerned in a small telescope. This does not necessarily impugn the
figure of the large speculum, the performance of which is entirely
dependent upon the condition of the air. The late Dr. Robinson, of
Armagh, who had the direction of the instrument for sometime, wrote
in 1871:—“A stream of heated air passing before the telescope, the
agitation and hygrometric state of the atmosphere, and any differences
of temperature between the speculum and the air in the tube are all
capable of injuring or even destroying definition, though the speculum
were absolutely perfect. The effect of these disturbances is, in
reflectors, as the cube of their apertures; and hence there are few
hours in the year when the 6-foot can display its full powers.” Another
of the regular observers, Mr. G. J. Stoney, wrote in 1878:—“The usual
appearance [of the double star γ^2 Andromedæ] with the best mirrors was
a single bright mass of blue light some seconds in diameter and boiling
violently.” On the best nights, however, “the disturbance of the air
would seem now and then suddenly to cease for perhaps half a second,
and the star would then instantly become two very minute round specks
of white light, with an interval between which, from recollection, I
would estimate as equal to the diameter of either of them, or perhaps
slightly less. The instrument would have furnished this appearance
uninterruptedly if the state of the air had permitted.” The present
observer in charge, Dr. Boeddicker, wrote the author in 1889:—“There
can be no doubt that on favourable nights the definition of the 6-foot
is equal to that of any instrument, as is fully shown by Dr. Copeland’s
drawings of Jupiter published in the ‘Monthly Notices’ for March 1874.
It appears to me, however, that the advantage in going from the 3-foot
to the 6-foot is not so great in the case of planets as in the case
of nebulæ; yet, as to the Moon, the detail revealed by the 6-foot on
a first-class night is simply astounding. The large telescope is a
Newtonian mounted on a universal joint. For the outlying portions of
the great drawing of the Orion nebula it was used as a Herschelian. As
to powers profitably to be used, I find no advantage in going beyond
600; yet formerly on short occasions (not longer than perhaps 1 hour
a night) very much higher powers (over 1000) have been successfully
employed by my predecessors.”
Mr. Lassell’s 4-foot reflector was taken to Malta, and while there its
owner, assisted by Mr. Marth, discovered a large number of nebulæ with
it, but it appears to have done nothing else. His 2-foot reflector,
which he had employed in previous years, seems to have been his most
effective instrument; for with this he discovered Ariel and Umbriel,
the two inner satellites of Uranus, Hyperion, the faintest satellite
of Saturn, and the only known satellite of Neptune. He also was one
of the first to distinguish the crape ring of Saturn. Mr. Lassell had
many years of experience in the use of large reflectors; and in 1871 he
wrote:—“There are formidable and, I fear, insurmountable difficulties
attending the construction of telescopes of large size.... These are,
primarily, the errors and disturbances of the atmosphere and the
flexure of the object-glasses or specula. The visible errors of the
atmosphere are, I believe, generally in proportion to the aperture
of the telescope.... Up to the size [referring to an 8-in. O.-G.] in
question, seasons of tranquil sky may be found when its errors are
scarcely appreciable; but when we go much beyond this limit (say to 2
feet and upwards), both these difficulties become truly formidable. It
is true that the defect of flexure may be in some degree eliminated,
but that of atmospheric disturbance is quite unassailable. These
circumstances will always make large telescopes _proportionately_ less
powerful than smaller ones; but notwithstanding these disadvantages
they will, on some heavenly objects, reveal more than any small ones
can.” Mr. Lassell’s last sentence refers to “delineations of the forms
of the fainter nebulæ,” to “seeing the inner satellites of Uranus, the
satellite of Neptune, and the seventh satellite of Saturn.” He mentions
that, when at Malta, he “saw, in the 2-foot equatoreal, with a power of
1027, the two components of γ^2 Andromedæ distinctly separated to the
distance of a neat diameter of the smaller one. Now, no telescope of
anything like 8-inches diameter could exhibit the star in this style.”
The large Cooke refractor of 24·8-inches aperture, which has been
mounted for about twenty years at Gateshead, has a singularly barren
record. Its atmospheric surroundings appear to have rendered it
impotent. The owner of this fine and costly instrument wrote the author
in 1885:—“Atmosphere has an immense deal to do with definition. I have
only had one fine night since 1870! I then saw what I have never seen
since.”
The Melbourne reflector of 4-feet aperture performed very indifferently
for some years, and little work was accomplished with it. Latterly its
performance has been more satisfactory; excellent photographs of the
Moon have been taken, and it has been much employed in observations
of nebulæ. The speculum having recently become tarnished, it has been
dismounted for the purpose of being repolished.
The silver-on-glass reflector of 47·2-in. diameter, at the Paris
Observatory, was used for some years by M. Wolf, who has also had the
control of smaller telescopes. He was in a favourable position to judge
of their relative effectiveness. In a lecture delivered at the Sardonne
on March 6, 1886, he said:—“During the years I have observed with the
great Parisian telescope I have found but one solitary night when the
mirror was perfect.” Further on, he adds:—“I have observed a great
deal with the two instruments [both reflectors] of 15·7 inches and
47·2 inches. I have rarely found any advantage in using the larger one
when the object was sufficiently luminous.” M. Wolf also avers that a
refractor of 15 inches or reflector of 15·7 inches will show everything
in the heavens that can be discovered by instruments of very large
aperture. He always found a telescope of 15·7-inch aperture surpass one
of 7·9 inches, but expresses himself confidently that beyond about 15
inches increased aperture is no gain.
The Washington refractor of 25·8 inches effected a splendid success
in Prof. Hall’s hands in 1877, when it revealed the two satellites of
Mars. But immediately afterwards these minute bodies were shown in
much smaller instruments; whence it became obvious that their original
discovery was not entirely due to the grasp of the 25·8-inch telescope,
but in a measure to the astuteness displayed by Prof. Hall in the
search. A good observer had been associated with a good telescope; and
an inviting research having been undertaken, it produced the natural
result—an important success. The same instrument, in the same hands,
enabled the rotation-period of Saturn to be accurately determined by
means of a white spot visible in December 1876 on the disk of the
planet, and which was subsequently seen by other observers with smaller
glasses. Good work in other directions has also been accomplished
at Washington, especially in observations of double stars and faint
satellites. But notwithstanding these excellent performances, Prof.
Hall expressed himself in rather disparaging terms of his appliances,
saying “the large telescope does not show enough detail.” He gave
a more favourable report in 1888; for we find it stated that “the
objective retains its figure and polish well. By comparison with
several other objectives which Prof. Hall has had an opportunity of
seeing during recent years, he finds that the glass is an excellent
one.”
Prof. Young, who has charge of the 23-inch refractor at Princeton, has
also commented on the subject of the definition of large telescopes. He
says:—“The greater susceptibility of large instruments to atmospheric
disturbances is most sadly true; and yet, on the whole, I find also
true what Mr. Clark told me would be the case on first mounting
our 23-inch instrument, that _I can almost always see with the
23-inch everything I see with the 9½-inch under the same atmospheric
conditions, and see it better_,—if the seeing is bad only a little
better, if good immensely better.” Prof. Young also mentioned that
a power of 1200 on the 23-inch “worked perfectly on Jupiter on two
different evenings in the spring of 1885 in bringing out fine details
relating to the red spot and showing the true forms of certain white
dots on the S. polar belt.”
The 26-inch refractor at the Leander McCormick Observatory, U. S.
A., is successfully engaged in observations of nebulæ, and many new
objects of this character have been found. It does not appear that the
telescope is much used for other purposes; so that we can attach no
significance to the fact that important discoveries have not been made
with it in other departments.
The great Vienna refractor of 27-inches aperture “does not seem to
accomplish quite what was expected of it,” according to Mr. Sawerthal,
who recently visited the Observatory at Währing, Vienna. The Director,
Dr. Weiss, states in his last report that “the 27-inch Grubb refractor
has only been occasionally used, when the objects were too faint for
the handier instruments.”
The still larger telescopes erected at the Observatories at Pulkowa and
Nice have so recently come into employment that it would be premature
to judge of their performance. In the Annual Report from Pulkowa
(1887) it is stated that Dr. H. Struve was using the 30-inch refractor
“in measuring those of Burnham’s double stars which are only seldom
measurable with the ‘old 15-inch,’ together with other stars of which
measures are scarce. He made 460 measures in eight or nine months, as
well as 166 micro metric observations of the fainter satellites of
Saturn and 15 of that of Neptune.” At Nice the 30-inch refractor was
employed by M. Perrotin in physical observations of Mars in May and
June 1888. The canal-shaped markings of Schiaparelli were confirmed,
and some of them were traced “from the ocean of the southern hemisphere
right across both continents and seas up to the north polar ice-cap.”
The 30-inch also showed some remarkable changes in the markings;
but these were not confirmed at other observatories. The telescope
evidently revealed a considerable amount of detail on this planet;
whence we may infer that its defining power is highly satisfactory.
The great Lick refractor, which appears to have been “first directed to
the heavens from its permanent home on Mount Hamilton on the evening of
January 3, 1888,” has been found ample work by the zealous astronomers
who have it in charge. Prof. Holden, in speaking of it, says:—“It needs
peculiar conditions, but when all the conditions are favourable its
performance is superb.” Mr. Keeler, one of the observers, writes that,
on January 7, 1888, when Saturn was examined, “he not only shone with
the brilliancy due to the great size of the objective, but the minutest
details of his surface were visible with wonderful distinctness. The
outlines of the rings were very sharply defined with a power of 1000.”
Mr. Keeler adds:—“According to my experience, there is a direct gain in
power with increase of aperture. The 12-inch equatoreal brings to view
objects entirely beyond the reach of the 6½-inch telescope, and details
almost beyond perception with the 12-inch are visible at a glance with
the 36-inch equatoreal. The great telescope is equal in defining power
to the smaller ones.” This is no small praise, and it must have been
extremely gratifying, not only to those who were immediately associated
with the construction of the telescope, but to astronomers everywhere
who were hoping to hear a satisfactory report. In its practical results
this instrument has not yet, it is true, given us a discovery of any
magnitude. It has disclosed several very small stars in the trapezium
of the Orion nebula, some difficult double stars have been found and
measured, and some interesting work has been done on the planets and
nebulæ. Physical details have been observed in the ring nebula, between
β and γ Lyræ, which no other telescope has ever reached before.
Mr. Common’s 5-foot reflector has been employed on several objects.
In the spring of 1889 Uranus was frequently observed with it, and
several minute points of light, suspected to be new satellites, were
picked up. Evidence was obtained of a new satellite between Titania
and Umbriel; but bad weather and haze, combined with the low altitude
of Uranus, interfered with the complete success of the observations.
“With only moderate powers, Uranus does not show a perfectly sharp
disk. No markings are visible on it, and nothing like a ring has been
seen round it.” Mr. Common, in a letter to the writer, dated November
9, 1889, says:—“The 5-foot has only been tried in an unfinished state
as yet, the mirror not being quite finished when put into the tube
last year. This was in order to gain experience and save the season.
It performed much better than I had hoped, and is greatly superior to
the 3-foot. I took some very fine photographs with it last year. It
has been refigured, or rather completed, this summer, and has just
been resilvered.” From this it is evident that Mr. Common’s large
instrument has not yet been fully tested; but it clearly gives promise
of successful results, and encourages the hope that it will exert an
influence on the progress of astronomy. Owing to the highly reflective
quality of silvered glass, the 5-foot speculum has a far greater
command of light (space-penetrating power) than the great objective
mounted at the Lick Observatory. Mr. Common’s mirror may therefore be
expected to grasp nebulæ, stars, satellites, and comets which are of
the last degree of faintness and quite invisible in the Lick refractor.
But we must not forget that the latter instrument is certainly placed
in a better atmosphere, and that its action is not therefore arrested
in nearly the same degree by haze and undulations of the air. With
equal conditions, the great reflector at Ealing would probably far
surpass the large refractor we have referred to, the latter having less
than one third of the light-grasping power of the former.
This rapid sketch of the performances of some of our finest telescopes
must suffice for the present in assisting us to estimate their value as
instruments of discovery. And it must be admitted that, on the whole,
these appliances have been disappointing. The record of their successes
is by no means an extended one, and in some individual cases absolute
failure is unmistakable. We must judge of large glasses by their
revelations; their capacity must be estimated by results. We often meet
with glowing descriptions of colossal telescopes: their advantages
are specified and their performances extolled to such a degree that
expectation is raised to the highest pitch. But it is not always that
such praise is justified by facts. The fruit of their employment is
rarely prolific to the extent anticipated, because the observers have
been defeated in their efforts by impediments which inseparably attend
the use of such huge constructions.
Our atmosphere is always in a state of unrest. Its condition is subject
to many variations. Heat, radiated or evolved from terrestrial objects,
rises in waves and floats along with the wind. These vapours exercise a
property of refraction, with the result that, as they pass in front of
celestial objects, the latter at once become subject to a rapid series
of contortions in detail. Their outlines appear tremulous, and all the
features are involved in a rippling effect that seriously compromises
the definition. Delicate markings are quite effaced on a disk which
is thus in a state of ebullition; and on such occasions observers are
rarely able to attain their ends. Telescopic work is, in fact, best
deferred until a time when the air has become more tranquil. In large
instruments these disturbances are very troublesome, as they increase
proportionately with aperture. They are so pronounced and so persistent
as to practically annul the advantage of considerable light-grasping
power; for unless the images are fairly well defined, mere brightness
counts for nothing. Reflectors are peculiarly susceptible to this
obstacle; moreover, the open tube, the fact that rays from an object
pass twice through its length, and that a certain amount of heat
radiated from the observer must travel across the mouth of the tube
all serve to impair the definition. A speculum, to act well, must be
of coincident temperature in every part. This is not always the case,
owing to the variableness of the weather or to unequal exposure of the
speculum. Large refractors, though decidedly less liable to atmospheric
influences, are yet so much at the mercy of them that one of the first
and most important things discussed in regard to a new instrument is
that of a desirable site for it.
The great weight of large objectives and specula tends to endanger the
perfect consistency and durableness of their figure, and imposes a
severe strain upon their cellular mounting. The glasses must obviously
assume a variety of bearings during active employment. This introduces
a possible cause of defective performance; for in some instances
definition has been found unequal, according to the position of the
glass. Specula are very likely to be affected in this manner, as they
are loosely deposited in their cells to allow of expansion, and the
adjustment is easily deranged. The slightest flaw in the mounting of
objectives immediately makes itself apparent in faulty images. Special
precautions are of course taken to prevent flexure and other errors of
the kind alluded to, and modern adaptations may be said to have nearly
eliminated them; but there is always a little outstanding danger, from
the ease with which glasses may be distorted or their adjustment become
unsettled.
Another difficulty formerly urged against telescopes of great size
was the trouble of managing them; but this objection can scarcely
be applied to the fine instruments of the present day, which are
so contrived as to be nearly as tractable as small ones. A century
ago, glass of the requisite purity for large objectives could not be
obtained; but this difficulty appears also to have quite disappeared.
And the process of figuring lenses of considerable diameter is now
effected with the same confidence and success as that of greatly
inferior sizes.
Let us now turn for a moment to the consideration of small instruments,
premising that in this category are included all those up to about
12-inches aperture. Modern advances have quite altered our ideas as to
what may be regarded as large and small telescopes. Sixty-five years
ago the Dorpat refractor, with a 9½-inch objective by Fraunhofer, was
considered a prodigy of its class; now it occupies a very minor place
relatively to the 30-inch and 36-inch objectives at Nice, Pulkowa, and
Mount Hamilton.
Prof. Hall remarked, in 1885:—“There is too much scepticism on the part
of those who are observing with large instruments in regard to what
can be seen with small ones.” This is undoubtedly true; but a mere
prejudice or opinion of this sort cannot affect the question we are
discussing, as it is one essentially relying upon facts.
Small instruments have done a vast amount of useful work in every
field of astronomical observation. Even in the realm of nebulæ, which,
more than any other, requires great penetrating power, D’Arrest showed
what could be effected with small aperture. Burnham, with only a
6-inch refractor, has equally distinguished himself in another branch;
for he has discovered more double stars than any previous observer.
Dawes was one of the most successful amateurs of his day, though his
instrumental means never exceeded an 8-inch glass. But we need not
particularize further. It will be best to get a general result from
the collective evidence of past years. We find that nearly all the
comets, planetoids, double stars, &c. owe their first detection to
comparatively small instruments. Our knowledge of sun-spots, lunar and
planetary features is also very largely derived from similar sources.
There is no department but what is indebted more or less to the
services of small telescopes: the good work they have done is due to
their excellent defining powers and to the facility with which they may
be used.
[Illustration: Fig. 11.
Refracting-Telescope, by Browning.]
We have already said that the record of discoveries made with really
large instruments is limited; but it should also be remarked that until
quite recently the number of such instruments has been very small.
And not always, perhaps, have the best men had the control of them.
Virtually the observer himself constitutes the most important part of
his telescope: it is useless having a glass of great capacity at one
end of a tube, and a man of small capacity at the other. Two different
observers essentially alter the character of an instrument, according
to their individual skill in utilizing its powers.
Large telescopes are invariably constructed for the special purpose of
discovering unknown orbs and gleaning new facts from the firmament.
But in attempting to carry out this design, obstacles of a grave
nature confront the observer. The comparatively tranquil and sharply
definite images seen in small instruments disappear, and in their
places forms are presented much more brilliant and expansive, it is
true, but involved in glare and subject to constant agitation, which
serve to obliterate most of the details. The observer becomes conscious
that what he has gained in light has been lost in definition. At
times—perhaps on one occasion in fifty—this experience is different;
the atmosphere has apparently assumed a state of quiescence, and
objects are seen in a great telescope with the same clearness of detail
as in smaller ones. It is then the observer fully realizes that his
instrument, though generally ineffective, is not itself in fault, and
that it would do valuable work were the normal condition of the air
suitable to the exercise of its capacity.
Those who have effected discoveries with large instruments have done
so in spite of the impediment alluded to. Relying mainly upon great
illuminating power, bad or indifferent definition has been tolerated;
and they have succeeded in detecting minute satellites, faint nebulæ,
clusters, and small companions to double stars. Telescopes of great
aperture are at home in this kind of work. But when we come to consider
discoveries on the surfaces of the Sun, Moon, and planets, the case
is entirely different; the diligent use of small appliances appears
to have left little for the larger constructions to do. There are
some thousands of drawings of the objects named, made by observers
employing telescopes from 3 up to 72 inches in diameter; and a careful
inspection shows that the smaller instruments have not been outdone in
this interesting field of observation. In point of fact they rather
appear to have had the advantage, and the reason of this is perhaps
sufficiently palpable. The details on a bright planetary object are apt
to become obliterated in the glare of a large instrument. Even with a
small telescope objects like Venus and Jupiter are best seen at about
the time of sunset, and before their excessive brilliancy on the dark
sky is enabled to act prejudicially in effacing the delicate markings.
Probably this is one of the causes which, in combination with the
undulations of the atmosphere, have restricted the discoveries of large
instruments chiefly to faint satellites, stars, and nebulæ.
Prof. Young ascribes many of the successes of small instruments to
exceptional cuteness of vision on the part of certain observers, and to
the fact that such instruments are so very numerous and so diligently
used that it is fair to conclude they must reap the main harvest of
discoveries. We must remember that for every observer working with an
aperture of 18 inches and more, there are more than a hundred employing
objectives or specula of from 5 to 12 inches; hence we may expect some
notable instances of keen sight amongst the latter. The success of men
like Dawes and others, who outstrip their contemporaries, and with
small glasses achieve phenomenal results, is to be ascribed partly
to good vision and partly to that natural aptitude and pertinacity
uniformly characteristic of the best observers. These circumstances go
far to explain the unproductiveness of large telescopes: in the race
for distinction they are often distanced by their more numerous and
agile competitors.
The objections which applied to the large reflecting instruments of
Herschel, Lassell, and Rosse scarcely operate with the same force
in regard to the great refractors of the present day, and for these
reasons:—Refractors are somewhat less sensitive to atmospheric
disturbances than reflectors. The modern instruments are mounted in
much improved style, and placed in localities selected for their
reception. In fact, all that the optician’s art can do to perfect such
appliances has been done, and Nature herself has been consulted as to
essentials; for we find the most powerful refractor of all erected on
the summit of Mount Hamilton, where the skies are clear and Urania ever
smiles invitingly.
Some observers who have obtained experience both with large and
small telescopes aver that, even on a bright planet, they can see
more, and often see it much better, with the larger glasses. But we
rarely, if ever, find them saying they can discern anything which is
absolutely beyond the reach of small instruments. It would be much
more satisfactory evidence of the super-excellence of the former if
definite features could be detected which are quite beyond the reach
of telescopes of inferior size; but we seldom meet with experiences of
this kind, and the inference is obvious.
There is undoubtedly a certain aperture which combines in itself
sufficient light-grasping power with excellent definition. It takes a
position midway between great illuminating power and bad definition on
the one hand, and feeble illuminating power and sharp definition on the
other. Such an aperture must form the best working instrument in an
average situation upon ordinary nights and ordinary objects. M. Wolf
fixes this aperture at about 15 inches, and he is probably near the
truth.
The quaint Dr. Kitchener, who, early in the present century, made
a number of trials with fifty-one telescopes, entertained a very
poor opinion of big instruments. In his book on ‘Telescopes,’ he
says:—“Immense telescopes are only about as useful as the enormous
spectacles suspended over the doors of opticians.” ... “Astronomical
amateurs should rather seek for _perfect_ instruments than _large_
ones. What good can a great deal of bad light do?”
We shall be in a better position a few years hence to estimate the
value of great telescopes; for the principal instruments of this class
have only been completed a short time. Judging from the statements
of some of the observers, who are men of the utmost probity and
ability, certain of the large instruments are capable of work far in
advance of anything hitherto done. Definition, they say, is excellent,
notwithstanding the great increase of aperture. The old stumbling-block
appears, therefore, to have been removed, and astronomy is to be
congratulated on the acquirement of such vastly improved implements
of research. Even should the large telescopes continue to prove
disappointing in certain branches, they may certainly be expected to
maintain their advantage in others. They will always be valuable as
a corrective to smaller and handier instruments. For special lines
of work in which very small or very faint objects are concerned,
considerable light-grasping power is absolutely required; and it is
chiefly in these departments that large instruments may be further
expected to augment our knowledge. In photographic and spectroscopic
work they also have a special value, which late researches have brought
prominently to the fore.
The telescopes of the future will probably surpass in dimensions
those of our own day. The University of Los Angelos, in California,
propose to erect a 42-inch refractor on the summit of Wilson’s Peak
of the Sierra Madre mountains, which is 6000 feet high and about 25
miles from Los Angelos. In reference to this contemplated extension
of size, it may be opportune to mention that large objectives do not
transmit light proportionately with their increased diameter, owing to
greater thickness of the lenses, which increases the absorption. The
Washington objective of 25·8-inch aperture is 2·87 inches in thickness,
and more than half the light which falls upon it is lost by absorption.
On the other hand, specula, with every enlargement of aperture, give
proportionately more light-grasping power, and their diameters might
be greatly increased but for the mechanical obstacles in the way of
their construction. Mr. Ranyard expresses the opinion that “with the
refractor we are fast approaching the practical limit of size.” After
referring to the Washington object-glass as above, he says:—“If we
double the thickness, more than three quarters of the light would be
absorbed and less than one quarter would be transmitted. The greatest
loss of light is only for the centre of the object-glass; but in all
parts the absorption is quadrupled for a lens of double aperture.”
If, therefore, future years see any great development in the sizes of
telescopes, it will probably be in connection with reflectors; for the
loss of light by absorption in the thick lenses of large refractors
must ultimately determine their limits. Mr. Calver says:—“The light of
reflectors exceeding 18 inches in diameter is certainly greater than
that of refractors of equal size, and for anything like 3 feet very
much greater.” He nearly obtained the order for a monster reflector for
the Lick Observatory, the Americans admitting that the reflector must
be the instrument of the future for power and light because there were
practically no limits to its size. But the reflector has not been much
used in America, and therefore is little known. For this reason the
authorities decided to erect a large refractor, and they appear to have
been justified in their selection, for the 36-inch objective has proved
excellent.
FOOTNOTES:
[5] Such objects show considerable glare in a very large instrument.
The advent of Jupiter into the field of the 6-foot has been compared to
the brightness of a coach-lamp. The outer satellite of Mars was seen
twice with this instrument in 1877, “but the glare of the planet was
found too strong to allow of good measures being taken.”
CHAPTER III.
_NOTES ON TELESCOPES AND THEIR ACCESSORIES._
Choice of Telescopes.—Refractors and Reflectors.—Observer’s
Aims.—Testing Telescopes.—Mounting.—Eyepieces.—Requisite
Powers.—Overstating Powers.—Method of finding the Power.—Field of
Eyepiece.—Limited Means no obstacle.—Observing-Seats.-Advantage
of Equatoreals.—Test-Objects.—Cheapness and increasing number of
Telescopes.—Utility of Stops.—Cleaning Lenses.—Opera-Glasses.—Dewing
of Mirrors.—Celestial Globe.—Observatories.
_Choice of Telescopes._—The subject of the choice of telescopes
has exercised every astronomer more or less, and the question as
to the best form of instrument is one which has occasioned endless
controversy. The decision is an important one to amateurs, who at
the outset of their observing careers require the most efficient
instruments obtainable at reasonable cost. It is useless applying to
scientific friends who, influenced by different tastes, will give
an amount of contradictory advice that will be very perplexing.
Some invariably recommend a small refractor and unjustly disparage
reflectors, as not only unfitted for very delicate work, but as
constantly needing re-adjustment and resilvering[6].
Others will advise a moderate-sized reflector as affording wonderfully
fine views of the Moon and planets. The question of cost is greatly in
favour of the latter construction, and, all things considered, it may
claim an unquestionable advantage. A man who has decided to spend a
small sum for the purpose not merely of gratifying his curiosity but
of doing really serviceable work, must adopt the reflector, because
refractors of, say, 5 inches and upwards are far too costly, and become
enormously expensive as the diameter increases. This is not the case
with reflectors; they come within the reach of all, and may indeed
be constructed by the observer himself with a little patience and
ingenuity.
_Refractors and Reflectors._—The relative merits of refractors and
reflectors[7] have been so frequently compared and discussed that we
have no desire to re-open the question here. These comparisons have
been rarely free from bias, or sufficiently complete to afford really
conclusive evidence either way. There is no doubt that each form of
instrument possesses its special advantages: aperture for aperture
the refractor is acknowledged to be superior in light-grasping power,
but the ratio given by different observers is not quite concordant.
A silver-on-glass mirror of 8-inches aperture is certainly equal to
a 7-inch objective in this respect, while as regards dividing power
and the definition of planetary markings, the reflector is equal to a
refractor of the same aperture. The much shorter focal length of the
reflector is an advantage not to be overlooked. A century ago Sir W.
Herschel figured his specula to foci of more than a foot to every inch
of aperture, except in the case of his largest instruments. Thus he
made specula of 18½-inches and 24-inches diameter, the former of which
had a focal length of 20 feet and the latter of 25 feet. The glass
mirrors of the present time are much shorter, and the change has not
proved incompatible with excellent performance. Calver has made two
good mirrors of 17-1/4-inches aperture, and only 8 ft. 4 in. focus. Mr.
Common’s 5-foot mirror is only 27½ feet, so that in these instances the
length of the tube is less than six times the diameter.
[Illustration: Fig. 12.
“The Popular Reflector” by Calver.]
It has long been proved that refractors and reflectors alike are, in
good hands, capable of producing equally good results; and we may
depend upon it that, in spite of all argument and experiment, both
kinds of telescope will continue to hold their own until superseded
by a new combination, which hardly seems likely. If the observer is
free from prejudice, he will have no cause to deplore the character
of his instrument, always supposing it to be by a good maker. Be
it object-glass or speculum, he will rarely find it lacking in
effectiveness. It happens only too often that the telescope or the
atmosphere is hastily blamed when the fault rests with the observer
himself. Let him be persistent in waiting opportunities, and let the
instrument be nicely adjusted and in good condition, and in the great
majority of cases it will perform all that can reasonably be expected
of it.
In choosing appliances for observational purposes, the observer will of
course be guided by his means and requirements. If his inclination lead
him to enter a particular department of research, he will take care to
provide himself with such instruments as are specially applicable to
the work in hand. Modern opticians have effected so many improvements,
and brought out so many special aids to smooth the way of an observer,
that it matters little in which direction he advances; he will scarcely
find his progress impeded by want of suitable apparatus. In size, as
also in character, the observer should be careful to discriminate
as to what is really essential. Large instruments and high powers
are not necessary to show what can be sufficiently well seen in a
small telescope with moderate power. Of course there is nothing like
experience in such matters, and practice soon renders one more or less
proficient in applying the best available means.
[Illustration: Fig. 13.
3-inch Refracting-Telescope, by Newton & Co.]
An amateur who really wants a competent instrument and has to
consider cost, will do well to purchase a Newtonian reflector. A
4½-inch refractor will cost about as much as a 10-inch reflector,
but, as a working tool, the latter will possess a great advantage.
A small refractor, if a good one, will do wonders, and is a very
handy appliance, but it will not have sufficient grasp of light for
it to be thoroughly serviceable on faint objects. Anyone who is
hesitating in his choice should look at the cluster about χ Persei
through instruments such as alluded to, and he will be astonished at
the vast difference in favour of the reflector. For viewing sun-spots
and certain lunar objects small refractors are very effective, and
star-images are usually better seen than in reflectors, but the latter
are much preferable for general work on account of three important
advantages, viz., cheapness, illuminating power, and convenience of
observation. When high magnifiers are employed on a refractor of small
aperture, the images of planets become very faint and dusky, so that
details are lost.
_Observer’s Aims._—If the intending observer merely requires a
telescope to exhibit glimpses of the wonders which he has seen
portrayed in books, and has no intention of pursuing the subject
further than as an occasional hobby, he will do well to purchase a
small refractor between 3 and 4 inches in aperture. Such instruments
are extremely effective on the Sun and Moon, which are naturally the
chief objects to attract attention, and, apart from this, appliances
of the size alluded to may be conveniently used from an open window.
The latter is an important consideration to many persons; moreover,
a small telescope of this kind will reveal an astonishing number of
interesting objects in connection with the planets, comets, &c., and
it may be employed by way of diversion upon terrestrial landscape, as
such instruments are almost invariably provided with non-inverting
eyepieces. Out-of-door observing is inconvenient in many respects, and
those who procure a telescope merely to find a little recreation will
soon acknowledge a small refractor to be eminently adapted to their
purposes and conveniences.
Those who meditate going farther afield, and taking up observation
habitually as a means of acquiring practical knowledge, and possibly
of doing original work, will essentially need different means. They
will require reflectors of about 8 or 10 inches aperture; and, if
mounted in the open on solid ground, so much the better, as there
will be a more expansive view, and a freedom from heated currents,
which renders an apartment unsuited to observations, unless with small
apertures where the effects are scarcely appreciable. A reflector
of the diameter mentioned will command sufficient grasp to exhibit
the more delicate features of planetary markings, and will show many
other difficult objects in which the sky abounds. If the observer be
specially interested in the surface configuration of Mars and Jupiter
he will find a reflector a remarkably efficient instrument. On the Moon
and planets it is admitted that its performance is, if not superior,
equal to that of refractors. If, however, the inclination of the
observer leads him in the direction of double stars, their discovery
and measurement, he will perhaps find a refractor more to be depended
upon, though there is no reason why a well-mounted reflector should not
be successfully employed in this branch; and the cost of a refractor
of the size to be really useful as an instrument of discovery must
be something very considerable—perhaps ten times as great as that of
a reflector of equal capacity. As far as my own experience goes the
refractor gives decidedly the best image of a star. In the reflector,
a bright star under moderately high power is seen with rays extending
right across the field, and these appear to be caused by the supports
of the flat.
_Testing Telescopes._—No amateur should buy an instrument, especially
a second-hand one, without testing it, and this is a delicate process
involving many points to be duly weighed. Experience is of great
service in such matters, and is, in fact, absolutely necessary.
Even old observers are sometimes misled as to the real worth of a
glass. In such cases, there is nothing like having a reliable means
of comparison, _i. e._ another telescope of acknowledged excellence
with which to test the doubtful instrument. In the absence of such a
standard judgment will be more difficult, but with care a satisfactory
decision may be arrived at. The Moon is too easy an object for the
purpose of such trials; the observer should rather select Venus or
Jupiter. The former is, however, so brilliant on a dark sky, and so
much affected with glare, that the image will almost sure to be faulty
even if the glass is a good one. Let the hour be either near sunrise or
sunset, and if the planet has a tolerably high altitude, her disk ought
to be seen beautifully sharp and white. Various powers should be tried,
increasing them each time, and it should be noticed particularly
whether the greater expansion of the image ruins the definition or
simply enfeebles the light. In a thoroughly good glass faintness will
come on without seriously impairing the definite contour of the object
viewed, and the observer will realize that the indistinctness is merely
occasioned by the power being relatively in excess of the light-grasp.
But in a defective telescope, a press of magnifying power at once
brings out a mistiness and confuses the details of the image in a very
palpable manner. Try how he will, the observer will find it impossible
to get rid of this, except, perhaps, by a “stop” which cuts off so much
light that the instrument is ineffective for the work required of it.
The blurred image is thought, at the moment of its first perception,
to be caused by the object being out of focus, and the observer vainly
endeavours to get a sharper image until he finds the source of error
lies elsewhere. A well-figured glass ought to come very sharply to a
focus. The slightest turn of the adjusting-screw should make a sensible
difference. On the other hand, an inferior lens will permit a slight
alteration of focusing without affecting the distinctness, because the
rays from the image are not accurately thrown to a point. Jupiter is
also a good test. The limbs of the planet, if shown clean and hard,
and the belts, if they are pictured like the finely cut details of an
engraving, will at once stamp a telescope as one of superior quality.
Saturn can also be examined though not, perhaps, so severe a test.
The belts, crape ring, Cassini’s division, ought to be revealed in
any telescope of moderate aperture. If, with regard to any of these
objects, the details apparently run into each other and there is a
“fuzzy” or woolly aspect about them which cannot be eliminated by
careful focusing, then either the atmosphere or the telescope is in
fault. If the former, another opportunity must be awaited. An observer
of experience will see at a glance whether the cause lies in the air
or the instrument. The images will be agitated by obnoxious currents,
if the defects are due to the atmosphere, but if the glass itself is
in error, then the objects will be comparatively tranquil but merged
in hazy outlines, and a general lack of distinctness will be apparent.
Perhaps the best test of all as to the efficiency of a telescope is
that of a moderately bright star, say of the 2nd or 3rd magnitude. With
a high power the image should be very small, circular, and surrounded
by two or three rings of light lying perfectly concentric with each
other. No rays, wings, or extraneous appearance other than the
diffraction rings should appear.
This, however, specially applies to refractors, for in reflectors the
arms of the flat occasion rays from any bright star; I have also seen
them from Mars, but of course this does not indicate an imperfect
mirror. If there is any distortion on one side of the image, then the
lenses are inaccurately centred though the instrument may be otherwise
good, and a little attention may soon set matters right. When testing a
glass the observer should choose objects at fairly high altitudes, and
not condemn a telescope from a single night’s work unless the evidence
is of unusually convincing character. If false colour is seen in a
silver-on-glass reflector it is originated by the eyepiece, though not
necessarily so in a refractor. The object-glass of the latter will be
sure to show some uncorrected colour fringing a bright object. A good
lens, when exactly focused, exhibits a claret tint, but within the
focus purple is seen and beyond the focus green comes out. In certain
cases the secondary spectrum of an object-glass is so inadequately
corrected that the vivid colouring of the images is sometimes
attributed by inexperienced observers to a real effect. A friend who
used a 3-inch refractor once called on me to have a glimpse of Jupiter
through my 10-inch With-reflector. On looking at the planet he at once
exclaimed “But where are the beautiful colours, Mr. Denning?” I replied
to his question by asking another, viz., “What colours?” he answered,
“Why, the bright colours I see round Jupiter in my refractor?” I said,
“Oh, they exist in your telescope only!” He looked incredulous, and
when he left me that night did not seem altogether pleased with the
appearance of Jupiter shorn of his false hues!
_Mounting._—Too much care cannot be given to the mounting of
telescopes, for the most perfectly figured glass will be rendered
useless by an inefficient stand; a faulty lens, if thoroughly
well mounted, will do more than a really good one on a shaky or
unmanageable mounting. Whatever form is adopted, the arrangement should
ensure the utmost steadiness, combined with every facility for readily
following objects. A man who has every now and then to undergo a
great physical exertion in bodily shifting the instrument is rendered
unfit for delicate work. The telescope should be provided with every
requisite for carrying on prolonged work with slight exertion on the
part of the observer. Unless the stand is firm there will be persistent
vibrations, especially if the instrument is erected in the open, for
there are very few nights in the year when the air is quite calm. These
contingencies should be provided against with scrupulous attention if
the observer would render his telescope most effective for the display
of its powers, and avoid the constant annoyance that must otherwise
follow.
[Illustration: Fig. 14.
Huygens’s negative eyepiece.]
[Illustration: Fig. 15.
Ramsden’s positive eyepiece.]
_Eyepieces._—Good eyepieces are absolutely essential. Many
object-glasses and specula have been deprecated for errors really
originated by the eyepiece. Again, telescopes have not unfrequently
been blamed for failures through want of discrimination in applying
suitable powers. A consistent adaptation of powers according to the
aperture of the telescope, the character of the object, the nature
of the observation, and the atmospheric conditions prevailing at the
time, is necessary to ensure the best results. If it is required to
exhibit a general view of Jupiter and his satellites to a friend, we
must utilize a low power with a large field; if, on the other hand,
we desire to show the red spot and its configuration in detail, we
must apply the highest power that is satisfactorily available. The
_negative_ or Huygenian eyepiece is the one commonly used, and it
forms good colourless images, though the field is rather small. The
_positive_ or Ramsden eyepiece gives a flatter and larger field, but
it is not often achromatic. A Kellner eyepiece, the feature of which
is a very large field, is often serviceable in observations of nebulæ,
clusters, and comets. Telescopes are sometimes stated to bear 100 to
the inch on planets, but this is far beyond their capacities even in
the very best condition of air. Amateurs soon find from experience
that it is best to employ those powers which afford the clearest and
most comprehensive views of the particular objects under scrutiny. Of
course when abnormally high powers are mentioned in connection with an
observation, they have an impressive sound, but this is all, for they
are practically useless for ordinary work. I find that 40, or at the
utmost 50 to the inch, is ample, and generally beyond the capacities
of my 10-inch reflector. A Barlow lens used in front of the eyepiece
raises the power about one third, and thus a whole set of eyepieces may
be increased by its insertion. It is said to improve the definition,
while the loss of light is very trifling. I formerly used a Barlow lens
in all planetary observations, but finally dispensed with it, as I
concluded the improved distinctness did not compensate for the fainter
image. A great advantage, both in light and definition, results from
the employment of a single lens as eyepiece. True, the field is very
limited, and, owing to the spherical aberration, the object so greatly
distorted near the edges that it must be kept near the centre, but, on
the whole, the superiority is most evident. By many careful trials I
find it possible to glimpse far more detail in planetary markings than
with the ordinary eyepiece. Dawes, and other able observers, also found
a great advantage in the single lens, and Sir W. Herschel, more than a
century ago, expressed himself thus:—“I have tried both the double and
single lens eye-glass of equal powers, and always found that the single
eye-glass had much the superiority in light and distinctness.”
_Requisite Powers._—For general purposes I believe three eyepieces
are all that is absolutely requisite, viz., a low power with large
field for sweeping up nebulæ and comets; a moderate power for viewing
the Moon and planets; and a high power for double stars and the more
delicate forms on the planets. For a 3-inch refractor, eyepieces of
about 15, 75, and 150 would be best, and for a 10-inch reflector 40,
150, and 300. For very difficult double stars a still higher power will
be occasionally useful, say 250 for the refractor, and 500 for the
reflector. The definition usually suffers so much under high powers,
and the tremors of the atmosphere are brought out so conspicuously,
that the greater expansion of the image of a planet does not
necessarily enable it to present more observable detail. The features
appear diluted and merged in hazy outlines, and there is a lack of the
bright, sharply determinate forms so steadily recognized under lower
magnifiers. In special cases great power may become essential, and,
under certain favourable circumstances, will prove really serviceable,
but, in a general way, it is admitted that the lowest power which shows
an object well is always the best. I have occasionally obtained very
fair views of Saturn with a power of 865, but find that I can perceive
more of the detail with 252. Some daylight observations of Venus
were also effected under very high power, and, though the definition
remained tolerably good, I found as the result of careful comparison
that less power answered more satisfactorily. But it would be absurd to
lay down inviolable rules in such cases. Special instruments, objects,
and circumstances require special powers, and observers may always
determine with a little care and experience the most eligible means to
support their endeavours. One thing should be particularly remembered,
that the power used must not be beyond the illuminating capacity of
the instrument, for planetary features appear so faint and shady under
excessive magnifiers that nothing is gained. To grasp details there
must be a fair amount of light. I have seen more with 252 on my 10-inch
reflector than with 350 on a 5-1/4-inch refractor, because of the
advantage from the brighter image in the former case.
_Overstating Powers._—It seems to be a fashionable imposition on the
part of opticians to overstate magnifying powers. Eyepieces are usually
advertised at double their true strength. My own 10-inch reflector
was catalogued as having four eyepieces, 100 to 600, but on trial I
found the highest was no more than 330. This custom of exaggerating
powers seems to have long been a privileged deception, and persons
buying telescopes ought to be guarded against it. Dr. Kitchiner says
it originated with the celebrated maker of reflectors, James Short,
and justly condemns it as a practice which should be discontinued.
I suppose it is thought that high powers advertised in connection
with a telescope have an exalted sound and are calculated to attract
the unwary purchaser; but good instruments need no insidious trade
artifices to make them saleable. The practice does not affect observers
of experience, because it is well understood, and they take good
care to test their eyepieces directly they get them. But the case is
different with young and inexperienced amateurs, who naturally enough
accept the words of respectable opticians, only to find, in many cases,
that they have been misleading and a source of considerable annoyance.
_Method of finding the Power._—The magnifying power of a telescope
may be determined by dividing the focal length of the object-glass or
mirror by the focal length of the eye-lens. Thus, if the large glass
has a focus of 70 inches and the eye-lens a focus of one inch, then
the power is 70. If the latter is only 1/4-inch focus, the resulting
power will be 280. But this method is only applicable to single lens
eyepieces. We may, however, resort to several other means of finding
the powers of the compound eyepieces of Huygens or Ramsden. Let the
observer fix a slip of white cardboard, say 1 inch wide, to a door or
post some distance off, and then (with a refractor) view it, while
keeping the disengaged eye open, and note the exact space covered by
the telescopic image of the card as projected on the door seen by the
other eye. The number of inches included in the space alluded to will
represent the linear magnifying power. A brick wall or any surface with
distinct, regularly marked divisions will answer the same purpose, the
number of bricks or divisions covered by the telescopic image of one
of them being equivalent to the power. But it should not be forgotten
that a telescope magnifies slightly less upon a celestial object than
upon a near terrestrial one owing to the shorter focus, and a trifling
allowance will have to be made for this. Another plan may be mentioned.
When the telescope is directed to any fairly bright object or to
the sky, and the observer removes his eye about 10 inches from the
eyepiece, a sharply defined, bright little disk will be perceived in
the eye-lens. If the diameter of this disk is ascertained and the clear
aperture of the object-glass or mirror is divided by it, the quotient
will be the magnifying power. Thus, if the small circle of light is ·2
inch diameter and the effective aperture of the large glass 5 inches,
then the power is 25. If the former is ·02 inch diameter and the
latter 7·5 inches, the power will be 375. The dynamometer is a little
instrument specially designed to facilitate this means of fixing the
magnifying power. It enables the diameter of the small luminous circle
in the eye-lens to be very accurately measured, and this is a most
important factor in deriving the power by this method.
[Illustration: Fig. 16.
BERTHON’S DYNAMOMETER. _HORNE & THORNTHWAITE LONDON._]
_Field of Eyepiece._—Observers often require to know the diameter of
the fields of their eyepieces. Those engaged in sweeping up comets,
nebulæ, or other objects requiring large fields and low powers, find
it quite important to have this information. They may acquire it for
themselves by simple methods. A planet, or star such as δ Orionis, η
or γ Virginis, or η Aquilæ, close to the equator, should be allowed to
run exactly through the centre of the field, and the interval occupied
in its complete transit from ingress to egress noted several times. The
mean result in min. and sec. of time must then be multiplied by 15,
and this will represent the diameter required in min. and sec. of arc
on the equator. A planet or star near the meridian is the best for the
purpose. If the object occupies 1 min. 27 sec. of time in passing from
the E. to the W. limit of the field, then 87 sec. × 15 = 1305″, or 21′
45″. A more accurate method of deriving the angle subtended by the
field is to let a star, say Regulus, pass through the centre, and fix
the time which lapses in its entire passage by a sidereal clock; then
the interval so found × 15 × cosine of the declination of Regulus will
indicate the diameter of the field. Suppose for instance, that the star
named occupies 2 min. 14 sec. = 134 sec. in its passage right across
the whole and central part of the field: then
134 log 2·127105
15 log 1·176091
Dec. of Regulus 12° 30′ log cos 9·989581
————————
1962″ log 3·292777
so that the diameter of the field of the eyepiece must be 32′ 42″,
nearly corresponding with the diameter of the Moon.
_Limited Means no Obstacle._—There are many observers who, having
limited means, are apt to consider themselves practically unable to
effect good work. This is a great illusion. There are several branches
of astronomy in which the diligent use of a small instrument may
be turned to excellent account. Perseverance will often compensate
for lack of powerful appliances. Many of the large and expensive
telescopes, now becoming so common, are engaged in work which could
be as well performed with smaller aperture, and when the manifold
advantages of moderate instruments are considered, amateurs may well
cease to deplore the apparent insufficiency of their apparatus. It is,
however, true that refractors have now attained dimensions and a degree
of proficiency never contemplated in former times, and that the modern
ingenuity of art has given birth to innumerable devices to facilitate
the work of those engaged in observation. In many of our best
appointed observatories the arrangements are so very replete with
conveniences, and so sedatory in their influences, that the observer
has every inducement to fall asleep, though we do not find instances
of “nodding” recorded in their annals. Further progress in the same
direction leads us to joyfully anticipate the time when, instead of
standing out in the frost, we may comfortably make our observations
in bed. This will admirably suit all those who, like Bristol people,
are reported to sleep with one eye open! But, to be more serious,
the work of amateurs is much hindered by lack of means to construct
observatories wherein they may conduct researches without suffering
from all the rigours of an unfavourable climate. Many of them have,
like William Herschel a century ago, to pursue their labours under
no canopy but the heavens above, and are exposed to all the trying
severity of frost and keen winds, which keep them shivering for hours
together, and very much awake!
[Illustration: Fig. 17.
Cooke and Son’s Educational Telescope.]
_Observing-Seats._—As to observing-seats, many useful contrivances have
been described from time to time in the ‘Astronomical Register’ and
‘English Mechanic.’ Some of these answer their design admirably, but I
believe a good chair, embodying all the many little requirements of the
observer, yet awaits construction. Those I have seen, while supplying
certain acknowledged wants, are yet deficient in some points which need
provision. With my reflector I find an ordinary step-ladder answers
the purpose very well. It is at once light, simple, and durable, and
enables observations to be secured at any altitude. It may be readily
placed so that the observer can work in a sitting posture, and the
upper shelves, while convenient to lean upon, may be so arranged as to
hold eyepieces, and are to be further utilized when making drawings
at the telescope. I find it possible to obtain very steady views of
celestial objects in this way. Everyone knows that during a critical
observation it is as essential for the observer to be perfectly still
as it is for the instrument to be free from vibration. A person
who stands looking through a telescope feels a desire to ensure a
convenient stability by catching hold of it. The impression is no doubt
correctly conveyed to his mind that he may obtain a better view in this
way; and so he would, were it not for the dancing of the image which
instantly follows the handling of the instrument. For this reason it is
absolutely necessary that no part of the observer touch the telescope
while in use. He must ensure the desired steadiness, which is really
a most important consideration, by other means; and an observer who
provides for this contingency will have taken a useful step in the way
of achieving delicate work.
_Advantage of Equatoreals._—Those who employ equatoreal mounting and
clock-work will manifestly command an advantage in tracing features
on a planet or other object requiring critical scrutiny. Common
stands, though often good make-shifts, require constant application
on the part of the observer, when his undivided attention should be
concentrated on the object. With an alt-azimuth stand nearly one half
the observer’s time is occupied in keeping the object near the middle
of the field. Though good views are obtainable, they are very fugitive.
Just as the delicate features are being impressed on the retina they
are lost in the ill-defined margin of the field, or from the necessity
of suddenly shifting the object back. A succession of hurried views of
this kind, during which the observer is frantically endeavouring to
grasp details which only require a steady view to be well displayed,
are often tantalizing and seldom satisfactory in their issues. This is
especially the case when a single lens and high powers are used, and
if the night is windy the difficulty is intensified. It is, therefore,
evident that a clock-driven telescope possesses marked advantages in
delicate work on faint objects, because the prolonged view better
enables the eye to gather in the details which are all but lost in
the elusive glimpses afforded by inferior means. Still we must not
forget that rough appliances do not present an effectual barrier to
success. The very finest definition comes only in momentary glimpses.
The sharply-cut outlines of planetary configuration cannot steadily
be held for long together. Only now and then the image acquires the
distinctness of an engraving, when the air and the focus of eye and
telescope severally combine to produce a perfect picture. Observers,
therefore, whose instruments are simply, though perhaps substantially
mounted in handy fashion, must profit by these moments of fine seeing,
and, when drawing, will find it expedient to fill in, little by little,
the delicate forms which reach the eye. This will take much time owing
to the drawbacks alluded to, but the outcome will more than justify its
expenditure, and the observer will gain patience and perseverance which
will prove a useful experience in the future.
Lenses out of centre or misplaced are, like other defects, calculated
to give rise to errors as numerous as they are various. But the most
striking of these apparently belong to a period when telescopes were
far less perfect and popular than at the present day. Indeed, it
is surprising that so very few false or imaginary discoveries are
announced when we consider the vast array of instruments that are
now employed. It is true we occasionally hear that a comet has been
discovered close to Jupiter, that several companions have been seen to
Polaris, or that some other extraordinary “find” has been effected, but
the age is dead when such announcements were accepted without suitable
investigation. The satellite of Venus has long since ceased to exist.
The active volcanoes on the Moon have become extinct. Even Vulcan will
have to be set aside, and, like many another sensation which caused
quite a _furóre_ in its day, must soon be altogether expunged from the
category of “suspects.”
_Test-objects._ Opticians sometimes advertise lists of
objects—generally double stars—which may be seen with their
instruments, but it does not appear to be sufficiently understood that
the character of a telescope is dependent in a great degree upon the
ability of the observer, who can either make or mar it, according to
the skill he displays in its management. Some men will undoubtedly
see more with 5 inches of aperture than others will with 10. Certain
observers appear to excel in detecting delicate planetary markings,
while others possess special aptitude for glimpsing minute objects such
as faint satellites, or _comites_ to double stars, and the explanation
seems to be that partly by experience and partly by differences in the
sensitiveness of vision, exceptional powers are sometimes acquired in
each of these departments. The various test-objects which have been
given by reliable authorities, though representing average attainments,
are not applicable to the abnormal powers of vision possessed by
certain observers. In fact, the capacity of a telescope cannot be
correctly assigned and its powers circumscribed by arbitrary rules,
because, as already stated, the character of the observer himself
becomes a most important factor in this relation. Climatic influences
have also considerable weight, though less so than the personal
variations referred to, for one man will succeed, where another meets
with utter failure. This is unquestionably due to differences in
eyesight, method, and experience. But whatever the primary causes may
be, everyone knows they induce widely discordant results, and occasion
many of the contradictions which become the subjects of controversy.
And, as a rule, amateurs should avoid controversy, because it very
rarely clears up a contested point. There is argument and reiteration,
but no mutual understanding or settlement of the question at issue.
It wastes time, and often destroys that good feeling which should
subsist amongst astronomers of every class and nationality. In cases
where an important principle is involved, and discussion promises to
throw light upon it, the circumstances are quite different. But paltry
quibblings, fault-finding, or the constant expression of negative
views, peculiar to sceptics, should be abandoned, as hindering rather
than accelerating the progress of science. Let observers continually
exercise care and discretion and satisfy themselves in every legitimate
way as to the accuracy of their results, and they may fearlessly give
them expression and overcome any objections made to their acceptance.
They should accord one another an equal desire for the promotion of
truth. Competition and rivalry in good spirit increase enthusiasm,
but there is little occasion for the bitterness and spleen sometimes
exhibited in scientific journals. There are some men whose reputations
do not rest upon good or original work performed by themselves, but
rather upon the alacrity with which they discover grievances and upon
the care they will bestow in exposing trifling errors in the writings
of their not-infallible contemporaries. Such critics would earn a more
honourable title to regard were they to devote their time to some
better method of serving the cause of science.
_Cheapness and increasing number of Telescopes._—A marked feature
of optical instruments is their increasing cheapness. Little more
than half a century ago Tulley charged £315 for a 10-inch Newtonian
reflector. At the present time Calver asks £50 for an instrument of the
same aperture, and sometimes one may be picked up, second-hand, for
half of that amount. Not only have telescopes become cheaper, but they
have greatly improved in performance since silvered glass superseded
the metallic speculum. Hence we find moderately-powerful instruments
in the hands of a very large number of observers. Astronomical
publications have proportionately increased, so that amateurs of to-day
can boast of facilities, both of making and recording observations,
which were scarcely dreamt of a century ago. It must be admitted,
however, that the results hardly do justice to the means available.
Such an enormous number of telescopes are variously employed that one
cannot avoid a feeling of surprise at the comparative rarity of new
discoveries, and, indeed, of published observations generally. It is
certain that the majority of existing telescopes are either lying idle
or applied in such a desultory fashion as to virtually negative the
value of the results. Others, again, are indiscriminately employed
upon every diversity of object without special aim or method, and with
a mere desire to satisfy curiosity. Now it is to be greatly deplored
that so much observing strength is either latent or misdirected.
The circumstances obviously demand that an earnest effort should
be made to utilize and attract it into suitable channels. To do
this effectually, the value of collective effort should be forcibly
explained, the interest and enthusiasm of observers must be aroused in
a permanent manner, and they must be banded together according to their
choice of subjects. An effort in this direction has been made by the
Liverpool Astronomical Society, and the results have proved distinctly
favourable; a considerable amount of useful work has been effected in
several branches and it forms the subject of some valuable reports
which have been annually published in the ‘Journal.’
_Utility of Stops._—There are a good many details connected with
observation which, though advice may be tendered in a general way,
are best left to the discrimination of observers, who will very
soon discover their influences by practical trial and treat them
accordingly. The employment of stops or diaphragms to contract the
aperture of telescopes is a question on which a diversity of opinion
has been expressed. It is often found, on nights of indifferent seeing,
that the whole aperture, especially of a faulty instrument, gives
bad images, and that, by reducing it, definition becomes immensely
improved. But Mr. Burnham, the double star observer, records his
opinion that a good glass needs no contraction, and that the whole
aperture shows more than a part unless there is defective figuring
at the outer zone of the lens, which will be cut off by the stop and
its performance thereby greatly improved. He seems to think that
a glass requiring contraction is essentially defective, but this
is totally opposed to the conclusions of other observers. It is
almost universally admitted that, on bad nights, the advantages of a
large aperture are neutralized by unsteady definition, and that, by
reducing the diameter, the character of the images is enhanced. As
regards instruments of moderate calibre the necessity is less urgent.
With my 10-inch reflector I rarely, if ever, employ stops, for by
reducing the aperture to 8 inches the gain in definition does not
sufficiently repay for the serious loss of light. But in the case of
large telescopes the conservation of light is not so important, and
a 14-inch or 16-inch stop may be frequently employed on an 18-inch
glass with striking advantage. The theory that only defective lenses
improve with contraction is fallacious, for in certain cases where
stops are regularly employed it is found that, under circumstances
of really good seeing, the whole aperture gives images which are as
nearly perfect as possible. It is clear from this that the fault
lies with the atmosphere, and that under bad conditions it becomes
imperative to limit its interference consistently with the retention of
sufficient light to distinguish the object well. In large reflectors,
particularly, the undulations of the air are very active in destroying
definition, and the fact will be patent enough to anyone who compares
the images given in widely different apertures. The hard, cleanly cut
disks shown by a small speculum or object-glass offer an attractive
contrast to the flaring, indefinite forms often seen in big telescopes.
_Cleaning Lenses._—As to wiping objectives or mirrors, this should be
performed not more often than absolute necessity requires; and in any
case the touches should be delicate and made with materials of very
soft texture. The owner of a good objective should never take the
handkerchief out of his pocket and, in order to remove a little dust
or dew, rub the glass until the offensive deposit is thought to be
removed. Yet this is sometimes done, though frequent repetition of such
a process must ultimately ruin the best telescope notwithstanding the
hardness of the crown glass forming the outer lens of the objective.
It will not bear such “rough and ready” usage and in time must show
some ugly scratches which will greatly affect its value though they may
not seriously detract from its practical utility. Good tools deserve
better treatment. When the glass really wants cleaning, remove it from
the tube and sweep its whole surface gently with a dry camel’s-hair
brush, or when this is not at hand get a piece of linen and “flick”
off the dust particles. Then wipe the lens, as soon as these have been
dislodged, with an old silk, or soft cambric handkerchief; fine chamois
leather is also a good material, and soft tissue paper, aided by the
breath, has been recommended. But whatever substance may be adopted it
must be perfectly clean and free from dust. When not in use it should
be corked up in a wide-necked bottle where it will be safe from contact
with foreign particles. In the case of mirrors there is an obvious need
that, when being repolished, the material used should be perfectly dry
and that the mirror also should be in the same state. It is unnecessary
to say here that in no case must the silver film be touched when it is
clouded over with moisture. This must first be allowed to evaporate in
a free current of air or before a fire; the former is to be preferred.
A suitable polishing-pad may be made with a square piece of washleather
or chamois in which cotton-wool is placed and then tied into a bag.
This may be dipped into a little of the finest rouge, and its
employment will often restore a bright surface to the mirror. But the
latter should be left “severely alone” unless there is urgent occasion
to repolish it, as every application of the rouged pad wears the film
and may take off minute parts of it, especially when dust has not been
altogether excluded. The precarious nature of the silvered surface
undoubtedly constitutes the greatest disadvantage of modern reflectors.
The polish on the old metallic mirrors was far more durable. Some of
Short’s, figured 150 years ago, still exist and are apparently as
bright as when they were turned out of the workshop! I have a 4-inch
Gregorian by Watson which must be quite a century old, and both large
and small specula seem to have retained their pristine condition.
With regard to the duration of the silver-on-glass films, much of
course depends upon the care and means taken to preserve them. Calver
says that sometimes the deposit does not last so long as expected,
though he has known the same films in use for ten years. A mirror
that looks badly tarnished and fit for nothing will often perform
wonderfully well. With my 10-inch in a sadly deteriorated state
I have obtained views of the Moon, Venus, and Jupiter that could
hardly be surpassed. The moderate reflection from a tarnished mirror
evidently improves the image of a bright object by eliminating the
glare and allowing the fainter details to be readily seen. When not
in use a tight-fitting cap should always be placed over the mirror,
and if a pad of cotton wadding of the same diameter is made to
inlay this cap it tends to preserve the film by absorbing much of
the moisture that otherwise condenses on its surface. The ‘Hints on
Reflecting-Telescopes,’ by W. H. Thornthwaite and by G. Calver, and the
‘Plea for Reflectors,’ by J. Browning, may be instructively consulted
by all those who use this form of instrument. The latter work is now,
however, out of print, and Mr. Browning tells me that he has quite
relinquished the manufacture of reflecting-telescopes. Mr. G. With
of Hereford, who formerly supplied the mirrors for his instruments,
has recently disposed of his reserve stock and entered an entirely
different sphere of labour. In the publications above alluded to
amateurs will find a large amount of practical information on the value
and treatment of glass mirrors.
_Opera-Glass._—A very useful adjunct, and often a really valuable one
to the astronomical amateur, is the Opera-Glass, or rather the larger
form of this instrument generally known as the Field-Glass. Of certain
objects it gives views which cannot be surpassed, and it is especially
useful in observations of variable stars and large comets. Whenever the
horizon is being scanned for a glimpse of the fugitive Mercury, or when
it is desired to have a very early peep at the narrow crescent of the
young Moon, or to pick up Venus at midday, or Jupiter before sunset,
all one has to do is to sweep over the region where the object is
situated, when it is pretty sure to be caught, and the unaided eye will
probably reach it soon afterwards. The opera-glass has the dignity of
being the first telescope invented, for even its binocular form is not
new; it is virtually the same pattern of instrument that was introduced
at Middleburg in 1609, though its compound object-glasses are of more
modern date. Anyone who entertains any doubts as to the efficacy of the
opera-glass or has had little experience in its use will do well to
look at the Pleiades and compare the splendid aspect of that cluster,
as it is there presented, with the view obtained by the naked eye, and
he will acknowledge at once that it constitutes a tool without which
the observer’s equipment is by no means perfect. The object-glasses
should have diameters of 2 or 2½ inches, and the magnifying power lie
between 4 and 6. There is a large field of view and the images are very
bright. The observer is enabled to enjoy the luxury of using both his
eyes, and when he directs the instrument upon a terrestrial landscape
he will be gratified that it does not turn the world upside down! It is
not surprising that an appliance, with recommendations so significant,
is coming more into favour every day, and for those branches suitable
to its means it is doing much useful work. A volume has been recently
published dealing expressly with the use of the opera-glass in
Astronomy; and in the ‘Journal of the L.A.S.’ vol. vii. p. 120, there
is an excellent paper by Major Markwick on the same subject. This
instrument will never, of course, by the nature of its construction,
be comparable to a modern telescope in regard to power, for Galilei,
when he augmented his magnifiers to 30, appears to have practically
exhausted the resources of this appliance. But in all those departments
requiring an expansive field and little power with a brilliant and
distinct image, the larger form of opera-glass is a great desideratum,
and its portability is not one of the least of its advantages.
_Dewing of Mirrors._—The disposition of mirrors to become clouded over
upon rises of temperature is a point meriting comment. When permanently
left in a telescope, fully exposed out of doors, the speculum undergoes
daily transitions. The heat generated in the interior of the tube by
the sun’s action causes a thick film of moisture to form upon the
silvered surface of the mirror, which remains in this state for a
considerable time, though the moisture evaporates before the evening.
The flat is similarly affected, and the result of these frequent
changes is that the coating of silver becomes impaired and presents
a crackly appearance all over the surface. Sometimes when a marked
increase of temperature occurs towards evening the speculum is rendered
totally unserviceable until it has been submitted to what Dr. Kitchiner
terms a process of “roasting.” The vapour will soon disappear when
the mirror is brought indoors and placed before a fire; but it is not
till some time after it has been remounted in the tube that it will
perform satisfactorily. Those who keep their mirrors in more equable
temperatures will not experience these inconveniences, which may also
in some measure be obviated by regularly placing a tight-fitting cap,
inlaid with cotton-wool, over the speculum at the conclusion of work.
This also protects the silver from the yellow sulphurous deposit which
soon collects upon it if used in a town. All sudden variations of
temperature act prejudicially on the performance of specula, and their
best work is only accomplished when free from such disturbing elements.
I have rarely found the flat to become dewed in a natural way during
the progress of observation. If on a cold night the observer puts his
hand upon its supports in order to alter its adjustment it instantly
becomes dewed, or if he stands looking down the tube it is almost sure
to be similarly affected; but in the ordinary course of work the flat
is little liable to become dewed in sensible degree. With refractors
dew-caps are very necessary, though they do not always prevent the
deposition of moisture on the object-glass, and this occasions frequent
wiping or drying, which in either case is very objectionable.
_Celestial Globe._—This forms another extremely useful addendum to
the appliances of the amateur. It enables a great many problems to be
solved in a very simple manner, and helps the young student to a lucid
comprehension of the apparent motions and positions of the fixed stars.
With ‘Keith on the Globes’ as a reference-book he may soon acquire
the method of determining the times of rising, southing, and setting
of any celestial object the place of which is known. He can also
readily find the height (altitude) and bearing (azimuth) at any time.
The distance in degrees between any two stars or between a star and
the Moon, a planet, or a comet may be found at a glance by laying the
quadrant of altitude on the pair of objects and reading off the number
of degrees separating them. If a new comet has been discovered, its
position should be marked in pencil upon the globe; and the observer,
after having noted its exact place relatively to neighbouring stars,
may proceed to identify the object with his telescope. If a large
meteor is seen, its apparent path amongst the constellations should be
projected on the globe and the points, in R.A. and Dec., of beginning
and ending of the flight read off and entered in a book. In many other
practical branches of astronomy this instrument will prove highly
serviceable, and is far preferable to a star-atlas. But the latter
is the most useful to the beginner who is just learning the names of
the stars and the configuration of the chief groups, because on the
globe the positions are all reversed east and west. The surface of the
globe represents the entire star-sphere reduced to a common distance
from the earth, and as seen from outside that sphere. The observer,
therefore, must imagine his eye to be situated in the centre of the
globe, if he would see the stars in the same relative places as he
sees them in the heavens. The reversion of the star-positions to which
we have been alluding is very confusing at first, and no doubt it
provokes mistakes, but a little experience will practically remove this
objection. The one great recommendation to a star-atlas is that it
displays the stars in the natural positions in which they are discerned
by the eye, thus enabling the student to become readily acquainted with
them, whereas the celestial globe affords no such facility. But in
other respects the latter possesses some valuable functions, and the
amateur who devotes some of his leisure to mastering the really useful
problems will attain a knowledge that will be of great benefit to him
in after years. A globe of 12-inches diameter will be large enough
for many purposes, but one of 18-inches will be the most effective
size. It should be mounted on a tall stand with single body and tripod
base. The stands, fitted with three parallel legs, in which the globe
is supported in the middle by weak connections from them, are not
nearly so durable. I have used several 18-inch globes mounted in this
manner, and the supports have quite given way under the pressure of
constant use; but this is impossible with the strong single body, which
is capable of withstanding any strain. Globes are frequently to be
obtained second-hand, and at trifling cost; but the observer must allow
for precession if he uses an old article. Many of the stars will be
1° or 2° east of the positions in which they are marked on the globe;
and it will be necessary to remember this if the appliance is to be
employed for exact results.
_Observatories._—Massive and lofty buildings have long gone out of
fashion, and lighter, drier structures have properly supplanted them.
Instruments of size are generally placed on or near the ground and
solidly supported to ensure stability, while the other erections are
made consistent with the necessity for pretty equable temperature and
freedom from damp. Amateurs will ordinarily find that a simple wooden
enclosure for the telescope, with suitable arrangements for opening
the top in any direction, is sufficient for their purpose and very
inexpensive. Some observers have, indeed, secured the desired shelter
for themselves and their telescopes by means of a canvas tent provided
with ready means for obtaining sky-room. Berthon has given a good
description of an amateur’s observing-hut in ‘The English Mechanic’ for
October 13th and 20th, 1871; and Chambers supplies some information
about amateur observatories in ‘Nature’ for November 19th, 1885[8].
Mr. Thornthwaite’s. ‘Hints on Telescopes’ may be usefully consulted
for details of the Romsey Observatory, which, like the Berthon model,
seems peculiarly adapted to the necessities of the amateur. The great
requirements in such structures are that they should be dry and not
obstruct any region of the firmament. They should also be large enough
to allow the observer perfect freedom in his movements and during the
progress of his observations. They are then decided advantages, and
will materially add to that comfort and convenience without which it
is rarely possible to accomplish really good work. When an observatory
is to be dispensed with it becomes necessary to erect a small wooden
house near the instrument, especially if placed at the far end of a
garden, in which the observer may keep certain appliances, such as a
lantern, celestial globe, step-ladder or observing-seat, oil, &c. Here
also he may record his seeings, complete his sketches, and consult
his working-list, star-charts, and ephemerides. A shelter of this
sort, apart from its practical helpfulness, avoids any necessity for
the observer to go in and out of doors, up and down stairs, &c., to
the annoyance of the rest of his family, who, on a frosty night, are
decidedly not of an astronomic turn, and vastly prefer house-warming to
stargazing!
FOOTNOTES:
[6] My 10-inch reflector by With-Browning was persistently used for
four years without being resilvered or once getting out of adjustment.
[7] In this and future references to reflectors the Newtonian form is
alluded to. The direct-vision reflectors of Gregory and Cassegrain
have gone out of use, and the present popularity of Newtonians may be
regarded as a case of the “survival of the fittest.”
[8] Chambers’s ‘Descriptive Astronomy,’ 4th ed. vol. ii., also contains
some useful references and diagrams.
CHAPTER IV.
_NOTES ON TELESCOPIC WORK._
Preparation.—Working-Lists.—Wind.—Vision.—Records.—Drawing.—Friendly
Indulgences.—Open-Air Observing.—Method.—Perseverance.—Definition in
Towns.—Photography.—Publications.—Past and Future.—Attractions of
Telescopic Work.
_Preparation._—An observer in commencing work in any department of
astronomy will find it a very great assistance to his progress if he
carefully reads and digests all that has been previously effected in
the same line. He will see many of the chief difficulties and their
remedies explained. He will further learn the best methods and be in
the position of a man who has already gained considerable experience.
If he enter upon a research of which he has acquired no foreknowledge
he will be merely groping in the dark, and must encounter many
obstacles which, though they may not effectually turn him from his
purpose, will at least involve a considerable expenditure of time and
labour. On the other hand, a person who relies upon guidance from prior
experimentalists will probably make rapid headway. He will be fortified
to meet contingencies and to avoid complications as they arise. He
will be better enabled to discriminate as to the most eligible means
and will confidently endeavour to push them to the furthest extent.
By adopting existing instructions for his direction and familiarizing
himself with the latest information from the best authorities he will
in a great measure ensure his own success or at least bring it within
measurable] distance. The want of this foreknowledge has often been
the main cause of failure, and it has sometimes led to misconceptions
and imaginary discoveries; for after much thought and labour a man
will overcome an impediment or achieve an end in a way for which he
claims credit, only to find that he has been anticipated years before
and that had he consulted past records, his difficulties would have
been avoided and he might have pressed much nearer the goal. Too much
importance cannot be attached to the acquisition of foreknowledge of
the character referred to, though we do not mean that former methods
or results are to be implicitly trusted. Let every observer judge for
himself to a certain extent and let him follow original plans whenever
he regards them as feasible; let him test preceding results whenever
he doubts their accuracy. We recommend past experiences as a guide,
not as an infallible precept. It would be as much a mistake to follow
the old groove with a sort of credulous infatuation as it would be to
enter upon it in utter ignorance of theoretical knowledge. An observer
should take the direction of his labours from previous workers, but be
prepared to diverge from acknowledged rules should he feel justified in
doing so from his new experiences.
[Illustration: Fig. 18.
Refracting-Telescope on a German Equatoreal.]
_Working-Lists._—Full advantage should be taken of good observing
weather. Sir John Herschel most aptly said that no time occupied in the
preparation of working-lists is ill-spent. In our climate the value
of this maxim cannot be overrated. If the 100 hours of exceptionally
good seeing, available in the course of the year, are to be profitably
employed, we must be continually prepared with a scheme of systematic
work. The observer should compile lists of objects it is intended to
examine, and their places must be marked upon the globe or chart so
as to avoid all troublesome references during the actual progress of
observation. If he has to consult ephemerides and otherwise withdraw
attention from the telescope he loses valuable time: moreover the
positions hurriedly assigned in such cases are frequently wrong and
entail duplicate references, involving additional waste of time; all
this may be avoided by careful preparation beforehand. If he has a
series of double or variable stars to observe he must tabulate their
places in convenient order so as to facilitate the work. If he intend
hunting up nebulæ or telescopic comets he must carefully mark their
positions relatively to adjoining stars. In the case of selenographical
objects or planetary markings he may equally prepare himself by
previous study. Adopting these precautions, objects may be readily
identified and the work expedited. When no such preparation is made
much confusion and loss of time result. On a cloudy, wet day observers
often consider it unnecessary to make such provision and they are taken
at a great disadvantage when the sky suddenly clears. A good observer,
like a good general, ought to provide, by the proper disposition of his
means, against any emergency. In stormy weather valuable observations
are often permissible if the observer is prompt, for the definition is
occasionally suitable under such circumstances. The most tantalizing
weather of all is that experienced during an anti-cyclone in winter.
For a week or two the barometer is very steady at a high reading, the
air is calm, and the sky is obscured with an impenetrable mass of
clouds.
_Wind._—The influence of wind on definition has been much discussed
in its various aspects, but it is scarcely feasible to lay down
definite rules on the subject. The east wind is rarely favourable to
good seeing, but the law is far from absolute. We must remember that
several distinct currents sometimes prevail, and the air strata at
various elevations are of different degrees of humidity and therefore
exercise different effects upon telescopic definition. A mere surface
breeze from the east may underlie an extensive and moist current from
the south-west, and telescopic definition may prove very fair under
the combination. Calm nights when there is a little haze and fog,
making the stars look somewhat dim, frequently afford wonderfully
good seeing. As a rule, when the stars are sparkling and brilliant,
the definition is bad; planetary disks are unsteady and the details
obliterated in glare. But this is not always so. I have sometimes
found in windy weather after storms from the west quarter, when the
air has become very transparent, that exceptionally sharp views may be
obtained; but unfortunately they are not without drawbacks, for the
telescope vibrates violently with every gust of wind and the images
cannot be held long enough for anything satisfactory to be seen. The
tenuous patches of white cirrous cloud which float at high altitudes
will often improve definition in a surprising manner, especially on the
Moon and planets. Of course this does not apply to nebulæ or comets,
which are objects of totally different character and essentially
require a _dark_ night rather than good definition before they may be
seen under the best conditions. As a rule, a steady, humid atmosphere
is highly conducive to good seeing, and it is rather improved than
impaired by a little fog or thin, white cloud. Some unique effects
of peculiar definition, such as oval or triangular star disks, have
been occasionally recorded, but we must content ourselves with a bare
reference to these phenomena. With regard to the general question it
may, however, be added that the character of the seeing often varies at
very short intervals in this climate. In the course of a night’s work
the definition will sometimes fluctuate in a most remarkable manner. An
observer who comes to the telescope and finds it impossible to obtain
satisfactory images should not entirely relinquish work at the first
trial. After an interval he should again test its performance, for
it frequently happens that a night ushered in by turbulent vapours,
improves greatly at a later period, and in the morning part becomes so
fine that it is worthy to be included in the select 100 hours assigned
by Sir W. Herschel as the annual limit. Those who reside in towns
will usually get the best definition after midnight, because there
is less interference then from smoke and heated vapours. It would
greatly conduce to our knowledge of atmospheric vagaries as affecting
definition, if observers, especially those employing large aperture,
preserved records as to the quality of the seeing, also direction of
wind and readings of the barometer and thermometer.
_Vision._—There are perhaps differences quite as considerable in
powers of vision as in quality of definition. It is not meant by
this that the same person is subject to great individual variations,
though some people are certainly liable to fluctuations, according to
state of health and other conditions. Some eyes, as already stated,
are less effective in defining planetary markings than in detecting
minute stars or faint satellites of distant planets. Of course the
natural capacity is greatly enhanced by constant practice, for the
human eye has proved itself competent to attain a surprising degree of
excellence by habitual training. Frequent efforts, if not overpressed
so as to unduly strain the optic nerves, are found to intensify rather
than weaken the powers of sight. Thus a distinguishing trait among
astronomers has been their keenness of vision, which, in many cases,
they have retained to an advanced age. It is true Dr. Kitchiner said
his “eye at the age of forty-seven became as much impaired by the
extreme exertion it had been put to in the prosecution of telescope
trials, as an eye which has been employed only in ordinary occupations
usually is at sixty years of age!—to cultivate a little acquaintance
with the particular and comparative powers of telescopes requires many
extremely eye-teasing experiments.” But the Doctor’s opinion is not
generally confirmed by other testimony, the fact being that the eye is
usually strengthened by special service of this character. To unduly
tax or press its powers must result in injury; but it is well known
that the capacities of our sight and other senses are enhanced by
their healthy exercise, and that comparative disuse is a great source
of declining efficiency. Before the observer may hope to excel as a
telescopist it is clear that a certain degree of training is requisite.
Many men exhibit very keen sight under ordinary circumstances, but
when they come to the telescope are hopelessly beaten by a man who has
a practised eye. On several occasions the writer was much impressed
with evidences of extraordinary sight in certain individuals, but upon
being tested at the telescope they were found very deficient, both as
regards planetary detail and faint satellites. Objects which were quite
conspicuous to an experienced eye were totally invisible to them. I
believe it is a good plan for habitual observers to employ method in
exercising their sight. In my own case I invariably use the right eye
on the markings of planets and the left on minute stars and satellites.
Practice has given each eye a superiority over the other in the special
work to which it has been devoted, and I fancy the practice might be
more generally followed with success.
It is an advantage to keep both eyes open when in the act of observing,
especially when surrounding objects are perfectly dark and there is no
distracting light from neighbouring windows or lamps. The slight effort
required to keep the disengaged eye closed interferes with the action
of the other, and though this is but trivial, critical work is not
efficiently performed under such conditions. Whenever light interferes
the observer may exclude it by a shade so arranged as to afford
complete protection to the unoccupied eye.
If faint objects are to be examined the observer should remain in a
dark situation for some little time previously, so that the pupil
of the eye may be dilated to the utmost extent and in a state most
suitable for such work. After coming from a brilliantly lit apartment,
or after viewing the Moon or a conspicuous planet, the eye is totally
unfit to receive impressions from a difficult object, such as a minute
star or faint nebula or comet; some time must be allowed to elapse
so that the eye may recover its sensitiveness. As a rule amateurs
will find it best to confine their attention to one class of objects
only on the same evening, for if the Moon is first examined and then
immediately afterwards the telescope is directed upon double stars
and nebulæ, the latter objects are little likely to be seen with good
effect. If faint objects generally are persistently studied night after
night and the observer refrains from solar and lunar work, his eye
will acquire greater sensitiveness and he will readily pick up minute
forms which are utterly beyond the reach of a man who indiscriminately
employs his eye and telescope upon bright and faint objects.
_Records._—With regard to records, every observer should make a note
of what he sees, and at the earliest possible instant after the
observation has been effected. If the duty is relegated to a subsequent
occasion it is either not done at all or done very imperfectly. The
most salient features of whatever is observed should be jotted down in
systematic form, so as to permit of ready reference afterwards. It is
useful to preserve these records in a paged book, with an index, so
that the matter can be regularly posted up. The negligence of certain
observers in this respect has resulted in the total loss of valuable
observations. Even if the details appear to possess no significance,
they should be faithfully registered in a convenient, legible form,
because many facts deemed of no moment at the time may become of
considerable importance. The observer should never refrain from such
descriptions because he attributes little value to them. Some men keep
voluminous diaries in which there is scarcely anything worth record;
but this is going to the other extreme. All that is wanted is a concise
and brief statement of facts. Some persons have omitted references to
features or objects observed because they could not understand them,
and rather distrusted the evidence of their eyes; but these are the
very experiences which require careful record and reinvestigation.
_Drawing._—Few observers are good draughtsmen; but it is astonishing
how seldom we meet with real endeavours to excel in this respect. Every
amateur should practise drawing, however indifferent his efforts may
be. Delineations, even if roughly executed, are often more effective
than whole pages of description. Pictorial representations form the
leading attraction of astronomical literature, and are capable of
rendering it more interesting to the popular mind than any other
influence. They induce a more apt conception of what celestial objects
are really like than any amount of verbal matter can possibly do.
For this reason it becomes the obvious duty of every observer to
cultivate sketching and drawing, at least in a rudimentary way. He will
frequently find it essential to illustrate his descriptions, so as
to ensure their ready comprehension. In fact, a thoroughly efficient
observer must of necessity become a draughtsman. It should, however,
be his invariable aim to depict just what he sees and in precisely
the form in which it impresses his eye. Mere pictorial embellishments
must be disregarded, and he should be careful not to include doubtful
features, possibly existing in the imagination alone, unless he intends
them simply for his own guidance in future investigations. If he sees
but little, and it is faithfully delineated, it will be of more real
value than a most elaborate drawing in which the eye and imagination
have each played a part. It is an undoubted fact that some of the most
striking illustrations in astronomical handbooks are disfigured by
features either wrongly depicted or having no existence whatever. There
is very great need for caution in representing such markings only as
are distinctly and unmistakably visible. In all cases where the object
is new or doubtful the observer should await duplicate observations
before announcing it. It is better that new features should evade
discovery than that delusive representations should be handed down to
posterity. As regards selenographical drawings I would refer the reader
to what Mr. Eiger advises on p. 21 and 22 of volume v. of the ‘Journal
of the Liverpool Astronomical Society.’ My own plan in sketching at
the telescope is to first roughly delineate the features bit by bit
as I successively glimpse them, assuring myself, as I proceed, as to
general correctness in outline and position; then, on completion, I go
indoors to a better light and make copies while the details are still
freshly impressed on the mind. To soften details a small piece of
blotting-paper must be wrapped round the pointed end of the pencil, and
the parts requiring to be smoothed gently touched or rubbed until the
desired effect is attained. This simple method, properly applied, will
enable delicate markings to be faithfully reproduced, and it certainly
adds in no small degree to the merit of a drawing.
_Friendly Indulgences._—Every man whose astronomical predilections
are known, and who has a telescope of any size, is pestered with
applications from friends and others who wish to view some of the
wonders of the heavens. Of course it is the duty of all of us to
encourage a laudable interest in the science, especially when evinced
by neighbours or acquaintances; but the utility of an observer
constituting himself a showman, and sacrificing many valuable hours
which might be spent in useful observations, may be seriously
questioned. The weather is so bad in this country that we can ill
spare an hour from our scanty store. Is it therefore desirable to
satisfy the idle curiosity of people who have no deep-seated regard for
astronomy, and will certainly never exhibit their professed interest
in a substantial manner? Assuredly not. The time of our observers is
altogether too valuable to be employed in this fashion. Yet it is an
undisputed fact that some self-denying amateurs are unwearying in their
efforts to accommodate their friends in the respect alluded to. My own
impression is that, except in special cases, the observer will best
consult the interests of astronomy, as well as his own convenience and
pleasure, by declining the character of showman; for depend upon it
a person who appreciates the science in the right fashion will find
ways and means to procure a telescope and gratify his tastes to the
fullest capacity. Some years ago I took considerable trouble on several
evenings in showing a variety of objects to a clerical friend, who
expressed an intention to buy a telescope and devote his leisure to the
science. I spent many hours in explanations &c.; but some weeks later
my pupil informed me his expenses were so heavy that he really could
not afford to purchase instruments. Yet I found soon after that he
afforded £30 in a useless embellishment of the front of his residence,
and it so disgusted me that I resolved to waste no more precious time
in a similar way.
_Open-Air Observing._—Night air is generally thought to be pernicious
to health; but the longevity of astronomers is certainly opposed to
this idea. Those observers who are unusually susceptible to affections
of the respiratory organs must of course exercise extreme care, and
will hardly be wise in pursuing astronomical work out of doors on
keen, wintry nights. But others, less liable to climatic influences,
may conduct operations with impunity and safety during the most severe
weather. Precautions should always be taken to maintain a convenient
degree of warmth; and, for the rest, the observer’s enthusiasm
must sustain him. A “wadded dressing-gown” has been mentioned as
an effective protection from cold. I have found that a long, thick
overcoat, substantially lined with flannel, and under this a stout
cardigan jacket, will resist the inroads of cold for a long time. On
very trying nights a rug may also be thrown over the shoulders and
strapped round the body. During intense frosts, however, the cold will
penetrate (as I have found while engaged in prolonged watches for
shooting-stars) through almost any covering. As soon as the observer
becomes uncomfortably chilly he should go indoors and thoroughly warm
his things before a fire. He may then return fortified to his work and
pursue it for another period before the frost again makes its presence
disagreeably felt. On windy nights a knitted woollen helmet to cover
the head, and reaching to the shoulders, is an excellent protection;
but an observer had better not wear it more often than is imperative,
or it becomes a necessity on ordinary nights. It is a great mistake to
suppose that “a glass of something hot” before going into the night air
is a good preventive to catching cold. It acts rather in the contrary
way. The reaction after the system has been unduly heated only renders
the observer more sensitive, and the inhalation of cold air is then
very liable to induce affections of the throat.
A telescope permanently erected in the open, and exposed to all
weathers, must soon lose its smart and bright appearance, but it need
lose none of its efficiency, which is of far more importance; for it is
intended for service, not for show. The instrument should be kept well
painted and oiled. I find vaseline an excellent application for the
screws and parts controlling the motions, as it is not congelative like
common oils. The observer, before a night’s work and before darkness
sets in, will do well to examine his instrument and see that it is in
the best condition to facilitate work. Whole tribes of insects take up
their habitation in the base or framework, and even in the telescope
itself if they can effect a lodgment; and I have sometimes had to
sweep away a perfect labyrinth of spiders’ webs from the interior of
the main tube. On one occasion I could not see anything through the
finder, try how I would. I afterwards discovered that a mason-wasp
(_Odynerus murarius_) had adopted the vacuity in front of the eye-lens
as a suitable site for her nest; and here she had formed her cells,
deposited her eggs, and enclosed the caterpillars necessary for the
support of the young when hatched. On another night I came hurriedly to
the telescope to observe Jupiter with my single-lens eyepiece, power
252, but could make nothing out of it but a confused glare, subject
to sudden extinctions and other extraordinary vagaries. I supposed
that the branches of a tree, waving in the wind, must be interposed
in the line of sight, but soon saw this could not possibly be the
explanation. Looking again into the eyepiece, I caught a momentary
glimpse of what I interpreted for the legs of an insect magnified
into gigantic proportions and very distinct on the bright background
formed by Jupiter much out of focus. On detaching the eyepiece and
carrying it indoors to a light, an innocent-looking sample of the
common earwig crawled out of it. The gyrations of the insect in its
endeavours to find a place of egress from its confinement had clearly
caused the effects alluded to. Telescopic observers are thus liable to
become microscopic observers before they are conscious of the fact,
and perhaps also in opposition to their intention. Other experiences
might be narrated, especially as regards nocturnal observing in country
or suburban districts, where the “serious student of the skies” may,
like myself, find diversion to his protracted vigils by the occasional
capture of a too-inquisitive hedgehog or some other marauding quadruped.
[Illustration: Fig. 19.
The Author’s Telescope: a 10-inch With-Browning Reflector.]
_Method._—Nearly all the most successful observers have been men of
method. The work they took in hand has been followed persistently and
with certain definite ends in view. They recognized that there should
be a purpose in every observation. Some amateurs take an incredible
amount of pains to look up an object for the simple satisfaction
of seeing it. But seeing an object is not observing it. The mere
view counts for nothing from a scientific standpoint, though it may
doubtless afford some satisfaction to the person obtaining it. A
practical astronomer, with his own credit at stake and the interests of
the science at heart, will require something more. In observing a comet
he will either fix its position by careful measurement with reference
to stars near, or critically examine its physical peculiarities, or
perhaps both. In securing these data he will have accomplished useful
work, which may quite possibly have an enduring value. In other
branches of observation his aim will be similar, namely to acquire new
materials with regard to place or to physical phenomena, according
to the nature of the research upon which he happens to be engaged.
Such results as he gathers are neatly tabulated in a form convenient
for after comparisons. There have been instances, we know, where
sheer carelessness has resulted in the loss of important discoveries.
Lalande must have found Neptune (and mathematical astronomy would
have been robbed of its greatest triumph) half a century before it was
identified in Galle’s telescope, but his want of care enabled it to
elude him just when he was hovering on the very verge of its discovery.
Numerous other instances might be mentioned. Failure may either arise
from imperfect or inaccurate records, from a want of discrimination,
from neglect in tracing an apparent discordance to its true source,
or from hesitation. I may be pardoned for mentioning a case within
my own experience. On July 11, 1881, just before daylight, I stood
contemplating Auriga, and the idea occurred to me to sweep the region
with my comet eyepiece, but I hesitated, thinking the prospect not
sufficiently inviting. Three nights later Schæberle at Ann Arbor,
U.S.A., discovered a bright telescopic comet in Auriga! Before sunrise
on October 4 of the same year I had been observing Jupiter, and again
hesitated as to the utility of comet-seeking, but, remembering the
little episode in my past experience, I instantly set to work, and
at almost the first sweep alighted upon a suspicious object which
afterwards proved itself a comet of short period. These facts teach one
to value his opportunities. They cannot be lightly neglected, coming
as they do all too rarely. The observer should never hesitate. He must
endeavour to at least effect a little whenever an occasion offers;
for it is just that little which may yield a marked success—greater,
perhaps, than months of arduous labour may achieve at another time.
_Perseverance._—Persistency in observation, apart from the value
derived from cumulative results, increases the powers of an observer
to a considerable degree. This is especially the case when the same
objects are subjected to repeated scrutiny. A first view, though it
may seem perfectly satisfactory in its conditions and results, does
not represent what the observer is capable of doing with renewed
effort. Let us suppose that a lunar object with complicated detail
is to be thoroughly surveyed. The observer delineates at the first
view everything that appears to be visible. But a subsequent effort
reveals other features which eluded him before, and many additional
details are gradually reached during later observations. Ultimately
the observer finds that his first drawing is scarcely more than a mere
outline of the formation as he sees it at his latest efforts. Details
which he regarded as difficult at first have become comparatively
conspicuous, and a number of delicate structures have been exhibited
which were quite beyond his reach at the outset. The eye has become
familiarized with the object, and its powers fairly brought out by
training and experience. This training is very serviceable, but
is seldom appreciated in the degree of its influence. Many a tyro
has abandoned a projected series of observations on finding that
his initiatory view falls wofully short of published drawings or
descriptions. He considers himself hopelessly distanced, and regards
it as impossible to attain—much less excel—the results achieved by
his predecessors. He does not realize that their work is the issue
of years of close application, and that it represents the collective
outcome of many successive nights. I need hardly say that it is a
great mistake to anticipate failure in this way. No telescopic work
has been done in the past that will not be done better in the future.
No observer can rate his capacity until he has rigorously tested it by
experience. The eye must become accustomed to an object before it is
able to do itself justice. Those who have been sedulously engaged in a
certain research will, as a rule, see far more than others who are but
just entering upon it—not from a natural superiority of vision, but
because of the aptitude and power acquired by practice. No matter how
meagre an observer’s primary attempts may be, he should by no means
relax his efforts, but rather feel that his want of success must be
remedied by experience. It is a common fault with observers that they
leave too much to their instruments, and rely upon them for the results
which really depend entirely upon their personal endeavours. A skilled
workman will do good work with indifferent tools; for after all it is
the character of the man that is evident in his results, and not so
much the resources which art places in his hands.
Much also depends upon the feelings by which the amateur is actuated
when he commences work. A few enter into it with a degree of energy
and determination that knows no wearying and will accept no defeat.
Others display a half-hearted enthusiasm, and are constantly doubting
either their personal ability or their instrumental means. Many
others, again, when the circumstances appear a little against them
regard failure as inevitable. It need hardly be said, however, that
every difficulty may be surmounted by perseverance, and that a man’s
enthusiasm is often the measure of his success, and success is rarely
denied to him whose heart is in his work.
_Definition in Towns._—The astronomical journals contain some
interesting references to the definition of telescopes in large towns.
Of course the purer the air the better for observational purposes.
But observers who reside in populous districts need not despair of
doing really useful work. The vapours hanging over a large city are
by no means so objectional as is commonly supposed. When they are
circulating rapidly across the observer’s field of view they will
prove very troublesome at times; but in a comparatively tranquil
state of the air definition is excellent. I have frequently found
planetary markings very sharp and steady through the smoke and fog of
Bristol. The interposing vapours have the effect of moderating the
bright images and improving their quality. When there is a driving
wind, and these heated vapours from the city are rolling rapidly past,
objects at once appear in a state of ebullition, and the work of
observation may as well be postponed. Smoke from neighbouring chimneys
is utterly ruinous to definition: a bright star is transformed into a
seething, cometary mass, and the planets undergo contortions of the
most astonishing character. Large instruments being more susceptible
to such influences—and, indeed, to atmospherical vagaries of all
kinds—are chiefly affected by the drawbacks we have alluded to; but
there are many opportunities when their powers may be fully utilized.
In sweeping for faint comets, or in other work (such as the observation
of nebulæ) where a dark sky is the first essential, a town station
has a manifest disadvantage because of the artificial illumination of
the atmosphere. But for general telescopic work the conditions do not
offer a serious impediment, especially if the observer is careful to
seize the many suitable occasions that must occur. The direction of the
wind relatively to his position and the central part of the city, will
occasion considerable differences to an observer who uses a telescope
in a suburban locality.
_Photography._—Upon this branch of practical astronomy not much will
be said in this volume, as it is rather beyond its scope, and possibly
also beyond the resources of ordinary amateurs, so far as really
valuable work is concerned. A reference must, however, be made to an
innovation which has deservedly assumed a very prominent place, and is
clearly destined to exert an accelerating influence on the progress of
_exact_ astronomy. At present it is impossible to foretell how far it
may be employed and extended, but judging from recent developments its
applications will be as manifold as they will be valuable. Photographic
records possess a great advantage over others, because they are more
accurate and therefore more reliable. They are pictures from Nature
taken by means free from the bias and error inseparable from mere
eye-estimations or hand-drawings. The latter are full of discordances
when compared one with another, and can seldom be implicitly trusted;
but in the photograph a different state of things prevails. Here we
have a faithful portrayal or reproduction of the object impressed by
itself upon the plate. Hence it can be depended upon, because there
has been no intermediate meddling either with its position or features
by what may be termed artistic misrepresentation. True, there may be
imperfections in the process; trifling flaws and obstructions will
invariably creep in wherever comparatively new and novel work is
attempted, but these will but little detract from the value of its
results. Photography is obviously a means of discovery as well as a
means of accurate record; for nebulæ and faint stars quite invisible to
the eye have been distinguished for the first time upon the negatives.
Those of our amateurs who intend working in this branch will find it a
productive one, and not decaying in interest; but the necessary outfit
will be expensive if thoroughly capable instruments are to be employed
in the service.
[Illustration:
LONDON STEREOSCOPIC CO. PHOTOMEZZOTYPE
STANMORE OBSERVATORY.
OUTSIDE VIEW]
_Publications._—The observer of to-day may esteem himself particularly
fortunate in regard to the number and quality of the astronomical
journals within his reach. Discoveries and current events receive
prompt notice in these, and readers are fully informed upon the
leading topics. Among the best of the periodicals alluded to are ‘The
Observatory’ (Taylor & Francis, London), ‘The Sidereal Messenger’
(Northfield, Minn., U.S.A.), and _L’Astronomie_ (Gautier-Villars,
Paris). The _Astronomische Nachrichten_ (Kiel, Germany) is a very old
and valued serial, and ‘The Astronomical Journal’ (Cambridge, Mass.,
U.S.A.) may also be favourably mentioned. The ‘Monthly Notices’ of the
Royal Astronomical Society and the ‘Journals’ of the Liverpool and
British Astronomical Societies contain many interesting materials.
‘Nature,’ ‘The English Mechanic,’ and ‘Knowledge’ are among the
English journals which devote part of their space to the science;
and the beautiful illustrations in the latter entitle it to special
recognition. It is evident, from this short summary, the amateur will
find that his literary appetite may be amply satisfied, and should he
desire a channel for recording his own work or ideas the publications
referred to offer him every facility and encouragement.
As to almanacks, the ‘Nautical’ which has been termed “The Astronomer’s
Bible,” includes a mass of tabular matter, some portion of which is of
utility to the amateur, but it does not give data which are to be found
in some other publications. I refer particularly to ephemerides of the
satellites of Mars, Saturn, Uranus, and Neptune, to the dates of max.
and min. of variable stars, to the times of rising and setting of the
Sun, Moon, and planets, to the epochs and positions of meteor-showers,
&c. The annual ‘Companion to the Observatory’ furnishes most of
these details, and ‘Whitaker’s Almanack’ and Brown & Sons’ ‘Nautical
Almanack’ each contain a large amount of serviceable information.
The latter, however, is chiefly devoted to topics connected with
Navigation, while ‘Whitaker’s Almanack’ is an extensive repertory of
general facts.
With respect to handbooks much depends upon the direction of the
observer’s labours, for he will obviously require works dealing
expressly with his special subject. As a reliable companion to the
telescope, Webb’s ‘Celestial Objects for Common Telescopes’ (4th edit.,
1881) is indispensable; as a work of reference, and one forming an
exhaustive conspectus of astronomical facts, Chambers’s ‘Descriptive
Astronomy’ (4th edit., in 3 vols., 1889) may be recommended.
Ledger’s ‘The Sun, its planets and their satellites’ is another good
descriptive work. The beginner will find Noble’s ‘Hours with a 3-inch
Telescope’ full of very instructive and agreeable material; while the
more experienced astronomer, requiring a masterly exposition of the
principles of the science, must procure Sir J. Herschel’s ‘Outlines’
(11th edit., 1871). In departmental work books of more exclusive
character will be necessary. Thus, students of solar physics will
want Young’s volume on ‘The Sun;’ observers of our satellite will
need Neison’s ‘Moon.’ Those who find double stars interesting should
get Crossley, Gledhill, & Wilson’s ‘Handbook’ and Chambers’s revised
edition of Admiral Smyth’s ‘Cycle;’ others working on variable stars
will need the Catalogues of Chandler and Gore. Jovian phenomena are
well represented in Stanley Williams’s ‘Zenographic Fragments.’ Comets
have been fully treated of in works by Cooper, Hind, and Guillemin;
while to the observer of eclipses Johnson’s ‘Eclipses Past and
Future’ is a valuable guide. Everyone interested in nebulæ will of
course require Herschel-Dreyer’s ‘General Catalogue,’ containing 7840
objects and published by the Royal Astronomical Society in 1888. As
to planetary observations, the several works of Webb, Chambers (vol.
i.), and Ledger, first cited, supply a large amount of detail, almost
obviating the necessity for further books.
_Past and Future._—Observers and telescopes go on increasing day
by day, and the future of astronomy has a most brilliant outlook.
Photography has latterly effected a partial revolution in observation,
though it can never entirely supersede old methods. Spectrum analysis,
too, has formed a valuable acquisition during the last quarter of a
century. With the new and refined processes, and with the gigantic
instruments which have been erected, we may confidently anticipate
many additions to our knowledge, especially in regard to very small
and faint bodies which the inferior appliances of previous years
have failed to grasp. And it is certain that some of the presumed
discoveries of past times must be expunged, because not verified by the
more perfect and powerful researches of a later date. Let us place in
parallel columns (1) a few of the suspected objects thus to be erased,
and (2) some of those which the future will probably add to our store:-
(1.) (2.)
Satellite of Venus. Satellites of Uranus and Neptune.
Vulcan. Ultra-Neptunian Planet.
Active Volcanoes on the Moon. Changes on the Moon.
Detached cusps of Venus and Mercury Rotation of Mercury, Venus,
indicating high mountains. Uranus, and Neptune.
Rings of Uranus and Neptune. Minor Planets.
Multiple companions to Polaris Periodical Comets.
and Vega. Nebulæ and Double Stars.
Whatever may be the direction of future enquiries or the departures
from old and tried methods, ordinary amateurs with small instruments,
though handicapped more heavily as regards the prospect of effecting
discoveries, may yet always be expected to accomplish useful work.
Even to him who simply makes the science a hobby and a source of
recreation in a leisure hour after the cares of business, the sky never
ceases to afford a means of agreeable entertainment. He may neither
achieve distinction nor seek it; but this he will assuredly do—gain
an instructive insight into the marvellous works of his Creator, and
acquire a knowledge which can only exercise an elevating tone to his
life. The observer who quietly, from his cottage window, surveys the
evening star or the new Moon through his little telescope often finds a
deeper pleasure than the proficient astronomer who, from his elevated
and richly appointed observatory, discovers new orbs with one of the
most powerful instruments ever made.
_Attractions of Telescopic Work._—In concluding our comments we may
briefly refer to the importance and pleasure attached to telescopic
work, and the growing popularity of observation in the attractive and
diverse field of astronomy. A telescope may either be employed as an
instrument of scientific discovery and critical work, or it may be
made a source of recreation and instruction. By its means the powers
of the eye are so far assisted and expanded that we are enabled to form
a clearer conception of the wonderful works of the Creator than could
be obtained in any other way. Objects which appear to natural vision in
dim and uncertain characters are resolved, even in telescopes of the
smallest pretentions, into pictures of well-defined outlines containing
details of configuration far exceeding what are expected. And it is
entirely owing to the exact measurements obtained under telescopic
power that many of the most important problems of astronomy have been
satisfactorily solved. To this instrument we are indebted, not only
in a great measure for our knowledge of the physical features of many
celestial bodies, but also for the accurate information we have gained
as to their motions, distances, and magnitudes. Apart from this it is
capable of affording ample entertainment to all those who are desirous
of viewing for themselves some of the absorbing wonders of astronomy as
described in our handbooks. And a demonstration of this practical kind
is more effective than any amount of description in bringing home to
the comprehension of the uninitiated the unique and picturesque side of
astronomy.
CHAPTER V.
_THE SUN._
Solar Observations.—Early notices of Spots.—Difficulties of the
old observers.—Small instruments useful.—Tinted glass.—Solar
Diagonal.—Structure of a Spot.—Methods of Drawing.—Ascertaining
Dimensions.—Observer’s aims.—Eclipses of the Sun.—Periodicity of
Spots.—Crateriform structure.—“Willow-Leaves.”—Rotation of the
Sun.—Planetary bodies in transit.—Proper motion of Sun-spots.—Rise
and decay of Spots.—Black Nuclei in the umbræ.—Bright objects near
the Sun.—Cyclonic action.—Sudden outbursts of Faculæ.—Shadows
cast by Faculæ.—Veiled Spots.—Recurrent disturbances.—Recurrent
forms.—Exceptional position of Spots.—The Solar prominences.
“Along the skies the Sun obliquely rolls,
Forsakes, by turns, and visits both the poles;
Diff’rent his track, but constant his career,
Divides the times, and measures out the year.”
The Sun is not an object comprehended in the title of this volume.
But to have omitted reference to a body of such vast importance,
and one displaying so many interesting features to the telescopic
observer, would have been inexcusable. We may regard the Sun as the
dominant power, the controlling orb, and the great central luminary of
our system. The phenomena visibly displayed on his surface assume a
particular significance, as affecting a body occupying so high a place
in the celestial mechanism.
The mean apparent diameter of the Sun is 32′ 3″·6, and his real
diameter 866,000 miles. The apparent diameter varies from a minimum
of 31′ 32″ at the end of June to a maximum of 32′36″; at the end of
December; and the mean value is reached both at the end of March and
September. The Sun’s mean distance from the Earth is about 92,900,000
miles, computed from a solar parallax of 8″·8, which appears to agree
with the best of recent determinations. At this distance the linear
value of 1″ of arc is 447 miles.
The Sun’s apparent diameter is as follows on the first day of each
month:—
′ ″
Jan. 1 32 36·0
Feb. 1 32 31·8
Mar. 1 32 20·4
April 1 32 3·8
May 1 31 48·0
June 1 31 36·4
July 1 31 32·0
Aug. 1 31 35·8
Sept. 1 31 47·0
Oct. 1 32 2·6
Nov. 1 32 19·2
Dec. 1 32 31·6
Solar observations may be pursued with a facility greater than that
attending work in some other departments of practical astronomy. The
Moon, planets, and stars have to be observed at night, when cold air,
darkness, and other circumstances are the cause of inconvenience;
but the student of the Sun labours only in the light and warmth of
genial days, when all the incidentals to observation may be agreeably
performed. There are, however, some drawbacks even in this pleasant
sphere of work. The light of the Sun is so great that much persistent
observation is apt to have an injurious effect on the eye, and will
certainly deaden its sensitiveness on faint objects. In the summer
months the observer experiences discomfort during a lengthy observation
from remaining so long in the powerful rays of the Sun, some of which
must fall upon his face unless measures are adopted to shield it.
During the progress of solar work the student should always provide
for himself as much shelter as possible from the glare, which must
otherwise disturb that equanimity of feeling in the absence of which no
delicate research is likely to be successfully conducted.
“Spots on the Sun” were remarked long before the telescope came into
service. In the early Chinese annals many references are made to these
objects; thus, in A.D. 188, February 14, it is recorded—“The colour
of the Sun reddish-yellow; a fleckle in the Sun (bird-shaped).” Other
ancient notices compare the spots to a flying bird, an apple, or an
egg. Many spots were seen in later years, especially in 321, 807, 840,
1096, &c. In 807 a large black spot upon the Sun was watched during a
period of eight days. It reflects much credit upon observers of a past
age that they performed so many useful feats of observation, though
relying simply upon the powers with which Nature alone had endowed
them. They anticipated the telescope in some important discoveries.
Large sun-spots are not, it is true, difficult features to perceive
with the naked eye under certain circumstances; for whenever there
is a fog or haze sufficiently dense to veil the lustre of the Sun in
suitable degree, they can be readily seen, presuming, of course, that
such spots are in existence at the time. They are sometimes observed,
in a purely casual way, by people who may happen to glance at the Sun
when he is involved in fog and looks like a dull, red ball suspended in
the firmament. On one occasion, near sunset, in the autumn of 1870, I
saw four large spots on different parts of the Sun, and these phenomena
were very numerous at about this time. When spots attain a diameter of
50″ or more they may be detected by persons of good sight; but if the
Sun is high and clear, coloured glass must be used to defend the eye.
Doubt hangs over the question as to the first telescopic observer of
the spots. It is certain that Fabricius, Galilei, Harriot, and Scheiner
all remarked them in about the year 1611; and of these Fabricius
perhaps deserves the chief praise, as the first who published a memoir
on the subject. Galilei appears undoubtedly to have had priority in
recognizing the bright spots, or _faculæ_. Scheiner discovered that the
black spots, or _maculæ_, are composed of a dark umbra and a fainter
outlying shade, called the penumbra. Arago quotes him as having also
described the Sun as “covered over its whole surface with very small,
bright, and obscure points, or with lively and sombre streaks of
very slender dimensions, crossing each other in all directions.” He
announced, too, that the spots were confined to a narrow zone on the
north and south sides of the equator, and this he termed the “Royal
Zone.”
Some grave difficulties appear to have marked the attempts of the
earlier observers; for they did not all use coloured glasses, and
the dazzling light of the Sun, intensified by their lenses, often
overpowered the sight, and so we find them awaiting opportunities when
fog partly obscured the Sun near his rising or setting. Thus Harriot,
who seems to have noticed and figured three sun-spots as early as 1610,
Dec. 8, says:—“The altitude of the Sonne being 7 or 8 degrees, and it
being a frost and a mist, I saw the Sonne in this manner.” His drawing
followed. On another occasion he says:—“A notable mist: I observed
the Sonne at sundry times, when it was fit.” Fabricius advised other
observers to commence their observations by admitting only a small
portion of the Sun into the field, so that the eye might be prepared to
receive the light of the entire disk. Galilei was equally unaware of
the advantage of tinted glass, and adopted the expedient of scanning
the Sun when placed in the vicinity of the horizon. He remarks that
“the spot of 1612, April 5 appeared at sunset;” and his writings
contain other references of similar import. Scheiner, however, appears
to have been more alive to the requirements of the work, and employed a
plain green glass placed in front of the object-lens of his telescope.
Under the various circumstances we have been alluding to, the views
obtained of the solar surface must necessarily have been of a very
defective character, and the old observers at least deserve our
sympathy in their exertions. No such obstacles confront the observer
now. He has everything provided for him. Instrumental devices rob the
Sun of his noonday brilliancy, and the eye serenely scans the details
of his expansive image without the slightest pain or effort.
Small telescopes are peculiarly well adapted for solar observations. A
good 3-inch refractor or 4-inch reflector will reveal an astonishing
diversity of structure in the spots, and show something of the
complicated _minutiæ_ of the general surface. If the aperture of
either instrument is 2 inches more than that stated, so much the
better; but further than this it is rarely advisable to go. When the
objective or mirror exceeds a diameter of 5 or 6 inches a stop often
improves the images, and even smaller instruments will perform better
when a little contracted. Definition is here the point to be desired;
of light we have a superabundance. But if the observer meditates a
critical analysis of the detail, either of a single spot, of a group of
spots, or of a small area of the luminous surface, then a fair amount
of aperture should be used, because greater aperture means greater
separating power, and the latter will be useful in resolving the
network of fibrous materials of which apparently the whole surface is
composed. But for the common requirements of the observer an instrument
of 3 or 4 inches will be found very effective, and it can either be
used on a short tripod stand, placed on a steady table near a window
having a south aspect, or it may be mounted on a tall garden stand and,
according to the owner’s pleasure, either fixed at his window or in
his garden. Two powers will be really necessary—one of about 60 and a
field of quite 33″ to contain the entire disk and give a good general
view, and another of 150 to which the observer will have recourse when
examining details. Additional eyepieces will be sometimes useful,
especially one of about 100; but the power of 60 previously recommended
will, if a Huygenian, answer the same purpose, for if the field-lens
is removed it will be increased to about 90. And should the observer
think that anything is to be gained by a higher magnifier than 150, let
him use the eye-lens only of that power. I have obtained many exquisite
views of sun-spots with a single lens, and, instead of purchasing
new eyepieces, a real advantage will be derived in adopting the plan
suggested. There will be a smaller field and more colour about the
image, but the improvement in definition is considerable, and more than
balances these disadvantages.
Tinted glass must always be employed, unless a dense fog prevails, in
which case the example of the old observers may be emulated. Several
coloured glasses, of various depths, are needed for use according as
the occasion requires. With a high Sun on a bright June day a darker
tint will be necessary than in the winter, when the Sun’s rays are
but feebly transmitted through the horizontal vapours. Red glass is
unsatisfactory, as there is much heat and glare with it; but when used
in combination with green the effect is excellent. Green alone is often
used, and answers well; but it is not always thick and dense enough for
the purpose. The plan of Sir W. Herschel, to interpose a glass trough
of diluted ink, has never become popular, though he found it to succeed
admirably. Smoked glass is also adapted for solar work, and recommends
itself as being always obtainable at a minute’s notice. Some observers
use a Barlow lens, with a thin film of silver deposited on the
surfaces. It is then sufficiently transparent to give a neutral tint
when held before a light, and sharp definition is said to be obtained
without additional protection. Mr. Thornthwaite has also employed a
coloured Barlow lens with effect.
A solar diagonal is a very necessary appliance if the observer would
ensure perfect safety; for any refractor exceeding 2-inches aperture
may, when turned on the Sun, focus enough heat to fracture the tinted
sun-glass. The diagonal, by preserving a part only of the solar rays
which are transmitted by the object-glass, enables observations to be
made in security. This little instrument is comparatively cheap, and no
telescope is complete without one. Dawes’s solar eyepiece serves the
same purpose in a different manner, but it is an expensive luxury. In
the latter construction there is a perforated diaphragm fixed near the
eyepiece and so arranged that the quantity of admitted light may be
modified consistently with the observer’s wishes.
In reflecting-telescopes with glass mirrors, effective views of the Sun
are obtainable by employing unsilvered mirrors; for sufficient light is
reflected by the glass surfaces to form good images of solar detail.
What, perhaps, interferes more than any other circumstance with
successful observation of the Sun, is the fact that the rays, falling
upon the telescope and objects near, induce a good deal of radiation,
the direct tendency of which is to impair the definition and give a
rippling effect to the disk. This is sometimes present in such force
that the spots are subject to an incessant commotion, which serves
to obliterate their more delicate features. A shady place is best,
therefore, for such work; and if the observer leaves his telescope
for a short time, intending to resume observations, it should never
be placed broadside to the Sun, or the tube wall get hot, and heated
currents must be generated in the interior, to the ruin of subsequent
views.
A large sun-spot consists of an apparently black nucleus, a brown
umbra, divided possibly by veins of bright matter or by encroachments
of the penumbra which surrounds it. The latter is of much lighter
tone than the umbra, though often similar in its general form. The
outer edges of the umbra are serrated or scalloped by rice-grain
protuberances. The inner region of the penumbra is much brighter than
the outer, and the latter often exhibits quite a dusky fringe, induced
by lines of dark material intervening with the brighter particles.
The filaments forming the penumbra—often grouped in a radial manner
with reference to the centre of a spot—would appear to be more widely
separated near the outer border of the penumbra, and sufficiently so to
allow sections of the umbral layer of the Sun to be observed through
the interstices. The lighter tint of the interior part of the penumbra
is stated to be due to contrast; but this is a mistake. The difference
is too definite and distinct to permit such an explanation. Mr. Maunder
says “that usually (not invariably) the penumbra darkens towards the
umbra, and that the phenomenon as ordinarily described is merely an
effect of contrast.” My own observations, however, appear to show that
there is an actual difference of detail in the outer and inner portions
of the penumbra, which gives a darker tone to the former.
In drawing the forms of sun-spots the observer must be expeditious,
because of the variations which are quickly and constantly affecting
them. In concluding a sketch I find it essential to make several
alterations in it, owing to the changes which have occurred in the
spots during the interval of a quarter of an hour or so since it was
commenced. The details must be filled in consecutively, each one
being the result of a careful scrutiny. When finished, the whole
sketch should be compared with the object itself and amended if found
necessary. The observer should also mark upon the sheet the measured
or estimated latitude and longitude of the spot, and make a finished
drawing from the basis of his sketch as soon as possible afterwards.
At Stonyhurst Observatory excellent delineations of solar phenomena
are made; and the late Father Perry, who lost his life in the cause
of science, thus described the method:—“On every fine day the image
of the Sun is projected on a thin board attached to the telescope,
and a drawing of the Sun is made, 10½ inches in diameter, showing the
position and outline of the spots visible. It is the first duty of the
assistant who makes the drawings to note the position of the spots,
and sketch their outlines. He then proceeds to shade in the penumbra
and to draw the finer details, comparing the drawing from time to time
by placing it alongside the projected image of the spots. The position
of the faculæ is then filled in with a red pencil, so that the eye can
at once recognize their grouping with respect to sun-spots, and the
other details drawn with a black pencil.” The same astronomer also
stated that, “as a general rule, careful drawings of the projected
image of the Sun give much more satisfactory pictures of the solar
surface than the photographs taken even at our best observatories. It
is quite true that occasionally an exquisite photograph on an enlarged
scale may be obtained, which exhibits features such as no pencil could
portray as accurately, but rarely indeed will the photograph furnish
all the details that a practised eye and hand, kept patiently at the
sketch-board, will detect and faithfully describe. And the reason
is not far to seek; for any experienced observer knows that, even
on the finest day, the definition is continually changing with the
sky, and that it is only at comparatively rare moments we can expect
those perfect conditions that enable the finest details to stand out
sharply, as Schiaparelli expresses it, like the faintest lines of a
steel engraving. A photograph may be accidentally taken during one
of these exceptionally favoured moments; but a patient draughtsman
is almost sure to secure several of these best opportunities at each
prolonged visit to his sketch-board. What would, therefore, be a great
acquisition at present is a series of careful solar drawings, taken at
short intervals of time, on days when characteristic spots are visible
upon the Sun; and this would be the surest way of adding much valuable
information to that already possessed concerning the changes that take
place in the solar photosphere.”
With regard to ascertaining the dimensions of sun-spots, very precise
results require accurate means of measurement and some mathematical
knowledge. For the general purposes of the amateur, who will only
want round numbers, simple methods may be adopted with success. I have
used, on a 4-inch refractor, a graduated piece of plane glass, mounted
suitably for insertion in the focus of the eyepiece, and marked with
divisions 1/200 of an inch apart. With power 65 I find the Sun’s disk
at max. distance covers 83 divisions of the graduated lens; so that one
division = 22″·8, the Sun’s min. diameter being 1892″. Each division,
therefore, is equal to 10,434 miles, the Sun’s real diameter being
866,000 miles.
[Illustration: Fig. 20.
Sun-spot of June 19, 1889, 2^h P.M.]
I viewed a large spot on June 19, 1889, and found its major axis
covered 2·6 divisions, = 59″·3[9]; so that its apparent length was
about 27,000 miles. For
1892″ : 866,000 miles :: 59″·3 : 27,143 miles.
The same method may be adopted if the image is thrown upon a screen.
Approximate values are to be obtained by means of fine cross wires
fixed in the eyepiece. Note the exact interval occupied by the Sun in
crossing the vertical wire, and also the interval occupied by the large
spot or group. If the Sun is 133 seconds in passing the wire, and the
group 6·5 seconds, then
133 seconds : 866,000 miles :: 6·5 seconds : 42,323 miles.
This plan is likely to be most successful when the Sun is near its
meridian passage; but it may be applied at any hour, if care is taken
to adjust the eyepiece so that the Sun’s motion is precisely at right
angles to the vertical wire. One other plan may be mentioned. Draw on
cardboard, with compasses, a circle about 10 or 12 inches diameter,
and divide this with 31 parallel lines. Subdivide each of the spaces
into 5, less prominently marked. Then, during observation, keep both
eyes open, and hold or fix the circular disk at a distance enabling it
to coincide with the telescopic image of the Sun. By carefully noting
how many divisions the group covers on the cardboard, its dimensions
may be readily found, because one division will be equal to about
5410 miles. Of course these methods[10] are simply approximate, and
only strictly applicable to objects not far removed from the central
regions of the Sun, because the spots are portions of a sphere, and not
angles subtended by a flat surface. When close to the E. or W. limbs,
foreshortening is considerable, though the polar diameter of a spot is
not affected by it then.
Presuming an observer to have his 3-or 4-inch telescope duly fitted
with a solar diagonal and tinted glass, he may naturally ask, after
his curiosity has been satisfied by the contemplation of his first
sun-spot, what he can do further: What special features is he to look
for? What changes ought to be recorded? What are the doubtful points
that require to be cleared up as regards the Sun’s physical appearance?
In what way are new and novel facts likely to be glimpsed? In a word,
he desires to know in what manner he may employ his eyes and instrument
usefully for science, while also gaining pleasure for himself.
Information like this is often needed by the young student, and
sometimes indeed by men who have already gained a little experience,
and who possess much larger instruments than we have intimated above.
In endeavouring to offer suggestions in response to such inquiries, I
would remark that the nature and direction of a research essentially
depend upon several conditions, viz. the observer’s inclination, his
instrumental equipment, his place of observation, and the amount of
time he can devote to the pursuit of his object. There are very few men
who, like Schwabe of Dessau, will confront the Sun on nearly every day
for more than forty years in order to learn something of its secrets.
Such extraordinary pertinacity is fortunately not required, except in
special cases. Amateurs may effect much valuable work in the short
intervals which many of them steal either from business or domestic
ties and offer at the shrine of astronomy.
There are quite a considerable number of attractive phenomena and
features on which the solar observer will find ample employment, and to
the principal of these it may be as well to make individual references.
_Eclipses of the Sun._—These phenomena deservedly rank amongst the most
important and impressive events displayed by the heavenly bodies, and
they are specially interesting to the possessors of small telescopes.
Solar eclipses have been so often made the subject of observation
and discussion, that our knowledge of the appearances presented may
be considered to be nearly complete. The various aspects of Nature
on such occasions have been so attentively studied in their manifold
bearings, that virtually nothing remains for the ordinary observer but
to reexamine and corroborate facts already well ascertained. He can
expect to glean few materials in a field where a plentiful harvest has
just been reaped. But the eclipsed Sun, if it has revealed most of
its secrets to previous investigators, has certainly not declined in
attractiveness; and the amateur will find the spectacle still capable
of exhibiting features which, though not full of the charms of novelty,
will be sufficiently striking and diversified to be remembered long
after the event has passed.
[Illustration: Fig. 21.
1891, June 6. 1899, June 8. 1900, May 28. 1905, Aug. 30.
1908, June 28. 1912, April 17. 1914, Aug. 21. 1916, Feb. 3.
At sunset.
1919, Nov. 22. 1920, Nov. 10. 1921, April 8. 1922, March 28.
At sunset. At sunset.
Solar Eclipses visible in England, 1891 to 1922.]
[Illustration: Fig. 22.
Total Solar Eclipse of August 19, 1887.]
Eclipses recur in cycles of 18 years and 10 days (= 6585 days). This
period was determined by the ancients, and called the _saros_. By its
means the times and magnitudes of eclipses were roughly computed long
before astronomy became an exact science.
A solar eclipse is really an _occultation_ of the Sun by the Moon; for
the word _eclipse_, in its usual reference, denotes the obscuration of
one body by its immersion in the shadow of another. During any single
year there are never less than two eclipses, nor more than seven.
Whenever there are two only, both are solar.
Since the fine solar eclipse of December 22, 1870, no large eclipse
of the Sun has been visible in England. It is remarkable that during
the thirty years from 1870 to 1900 these phenomena are all of an
unimportant, minor character. Within the thirty years following 1891
there will be twelve solar eclipses, for which the Rev. S. J. Johnson
has given projections (as shown on p. 98) for the period of greatest
obscuration.
Total eclipses are extremely rare as regards their visibility at a
given station. Thus between 878 and 1715 not one was observed at
London, and during the next 500 years there will be a similar absence
of such a phenomenon. The observer of total eclipses must perforce
journey to those particular tracts of the earth’s surface over which
the band of totality passes. On such occasions photography plays an
important part; and the corona, the red flames, the shadow-bands, and
numerous other features become the subjects of necessarily hurried
observation and record, for totality endures for very few minutes[11].
As regards ordinary partial eclipses, amateurs usually find ample
entertainment in noting the serrated aspect of the Moon’s contour
projected on the bright Sun. It is also interesting to watch the
disappearance and reappearance of the solar spots visible at the time.
Rather a low magnifying power, with sufficiently expansive field to
include the entire disk, is commonly best for the purpose of these
observations.
_Periodicity of Spots._—This detail may be said to have been fully
investigated. Schwabe and Wolf have accomplished much in this
direction. A work of this kind must, by the nature of it, extend over
many years and entail many thousands of observations. It is therefore
more suited to the professional astronomer than to the amateur, whose
attention is more or less irregular owing to other calls. The sun-spot
cycle is one of about 11 years, during which there are alternately
few and many spots on the Sun. There appear to be some curious
fluctuations, disturbing the regular increase and decrease in the
number of spots; and these variations are worthy of more attention.
The following are the years of observed maxima and minima of sun-spot
frequency:—
Maxima. Minima.
1828. 1833.
1837. 1843.
1848. 1854.
1860. 1867.
1870. 1878.
1883-4. 1890 (?).
These phenomena have been rare during the past few years. The next
maximum may be expected in about 1894, when solar observers will
probably have an abundance of new materials to study.
_Crateriform Structure._—In 1769 Prof. Wilson, of Glasgow, while
watching a sun-spot with a Gregorian reflecting-telescope, remarked
that, as it approached near the limb, the penumbra became much
foreshortened on the interior side. He inferred from this that the
spots were cavities, and the idea has been generally accepted; so
that these objects are sometimes termed solar craters, and commonly
regarded as openings in the luminous atmosphere of the Sun. But the
conclusion appears to be based on data not uniformly supporting it. In
1886 the Rev. F. Howlett published some observations which “entirely
militate against the commonly received opinion that the spots are
to any extent sunk in the solar surface as to produce always those
effects of perspective foreshortening of the inner side of the penumbra
(when near the limb) which have been described in various works on
astronomy.” In a number of instances the penumbra is wider on the side
nearest the Sun’s centre, whereas the converse ought to be the case on
the cavity theory. The fine sun-spot of July 1889 offered an example
of this; for when it was near the W. limb the W. side of the penumbra
was obviously much narrower than the E. side, so that the appearance
would indicate the object as an elevation rather than a depression. The
observer should keep a register of the aspect of all pretty large spots
near the limb, and note the relative widths of the E. and W. sides of
the penumbra. An extensive table of such results would be interesting,
and certain to throw some light on the theory of spot-structure. It is
of course possible that occasionally the inner side of the penumbra is
broader than the outer, and thus appears wider even on the limb, though
really forming the side of a shallow depression.
“_Willow-Leaves._”—In 1861 the late Mr. Nasmyth announced that the
entire solar surface was composed of minute luminous filaments in
the shape of “willow-leaves,” which interlaced one another in every
possible variety of direction. This alleged discovery only met with
doubtful corroboration. The objects were stated by some authorities to
be simply identical with the “corrugations” and “bright nodules” of
Sir W. Herschel. Mr. Stone called them “rice-grains.” The eagle-eyed
Dawes thought “granulations” a more appropriate term, as it implied
no consistency of form and size. Secchi referred to them as oblong
filaments, and “rather like bits of cotton-wool of elongated form.” The
Rev. F. Howlett described the Sun as presenting a granulated, mottled
appearance in a 3-inch Dollond refractor, and mentioned that on the
morning of June 9, 1865, the aspect of its surface was like that of
new-fallen snow, the objects “being not rounded but sharply angular.”
The opinions of observers were thus singularly diverse, and the result
of several animated discussions at the Royal Astronomical Society was
that little unanimity was arrived at, except as to the fact that the
Sun’s surface was crowded with small luminous filaments of elongated
form, and either rounded or angular at the ends. There was no accord
as to their precise forms or distinctive manner of grouping. Some of
the observers averred that the “willow-leaves” or “rice-grains” had no
title whatever to be regarded as a new discovery, the same appearances
having been recognized long before. Gradually the contention ceased,
and though more than a quarter of a century has passed since the
discussion arose there has been little new light thrown on the subject.
Amateurs will therefore do well to probe deeper into this promising
branch of solar observation. As Mr. Nasmyth himself stated,
considerable telescopic power is required, combined with a good
atmosphere. But comparatively small instruments will also be useful,
because of their excellent definition and efficacy in displaying
details on a brilliant orb like the Sun. A power of 150 should be
employed in examining small regions of the general surface, and also
the edges of the umbra and penumbra of sun-spots. When definition is
unusually sharp, and the details very distinct, the magnifying power
should be increased if it can be done with advantage; and the observer
should utilize an occasion like this to the utmost extent. On a really
excellent day more may be sometimes detected than during several weeks
when the atmosphere is only moderately favourable. The observations,
being of a critical nature, should not be attempted in winter, when
the Sun is low. I have frequently secured fine views of the delicate
structure of the solar surface between about 8 and 9 A.M. in the summer
months; and this is often a convenient time for amateurs to snatch a
glimpse, before going to business.
With reference to the general question as to the existence of the
“willow-leaves,” my conception of the matter is that the features
described by Mr. Nasmyth are not new. His drawing of a spot in Sir J.
Herschel’s ‘Outlines’ and Chambers’s ‘Descriptive Astronomy’ exhibits
objects extremely uniform in shape and size, and this uniformity I
have never observed in the penumbra of spots. As to the engraving in
the ‘Outlines,’ showing the aspect of the interlaced “willow-leaves”
on the general surface, this is also not realized in observation. The
“corrugations” and “bright nodules” of Sir W. Herschel aptly represent
what is seen, and they are possibly identical with the “very small
bright and obscure points” and “lively and sombre streaks” of Scheiner,
though seen much better and in more profusion of detail through the
improved modern telescopes. The so-called “willow-leaves” are rounded
at the ends, and are consistent neither in size nor shape. They
encroach upon the umbra of the spots, and give a thatched appearance
to the edges. The penumbra also shows this in its outer limits, where
it is also fringed with lenticular particles. Drawings by Capocci and
Pastorff seventy-five years ago, and published in Arago’s ‘Popular
Astronomy,’ show the thatching at the edges of the umbra quite as
palpably as it is represented in recent drawings.
[Illustration: Fig. 23.
Belts of Sun-spots, visible October 29, 1868.]
_Rotation of the Sun._—By noting when the same individual spots return
to the same relative places on the disk, the approximate time of
rotation is easily deduced. This varies according to the latitude of
the spots[12]; whence it is evident the solar atmosphere is affected by
currents of different velocities, causing the spots to vary in their
longitudes with reference to each other. The Earth’s motion round
the Sun causes the spots to travel apparently more slowly than they
really do; for observations prove that a spot completes a rotation in
27 days 5 hours, whereas the actual time, after making allowance for
the earth’s orbital motion, is about 25 days 7-3/4 hours. The period of
rotation may be roughly found as follows, supposing a spot to return to
precisely the same part of the disk in 27 days 5 hours:—
365^d 5^h 49^m + 27^d 5^h = 392 10^h 49^m.
Then
392^d 10^h 49^m (= 565,129^m) : 365^d 5^h 49^m (= 525,949^m)
:: 27^d 5^h (= 39,180^m) : 25^d 7^h 44^m (= 36,464^m).
For exact results several circumstances have to be considered, such as
the direction of the spot-motions across the disk, as the chords vary
according to the season; thus in June and December the spots traverse
straight lines, while in March and September their paths are curved,
like a belt on Saturn when the planet is inclined. Some of the spots
display considerable proper motion; so that it is best to observe a
number of these objects, and reduce the times to a mean result. They
are not very durable, rarely lasting longer than a few weeks; but
some of the more extensive disturbances are sustained for several
months, during which many singular changes are effected. The period of
rotation, as determined by several observers, is as follows:—
d h m
1678. Cassini 25 13 55
1718. Bianchini 25 7 48
1775. Delambre 25 0 17
1841. Laugier 25 8 10
1846. Kysæus 25 2 10
1852. Böhm 25 12 29
1863. Carrington 25 9 7
1865. Schwabe 25 5 0
1868. Spörer 25 5 31
1888. Wilsing 25 5 47
The motion of rotation is similar in direction to that in which the
planets move found the Sun, namely from west to east. Hence the spots
come into view on the east limb of the Sun, and disappear at the west.
_Planetary Bodies in transit._—During observation the observer should
particularly watch any very dark, small spots that may be visible,
such as are isolated and pretty circular and definite in outline. If
an object of this character is seen it should be examined with a high
power, and its aspect critically noted. Should the observer entertain
any suspicion of its being of a planetary nature, he should carefully
determine its position on the disk, and, after a short interval,
re-observe it for traces of motion. If it remains stationary, its true
solar origin will be proved. If motion is shown, then the successive
positions of the object during its transit, and its place of egress,
with the time of each observation, should be recorded. In such a case
it would be a good plan to project the Sun’s image, and mark the place
of the suspicious object and chief sun-spots at short intervals. This
would be more accurate than mere eye-estimation. The observer who
scans the solar surface for intra-Mercurial planets must remember
that, if any such bodies exist, they will probably be very diminutive.
Venus, when on the Sun in December 1882, was a spot 63″ in diameter,
and easily perceptible to the naked eye. Mercury, at the transits of
1861, 1868, and 1881, was a little less than 10″, but in 1878 was
12″. If “Vulcan,” the suspected interior planet, has any existence
it may possibly be much smaller than Mercury, and will thus escape
observation, unless the observer exercises great care in the search.
The mobile, planetary spots asserted to have been seen on the Sun in
past years prove nothing definite, and appear to have been illusory.
_Proper Motion of Sun-spots._—This feature is one deserving more
investigation. The distances separating individual spots should either
be measured with a micrometer or determined by transits across a wire,
and the displacement recorded from hour to hour or from day to day.
Spots in different latitudes will almost certainly exhibit some change
of relative place; and objects in the same latitude must be watched,
for similar variations probably affect them. The physical peculiarities
of such spots should be remarked, and also the alterations of
appearance they undergo during the time they approach or recede from
each other.
_Rise and Decay of Spots._—Occasionally large spots are formed in an
incredibly short time, and the disappearance of others has been equally
sudden. Schwabe found, from many observations, that the western spots
of a group are obliterated first; but authorities differ. I have
usually observed that the smaller, outlying members of a group vanish
before the larger spot, which then contracts and is invaded by tongues
of faculæ; so that its effacement soon follows, and nothing remains to
indicate the disturbance but bright ridges of faculæ, which are very
conspicuous near the limb.
_Black Nuclei in the Umbræ._—Dawes was the first to announce that the
umbra sometimes included a much darker area or nucleus. This is present
in nearly all large spots. A part of the umbra seems covered or veiled
by a slightly luminous medium, and the portion unaffected looks black
by contrast. On October 1, 1881, with a 2½-inch refractor, I saw a
large sun-spot, the umbra of which was broken up into 7 fragments, and
the S. preceding part appeared very black while the others showed a
much lighter tint. In the fine spot of June 1889 a nucleus was also
distinctly apparent; and this feature is sometimes so obvious in large
spots that it may be observed with an instrument of only 2-inches
aperture. I have usually remarked the nucleus on one side of the
umbra, and abutting the penumbra. It may be formed by light patches of
transparent material floating over the umbra, and leaving a part free
where the Sun’s dark body is fully exposed. This light material is
possibly suspended far above the umbra and inconstant in its position;
so that the place and form of the nucleus should always be noted for
traces of change. It is necessary that such details should be closely
watched during an entire day, or several days; for the variations could
then be followed, and perhaps reduced to some law. This persistence
is very necessary, in order to solve many of the peculiarities of
sun-spots, which, though pretty well known in appearance, have not been
thoroughly studied in their various developments.
_Bright Objects near the Sun._—Small, rapidly moving bodies have been
occasionally reported as seen passing over the Sun. In several cases
these have been prematurely assigned a meteoric origin. They have been
described as luminous bodies of irregular shape, as moving in a common
direction, and as being very distinct when projected on the dark sky
just outlying the bright limb of the Sun. There is little doubt they
are either the pappus of different kinds of seed, or convolutions of
gossamer, which have been lifted to great heights in the air, and
are rendered bright by reflection from the bordering Sun. In this
connection I may mention some observations of my own with a 4-inch
refractor:—
“1889, MAY 20, 0^h 30^m P.M.—Bright points and little misty forms kept
passing from the Sun’s limb, at the average rate of 13 in a minute.
They moved in the same direction as the clouds and wind. Some of them
were followed by tails, which were far from straight. I saw them best
when I focused the telescope for an object much nearer than the Sun.
One of these forms would occasionally halt and pursue an irregular
flight. It was evident they were terrestrial objects, with motions
controlled by the wind.
“3^h P.M.—Many bright objects still passing from the Sun’s limb.”
“1889, MAY 22, 9^h A.M.—Observed vast numbers of luminous particles
floating about contiguous to the Sun’s margin. They were clearly
carried along by the wind; but this being very slight, their motions
were extremely slow, and now and then many of them became nearly
stationary. Their directions were far from uniform, though the general
tendency was obviously in a common line of flight. I watched them for
some time passing in a plentiful shower.”
These objects are always noticed in summer-time, and I believe they
would much more frequently attract remark but for the fact that they
require a longer focus than the Sun and cannot be recognized when on
the disk, to which the observer is usually giving the whole of his
attention. Those who are often employed in solar work will find it an
interesting diversion to look for these bodies. The instrument should
be focused as for a distant terrestrial object, and only a part of
the Sun’s limb should be retained in the field of view of an eyepiece
of moderately low power. Then, looking intently at the dark sky near
the limb, the bright objects will be sometimes seen sailing past in
considerable numbers.
_Cyclonic Action._—The appearance in detail of certain spots, coupled
with evidences of rotatory motion round their own centres, has induced
the belief that they are liable to action in some degree similar to the
cyclonic storms[13] which disturb and rend the terrestrial atmosphere.
Such indications should be looked for in fairly conspicuous spots,
and any peculiarities of the nature alluded to made the subject of
close investigation. A spot showing features having a spiral tendency
may not, however, have a gyratory movement about its centre. This can
only be determined by critically noting the details, and frequently
reobserving them for traces of motion. The penumbra always shows
radiations converging on the umbra as a centre; but this is merely a
form of structure, and proves nothing in evidence of a revolving storm.
_Sudden Outbursts of Faculæ._—In September 1859 Carrington and Hodgson
independently observed a striking outburst of faculæ in front
of a large group of spots which they were examining. It remained
visible about five minutes, during which interval several patches
of light travelled over a region nearly 34,000 miles in extent. An
extraordinary magnetic disturbance was simultaneously recorded at the
Kew Observatory, and sixteen hours afterwards there followed a magnetic
storm of unusual severity. On another occasion Dr. Peters observed
flashes of light cross and recross the umbra of a prominent spot with
electric velocity. Some other startling observations of solar phenomena
have been effected, and there is no question as to their having been
matters of fact. In the presence of effects so sudden, so obvious, and
so unexpected, no wonder the observers at first doubted the evidence
of their eyes and suspected the cause to lie in a fractured glass or
a fault of adjustment. But the corroboration afforded the clearest
proof as to the actuality of the events described. They will doubtless
occur again; but these phenomena cannot be definitely predicted as to
time, so that students of the solar surface should be prepared for a
repetition of them whenever they may occur.
[Illustration: Fig. 24.]
_Shadows cast by Faculæ._—M. Trouvelot, while examining a large
sun-spot on May 26, 1878, noticed that it was “completely surrounded
by very brilliant and massive faculæ.” “On one part of the penumbra
an extraordinary appearance was perceived, which resembled so closely
a shadow, as it would have been cast by the overhanging faculous mass,
that it seemed useless to seek, and it was impossible to admit, any
other explanation. This shadow, the outline of which was a little
diffused, had the same shape as, and reproduced with great exactness,
the outline of the faculous mass situated above it. It was not so
black as the opening in spots called the umbra, but of a very dark
tint.” A similar feature was seen by Kirk and Maclean on May 2, 1884,
and the ‘Observatory,’ vol. vii. pp. 146, 170, and 197, contains some
interesting particulars on this subject. Fig. 24 is a drawing by
Kirk, in which the shadow is represented by A, B; at C “it accurately
followed the outline of the intensely white margin of the spot.”
_Veiled Spots._—The late Father Perry described these objects at the
R.A.S. meeting on May 9, 1884, and said they are to be seen all over
the face of the Sun. They only exist for two or three minutes, and then
disappear. In one instance he observed a train of these veiled spots
stretching over “a tenth part of the Sun’s diameter, which was nearly
as obvious to the eye as the penumbra of an ordinary spot; it split
into two throughout its whole length, and disappeared in a minute.
The veiled spots seem to be of two classes: the one appear like small
greyish clouds, which disappear after a few minutes, as if they were
formed and rapidly evaporated by the Sun’s heat, and the others seem
to be connected with the umbra of ordinary spots; they appear about
them, and are more permanent than the ordinary veiled spots, lasting
sometimes two or three days, but never longer.” These markings appear
to have been first detected by Trouvelot in 1875, and he gives some
information as to this class of phenomena in the ‘Observatory,’ vol.
viii. pp. 228 _et seq._
_Recurrent Disturbances_[14].—It is supposed, and with good evidence
affirming the idea, that certain regions of the Sun’s surface are
subject to frequent outbursts of spots, which are possibly due to
forces acting from below the Sun’s bright atmosphere. After the
disappearance of large groups or isolated spots it is therefore
advisable to watch the same region for some time afterwards, to find
whether it remains perfectly quiescent, or whether it soon again
becomes a seat of activity and change.
_Recurrent Forms._—Certain spots observed at different times have
exhibited appearances so nearly resembling each other that it has
been considered the likeness may be due to something more than mere
accident. Whenever such suggestive coincidences are recognized the
observer should note them particularly, and secure drawings. It should
be his aim to determine the exact intervals elapsing between the
presentation of spots or groups of this character, and also whether
they occupy the same latitude and longitude on the Sun’s disk.
_Exceptional Position of Spots._—The ordinary spots are rarely seen
more than 35° distant from the solar equator or within 8° of it. They
usually appear in the zones from 8° to 20° N. and S. of the equator.
A few exceptions may be mentioned[15]. Mechain saw a spot in July
1780 having a latitude of 40-1/3°; in April 1826 Capocci recorded one
having 49° of S. latitude; Schwabe and Peters observed spots 50° from
the equator. Lahire, in the last century, described a spot as visible
in a latitude of 70°; but the accuracy of this observation has been
questioned. Whenever a spot is seen near the equator, or very far
removed from it, measures should be taken of its exact place; for
it is desirable to learn something more of those disturbances which
occasionally affect the more barren regions of the solar envelope.
_The Solar Prominences._—Those amateurs who have included a
spectroscope in their instrumental outfit will find the study of
the chromosphere and prominences a most productive one. Huggins and
Zöllner were the first to apply the “open-slit” method; and the study
of the shape of the hydrogen prominences commenced in 1869. Tupman
details (‘Monthly Notices R.A.S.,’ vol. xxxiii. p. 106) a series of
observations which he secured in 1872 with a refractor of 3-inches
aperture and a direct-vision spectroscope of five prisms. He mentions
the cost of the entire apparatus as only £18, and says he entertains
“no doubt that an equally effective instrument could be made for much
less.” The prominences appear to be of different kinds, and are known
as “cloud”- and “flame”-prominences. Both are liable to rapid changes.
Trouvelot, in June 1874, noticed “a gigantic comma-shaped prominence,
82,000 miles high, which vanished from before his eyes by a withdrawal
of light as sudden as the passage of a flash of lightning.” Since
the study of these remarkable forms was rendered feasible by using a
greater dispersion to open the slit of the spectroscope wide enough to
see them, they have been made the subject of daily study and record.
The results, so far as they have been investigated, show that the
region of the Sun’s limb in which the prominences are most frequent
reaches to some 40° on either side of the equator, which is somewhat
greater than the area of sun-spot frequency. The chromosphere itself
is probably of much the same character as the erupted prominences,
and formed of little flames arranged thickly together like “blades of
grass.”
In observing the Sun with a telescope the amateur will soon notice
that the surface is far more brilliant in the central parts than round
the margin of the disk. Vögel has estimated that immediately inside
the edges the brightness does not amount to one seventh that of the
centre. The difference is entirely due to the solar atmosphere, which
is probably very shallow relatively to the great diameter of the Sun.
FOOTNOTES:
[9] The Rev. F. Howlett measured this spot on the following day,
June 20, and found it 63″ in its largest diameter. He used a small
refractor, and projected the Sun’s image on to a screen sufficiently
distant for it to have a diameter of 3 feet.
[10] On May 13, 1890, at 3^h, I tested the three methods alluded
to on a scattered train of small spots, and derived the following
measurements of length:—
By glass micrometer 76,570 miles.
" cross wires 76,610 "
" cardboard disk 75,770 "
In this comparison I used an excellent 4-inch Cooke refractor,
belonging to a friend.
[11] The maximum duration of totality, under every favouring
circumstance, appears to be about 8 minutes. The great eclipse which
occurred on August 18, 1868, maintained the total phase for nearly 6
minutes 50 seconds in the Gulf of Siam. In reference to this eclipse,
Dr. Weiss says:—“In the records of ancient eclipses there are to be
found only two which may be compared in size with that of August 18,
1868, but none in which the totality lasted so long. The first of these
is the eclipse of Thales (28 May, 585 B.C.), which is said to have
been the first predicted, and to have terminated a bloody war between
the Lydians and the Medes. The second was visible on June 17, 1433, in
Scotland, and the time of its occurrence was long remembered by the
people of that country as ‘the black hour.’”
[12] Carrington found that spots near the equator gave a shorter
rotation-period than those far removed from it. This offers an analogy
to the spots on Jupiter, which move with greater celerity near the
equator, though the rule is not absolute.
[13] In 1852 Dawes observed and measured a rotatory motion affecting a
spot at the rate of about 17° per day.
[14] Lalande, in 1778, asserted that “there are spots of very
considerable magnitude, which, reappear in the same physical points of
the solar disk.”
[15] A spot was visible on June 30, 1889, in 40° South latitude.
Its recorded duration was 2 days. This object was observed at the
Stonyhurst Observatory and at a station in North America.
CHAPTER VI.
_THE MOON._
Attractive aspect of the Moon.—Absence of air and water.—Only one
Hemisphere visible.—Earthshine.—Telescopic observations of the
lunar surface.—Eclipses.—Lunar changes.—Formations.—Plato and
other objects described.—Table of Moon’s age and formations near
terminator.—Occultations of stars.—Visibility of the new Moon.
“The western Sun withdraws: meanwhile the Moon,
Full orb’d, and breaking through the scatter’d clouds,
Shows her broad visage in the crimson’d east.”
Early in autumn, when the evenings are frequently clear, many persons
are led with more force than usual to evince an interest in our
satellite, and to desire information which may not be conveniently
obtained at the time. The aspect of the Moon at her rising, near the
time of the full, during the months of August, September, and October,
is more conspicuously noticeable than at any other season of the year,
on account of the position she then assumes on successive nights,
enabling her to rise at closely identical times for several evenings
together. The appearance of her large, ruddy globe at near the same
hour, and her increasing brilliancy as her horizontal rays give way
under a more vertical position, originated the title of “Harvest Moon,”
to commemorate the facility afforded by her light for the ingathering
of the corn preceding the time of the autumnal equinox.
It will be universally admitted that the Moon possesses special
attractions for us, as being situated nearer than any other celestial
body, and forming the inseparable companion or tributary world to the
Earth. The many important influences she exercises have led to her
becoming the object of close investigation; so that her motions and
physical appearances have been ascertained with a remarkable degree
of exactness and amplitude. Her movements regulate the tides; her
positions are of the utmost moment to the mariner; her light is the
welcome beacon of the wayfarer, and its picturesque serenity has ever
formed the theme of poets. To the practical astronomer she constitutes
an orb perfectly unique as regards extent and variety of detail; and
questions relating to the physical condition of her surface, now and in
past ages, supply a fund of endless speculation to the theorist.
The mean apparent diameter of the Moon is 31′ 5″, and it varies from
29′ 21″ at perigee to 33′ 31″ at apogee. Her real diameter is 2160
miles, and her mean distance slightly exceeds 237,000 miles. Her
revolution round the Earth (= sidereal period) is performed in 27^d 7^h
43^m 11^s·46, but the time from one new moon to another (= synodical
period) is 29^d 12^h 44^m 3^s. The Moon’s motion through the firmament
is at the rate of 13° 10′ 35″ per day and 32′ 56″ per hour. Thus she
travels over a space slightly exceeding her own diameter in one hour.
The linear value of 1″ at the distance of our satellite is 1·16 mile,
and of 1′ 69½ miles.
When we critically survey the face of the Moon with a good telescope,
we see at once that her surface is broken up into a series of craters
of various sizes, and that some very irregular formations are scattered
here and there, which present a similar appearance to elevated
mountain-ranges. The crateriform aspect of the Moon is perhaps the more
striking feature, from its greater extent; and we recognize in the
individual forms a _simile_ to the circular cavities formed in slag or
some other hard substances under the action of intense heat. In certain
regions of the Moon, especially those near the south pole, the disk
is one mass of abutting craters, and were it not for the obvious want
of symmetry in form and uniformity of size, the appearance would be
analogous to that of a gigantic honeycomb. These craters are commonly
surrounded by high walls or ramparts, and often include conical hills
rising from their centres to great heights. While the eye examines
these singular structures, and lingers amongst the mass of intricate
detail in which the whole surface abounds, we cannot but feel impressed
at the marvellous sharpness of definition with which the different
features are presented to our view. It matters not to what district we
direct our gaze, there is the same perfect serenity and clearness of
outline. Not the slightest indication can be discerned anywhere of mist
or other obscuring vapours hanging over the lunar landscape.
_Absence of Air and Water._—Now it is palpable from this that the Moon
has no atmosphere of sufficient density to render itself appreciable;
for such an appendage, if it existed in any visible form, would at once
obtrude upon the attention, and we should probably recognize some of
the characteristics common to the behaviour of our terrestrial clouds.
But nothing of the kind is apparent on the Moon: there is an unbroken
transparency spread over the whole extent of the Moon’s scenery; whence
we conclude that if any air exists on the surface it is of extremely
attenuated nature, and possibly confined to the bottom of the craters
and low-lying formations, which are arranged in such prolific manner on
our satellite.
Nor is there any perceptible intimation of water upon the Moon. It is
true that several dark grey patches have been given names, leading
one directly to the inference that lakes and seas comprise part of
the surface phenomena. Thus there is the _Mare Serenitatis_ (“the sea
of serenity”) and many other designations of similar import, which
we cannot but insist are wrongly applied and calculated to lead to
misapprehension. Before the invention of the telescope furnished us
with the means of accurately determining the character of the lunar
features, such apellations may have been considered eligible; but now
that the non-existence of water in any extensive form is admitted,
the titles are rendered obsolete. Still their retention is in some
respects advisable, for any sweeping change in a recognized system of
nomenclature must cause confusion, and the names alluded to serve a
useful end in facilitating reference; so that, under the circumstances,
it would perhaps be unwise to attempt reform, or to introduce an
innovation which must occasion many difficulties.
_Only one Hemisphere visible._—In discussing the nature and appearances
of the lunar formations, it must be distinctly understood that our
remarks apply to those visible on the side invariably turned towards
the Earth. For, in point of fact, there is a considerable expanse
of the lunar disk never perceptible from the Earth at all. This is
occasioned by the circumstance that the Moon rotates upon her axis
in precisely the same time as she revolves around the Earth, and is
therefore enabled to present the same side towards us on all occasions.
A slight tilting (called libration) takes place, so that we are
allowed a glimpse of fragments of the side normally invisible, and its
analogous aspect leads us to suppose that there is no great distinction
between the features of the Moon’s visible and invisible hemispheres.
From exact computations it appears that we are enabled to see a
proportion of 59/100 of the surface, and that the remaining 41/100 are
permanently beyond our reach.
_Earthshine._—A few mornings before new moon, and on a few evenings
after it, the whole outline of the dark portion of the lunar globe
may be distinctly perceived. A feeble illumination like twilight
pervades the opaque part, and this is really earthlight thrown upon
our satellite, for near the times of new moon the Earth appears at
her brightest (her disk being fully illuminated) as seen from the
Moon. The French term for this light is _la lumière cendrée_, or “the
ashy light.” The appearance is often popularly referred to in our own
country as “the old Moon in the new Moon’s arms.” Some of the old
observers remarked that the waning Moon showed this earthlight more
strongly than the new Moon.
_Telescopic Observations of the Lunar Surface._—Our telescopes give
by far the most pleasing view of the Moon when she is in a crescent
shape. At such a period the craters and mountains, with their dark
shadows, are splendidly displayed. A good view is also obtainable
with the Moon at first or last quarter, or when the disk is gibbous.
But the full Moon is decidedly less attractive; for the shadows have
all disappeared, and the various formations have quite lost their
distinctive character. The disk is enveloped in a flood of light,
causing glare, and though there is a large amount of detail, including
systems of bright rays, many differences of tint, and bright spots,
yet the effect is altogether less satisfactory than at the time of a
crescent phase.
The nature of the work undertaken by the amateur must largely depend
upon his opportunities and the capacity of his appliances. It is
evident that in the investigation of lunar details it is essential to
be very particular in recording observations; for unless the conditions
of illumination are nearly the same, lunar objects will present little
resemblance. He should therefore examine the formations at intervals of
59^d 1^h 28^m, when the terminator is resting on nearly identical parts
of the surface. In periods of 442^d 23^h (= 15 lunations) there is
another repetition of similar phase; also in periods of 502^d 0^h 28^m
(= 17 lunations).
The observer, in entering results into his note-book, should state
the Moon’s age to the nearest minute, and give aperture and power of
telescope and state of sky. Those objects which he has recorded at one
lunation should be re-observed after an intervening lunation, or at
intervals of 59^d 1^h 28^m. He will then find his notes and drawings
are comparable. By the persistent scrutiny of special structures he
will discern more and more of their details; in other words, he will
find his eye soon acquires power with experience and familiarity with
the object. Comparisons of his own work with the charts and records
of previous observers will be sure to interest him greatly, and the
differences which he will almost certainly detect may exert a useful
influence in inciting him to ascertain the source of them. He must not
be premature in attributing such discordances to actual changes on the
Moon; for he must remember that perfect harmony is rarely to be found
in the experiences of different observers. But whenever his own results
are inconsistent with those of others, the fact should be carefully
noted and the observations repeated and rediscussed with a view to
reconcile them. The charts and descriptions of former selenographers
are excellent in their way, and the outcome of much zealous labour; but
they contain omissions and inaccuracies which it has been impracticable
to avoid. The amateur who discovers a mountain, craterlet, or rill not
depicted on his lunar maps must therefore neither regard it as a new
formation or as a new discovery; for it may have been overlooked by
some of the previous observers, and is possibly drawn or described in
a work which he does not happen to have consulted. Such differences
should, however, always be announced, as they clear the way for others
working in the same field.
A small instrument, with an object-glass of about 2½ inches, will
reveal a large amount of intricate detail on the surface of our
satellite, and will afford the young student many evenings of
interesting recreation. But for a more advanced survey of the
formations, with the view to discover unknown objects or traces of
physical change in known features, a telescope of at least 8 or 10
inches aperture is probably necessary, and powers of 300, 350, and more.
_Eclipses of the Moon._—These phenomena comprise a variety of
interesting aspects. They are less numerous, in actual occurrence, than
solar eclipses in the proportion of about 2 to 3; but they are more
frequently visible, because they may be witnessed from any part of an
entire hemisphere, whereas eclipses of the Sun are only observable from
a tract of the Earth’s surface not exceeding 180 miles in breadth. The
Moon may remain totally eclipsed for a period of 2 hours 4 minutes, and
the whole duration, including the penumbral obscuration from its first
to its last projection, is about 6 hours. Sometimes the Moon suffers
total eclipse twice in the same year, and both may be visible, as in
1844, 1877, 1964, &c. It is possible for three such eclipses to occur
within a single year, as in 1544. In 1917 there will be three total
lunar eclipses, but not all visible in England. In the latter year
there will be no less than seven eclipses, as in 1935.
On the last two occasions—Oct. 4, 1884, and Jan. 28, 1888—when the
Moon was totally immersed in the Earth’s shadow, the atmosphere was
very clear; and it is hoped equally favourable conditions will attend
the similar phenomena of Nov. 15, 1891, Sept. 4, 1895, and Dec. 27,
1898. One of the most interesting features during these temporary
obscurations of our satellite is the occultation of small stars. Prof.
Struve compiled a list of no less than 116 of these objects that would
pass behind the Moon’s shadowed limb during the eclipse of Oct. 4,
1884.
Another important effect is the variable colouring on the Moon. This
differs considerably in relative intensity as seen during successive
eclipses, and the cause is not perhaps fully accounted for. Kepler
thought it due to differences in humidity of those parts of the Earth’s
atmosphere through which the solar rays pass and are refracted to the
eclipsed Moon. The intense red hue which envelopes the lunar surface
on such occasions is due to the absorption of the blue rays of light
by our atmosphere. The sky at sunset is often observed to be similarly
coloured, and from the operation of similar causes. Sometimes the Moon
entirely disappears when eclipsed, but on other occasions remains
distinctly obvious, like a bright red ball suspended in the firmament.
On May 5, 1110, Dec. 9, 1620, May 18, 1761, and June 10, 1816, our
satellite is said to have become absolutely imperceptible during
eclipse. Wargentin, who described the appearance in 1761, remarks:—“The
Moon’s body disappeared so completely that not the slightest trace of
any portion of the lunar disk could be discerned, either with the naked
eye or with the telescope.” On Oct. 4, 1884, I noticed that the opacity
was much greater than usual; at the middle period of the eclipse the
Moon’s diameter was apparently so much reduced that she looked like a
dull, faint, nebulous mass, without sharply determinate outlines. The
effect was similar to that of a star or planet struggling through dense
haze. Yet, on March 19, 1848, the Moon “presented a luminosity quite
unusual. The light and dark places on the face of our satellite could
be almost as well made out as on an ordinary dull moonlight night.” On
July 12, 1870, Feb. 27 and Aug. 23, 1877, and Jan. 28, 1888, the Moon,
as observed at Bristol, was also fairly bright when totally immersed
in the Earth’s shadow. In explanation of these singular differences,
Dr. Burder has suggested that Kepler’s views seem inadequate, and
that the solar corona is probably implicated in producing light and
dark eclipses. He concludes that, as the corona sometimes extends
to considerable distances from the Sun, and is very variable in
brightness, it may have sufficient influence to occasion the effects
alluded to.
_Lunar Changes._—The question as to whether physical changes are
occurring in the surface-formations of our satellite is one which
offers attractive inducements to telescopic observers. Though the Moon
appears to have passed the active state, it is very possible that
trivial alterations continue to affect some of her features. In April
1787 Sir W. Herschel wrote:—“I perceive three volcanoes in different
places of the dark part of the new Moon. Two of them are already nearly
extinct, or otherwise in a state of going to break out; the third
shows an eruption of fire or luminous matter.” Schröter, however, was
correctly of opinion that these appearances were due to reflected
light from the Earth falling upon elevated spots of the Moon having
unusual capacity to return it. Schröter himself thought he detected
sudden changes in 1791. He says that, on the 30th of December, at 5^h
P.M., with a 7-foot reflector magnifying 161 times, he perceived the
commencement of a small crater on the S.W. declivity of the volcanic
mountain in the Mare Crisium, having a shadow of at least 2′ 5″. On
the 11th of January, 1792, at 5^h 20^m P.M., on looking at the place
again he could see neither the new crater nor its shadow. In this
case the disappearance was doubtless an apparent one, merely due to
the reversed illumination under which the object was examined in the
interval of 12 days.
Many other observers besides Herschel have been struck with the
brightness of certain spots situated in the opaque region of the lunar
disk; but there is no doubt the cause has been uniformly one and the
same, viz. the highly reflective properties of some of the mountains
(notably of one named Aristarchus), which are distinctly visible as
luminous spots amid the relatively dark regions surrounding them. They
afford no certain evidence of existing volcanic energy, and in the
light of modern researches such an idea cannot be entertained.
On June 10, 1866, Temple noticed a remarkable light appearance,
agreeing with the position of Aristarchus, upon the dark side of the
Moon, faintly illuminated by earthshine. The object did not exhibit
a faint white light analogous to that of other craters in the dark
side, but it was star-like, diffused, in colour reddish yellow, and
evidently dissimilar to other bright spots. He wrote, in reference to
this matter:—“Of course I am far from surmising a still active chemical
outbreak, as such an outbreak supposes water and an atmosphere, both
of which are universally allowed not to exist on the Moon, so that
the crater-forming process can only be thought of as a dry, chemical,
although warm one.”
On November 17, 1866, Schmidt announced that the lunar crater Linné,
about 5½ miles in diameter, and situated in the Mare Serenitatis, had
disappeared! He averred that he had been familiar with the object as
_a deep crater_ since 1841, but in October 1866 he found its place
occupied by _a whitish cloud_. This cloud was always visible, but the
crater itself appeared to have become filled up, and was certainly
invisible under its former aspect. Such a definite statement, emanating
as it did from a diligent and experienced student of selenography,
naturally aroused keen interest, and Linné at once became the object
of wide-spread observation. But a reference to Schröter’s results,
obtained in the latter part of the last century, threw some doubt
upon the alleged change. This observer had figured Linné on November
5, 1788, as a round white spot, and there is nothing in his drawing
indicating a crateriform aspect. His description of Linné was:—“A
flat, somewhat doubtful crater, which appears as a round white spot.”
Mr. Huggins regarded Schröter’s observations as correctly expressing
the appearance of this object in 1867 under the same conditions of
illumination. On the other hand, Lohrmann (1823) and Mädler (1831)
referred to Linné as a deep crater, and in terms inconsistent both
with Schröter’s drawing and with the present aspect of the object.
The outcome of the many fresh observations that were collected was
that Linné appeared as a white cloud, with a small black crater within
a large shallow-ringed depression. But as usual in such cases, the
observers were far from being unanimous as to the details of the
formation; and certainly in regard to a lunar object this need occasion
no surprise, for slight differences in the angle of illumination
produce marked changes in the aspect of lunar features. The fact of
actual change could not be demonstrated, and the negative view appears
to have subsequently gained weight.
Another instance of alleged activity on the Moon was notified by Dr.
Klein in the spring of 1877. He saw a deep black crater about 18
miles to the W.N.W. of Hyginus, and in a particular place where he
had previously recognized no such object, though he had frequently
examined the region and was perfectly familiar with it. Forthwith every
telescope was directed to this part of the Moon. The maps of earlier
observers were eagerly consulted, and lunar photographs scanned for
traces of the new object. Many drawings were made of the district
near Hyginus and of the remarkable rill or cleft connected with it;
but amongst both old and new records some puzzling discordances were
detected. Many of the observers, instead of finding Dr. Klein’s new
formation a sharply-cut, deep crater, saw it rather in the character of
a saucer-like depression; and I drew it under this aspect on several
occasions with a 10-inch reflector. The fact, therefore, of its being
a new feature admitted of no valid and convincing proofs, and thus the
same uncertainty remains attached to this object as to Linné, nothing
being absolutely proved[16]. The problem as to whether the Moon is
still the seat of physical activity has yet to be solved.
Many circumstances are antagonistic to the discovery of changes on
the Moon. As the Sun’s altitude is constantly varying with reference
to lunar objects, they assume different aspects from hour to hour.
In a short interval the same formations become very dissimilar.
When the Sun is rising above the more minute craters they are often
distinguished in their true characters; but near the period of full
Moon they are visible as bright spots, and it is impossible to tell
whether they represent craters or conical hills. With a vertical Sun,
as at the full, all the shadows have disappeared—in fact, the entire
configuration has been transformed, and many of the interesting
lineaments displayed at the crescent phase are no longer seen. The
Moon’s libration also introduces slight differences in the appearance
of objects. And these are not the only drawbacks; for observations,
in themselves, are seldom accordant, and it is found that drawings
and descriptions are not always to be reconciled, though referring to
identical and invariable features. The lunar landscape must be studied
under the same conditions of illumination and libration, with the same
instrument and power, and in a similar state of atmosphere, if results
are to be strictly comparable. But it is very rarely that observations
can be effected under precisely equal conditions; hence discordances
are found amongst the records.
The whole of the Moon’s visible sphere exhibits striking imprints of
convulsions and volcanic action in past times, though no such forces
appear to operate now. The surface seems to have become quiescent,
and to have assumed a rigidity inconsistent with the idea of present
energy. But we cannot be absolutely certain that minute changes are not
taking place, and, being minute, the prospect of their detection is
somewhat remote. Students of lunar scenery will probably have to watch
details with scrupulous care and for long periods before an instance of
real activity can be demonstrated.
_Lunar Formations._—The Moon abounds in objects of very diversified
character, and they have been classified according to peculiarities
of structure. The names of eminent astronomers have been applied
to many of the more definite features—a plan of nomenclature which
originated with Riccioli, who published a lunar map at the middle of
the seventeenth century. The following brief summary comprises many of
the principal formations:—
_Mare._ A name applied by Hevelius to denote the large and relatively
level plains on the Moon, which present some similarity to terrestrial
seas. They are visible to the naked eye as dusky spots, and in a
telescope show many craters, hills, and mounds, and some extensive
undulations of surface.
_Palus_ (Marsh) and _Lacus_ (Lake) were titles given by Riccioli to
minor areas of a dark colour, and exhibiting greater variety of detail
and tint than the _Maria_.
_Sinus_ (Bay) has been applied to objects like deep bays on the borders
of the _Maria_.
_Walled Plains_ extend from 40 to 150 miles in diameter, and are
commonly surrounded by a terraced wall or mountain-ranges. The
interiors are tolerably level, though often marked with crater-pits,
mounds, and ridges.
_Mountain-Rings._ These represent rings of mountains and hills,
enclosing irregularities, possibly furnished by the debris of the
crumbling exterior walls, which, in certain instances, appear to have
fallen inwards.
_Ring-Plains_ are more circular and regular in type than the walled
plains, and consist of a moderately flat surface surrounded by a single
wall. _Crater-Plains_ are somewhat similar, and seldom exceed 20 miles
in diameter. They “rise steeply from the mass of debris around the foot
of their walls to a considerable height, and then fall precipitously
to the interior in a rough curved slope, whilst on their walls,
especially on the exterior, craterlets and crater-cones often exist in
considerable numbers.”
_Craters, Craterlets, and Crater-Pits._ Usually circular in form, and
severally offering distinctions as to dimensions and shape. The craters
are surrounded by walls, rising abruptly to tolerable heights, and
pretty regular in their contour. When the Sun is rising the shadow of
the walls falls upon the interior of the craters, and many of these
dark conspicuous objects are to be seen near the Moon’s terminator.
With a high Sun some of the craters are extremely bright. In proof
of the large number of these objects, it may be noted here that in
Mädler’s lunar map (1837) 7735 craters are figured, while in Schmidt’s
(1878) there are no less than 32,856!
_Crater-Cones._ Conical hills or mountains, visible as small luminous
spots about the period of full Moon. They are from 1/2 to 3 miles in
diameter, and show deep central depressions. It is somewhat difficult
to distinguish them from the ordinary mountain-peaks and white spots,
and they are not unlike the cones of terrestrial volcanoes.
_Rills or Clefts._ These are very curious objects. They were first
discovered by Schröter in 1787, and some of them are to be traced over
a considerable extent of the lunar surface, their entire length being
200 or 300 miles. They have the appearance of cuttings or canals, and
are sometimes straight, sometimes bent, and not unfrequently develop
branches which intersect each other. They apparently run without
interruption through many varieties of lunar objects. The bottoms of
these rills are nearly flat, and look not unlike dried riverbeds. Some
observers have regarded them as fractures or cracks in the Moon’s
surface; but their appearance and circumstances of arrangement are
opposed to such a view. Our present knowledge includes more than 1000
of these rills.
_Mountain-Ranges_ are chains of lofty peaks and highlands, sometimes
divided by rills and numerous ravines and cross valleys. Some of these
ranges are of vast magnitude, and the summits of the mountains reach
altitudes between 15,000 and 20,000 feet, and sometimes even more.
_Mountain-Ridges_ are to be found scattered in the greatest abundance
in the most disturbed localities of the lunar surface. They sometimes
connect several formations, or surmount ravines or depressions of large
extent. Peaks attaining altitudes of more than 5000 feet rise from
them, and they range in several cases over 100 miles.
_Ray-Centres._ Systems of radiating light-streaks, having a
mountain-ring as the centre of divergence, and stretching to distances
of some hundreds of miles round. Tycho, Copernicus, Kepler, Anaxagoras,
Aristarchus, and Olbers are pronounced examples of this class.
In Beer and Mädler’s chart of the Moon the names are attached to the
various formations, as they are also in Neison’s maps and in some other
works. One of these will be absolutely necessary to the student in
prosecuting his studies. He will then have a ready means of acquainting
himself with the various formations, and making comparisons between his
new results and the drawings of earlier selenographers. I would refer
the reader to Neison’s and Webb’s books for many references in detail
to lunar features, and must be content here with a brief description of
a few leading objects:—
_Plato_ is an extensive walled plain, 60 miles in diameter, and
situated on the N.E. boundary of the Mare Imbrium. Nasmyth and
Carpenter describe the wall as “serrated with noble peaks, which cast
their black shadows across the plateau in a most picturesque manner,
like the towers and spires of a great cathedral.” It has received
a large amount of attention, with a view to trace whether changes
are occurring in the numerous white spots and streaks lying in its
interior. In 1869-71 Mr. Birt collected many observations, and on
discussing them was led to believe that “there is strong probability
that activity, of a character sufficient to render its effects visible,
had been manifested.” The inquiry was renewed by Stanley Williams in
1882-84, and he concluded that the results were strongly confirmatory
of actual change having occurred since 1869-71. The relative visibility
of several of the bright spots had altered in the interim, and
the curious intermingling bright streaks also exhibited traces of
variation. At sunrise the interior of Plato is pure grey; but with
the sun at a considerable height above it, the plain becomes a dark
steel-grey. The change is an abnormal one, and difficult to explain.
South of Plato there is a fine example of an isolated peak, named Pico,
which is about 8000 feet high.
[Illustration: Fig. 25.
Light-spots and streaks on Plato, 1879-82. (A. Stanley Williams.)]
_Great Alpine Valley._ This object, supposed to have been discovered
by Bianchini in 1727, and having a length, according to Mädler, of 83
miles and a breadth varying from 3½ to 6 miles, is a very conspicuous
depression situated near Plato, and running from the Mare Frigoris
to the Mare Imbrium. It exhibits at its southern extremity an oval
formation, and a narrow gorge issues from it to the northward, opening
out further on, and imparting to the whole appearance a shape which
Webb likened to a Florence oil-flask. Elger has fully described this
singular structure. “It is only when far removed from the terminator
that its V-shaped outlet to the Mare Imbrium flanked on either side
by the lofty Alps can be traced to advantage, or the flask-like
expansion with the constricted gorge leading up to it from the N.W.
satisfactorily observed. At other times these features are always more
or less concealed by the shadows of neighbouring heights. The details
of the upper or more attenuated end of the valley are, however, best
seen under a setting sun, when many striking objects come to light, of
which few traces appear at other times.”
_Archimedes._ One of the most definite and regular of the walled
plains. It is 60 miles in diameter, with a wall rising about 4200 feet
above the surface. Some small craters and various streaks diversify its
centre.
_Tycho._ A grand ring-plain, 54 miles in diameter and about 17,000
feet (= nearly 3 miles) deep, and forming the centre of the chief
ray-system of the Moon. The light-radiations stretch over one fourth of
the visible hemisphere at the full, but they are imperceptible with the
Sun’s altitude below 20°. These remarkable radiations from Tycho form a
striking aspect of lunar scenery, and any small telescope reveals them.
Webb has termed Tycho “the metropolitan formation of the Moon;” and the
idea embodied in the expression must strike observers as very apposite.
This object is visible to the naked eye at the time of full. A fine
hill rises from its centre to a height of 5500 feet.
_Copernicus._ A magnificent ring-plain, 56 miles in diameter, and
surrounded by a wall (in which there are terraces and lofty peaks,
separated by ravines) attaining an elevation of 11,000 feet. There
is a central hill of nearly 2500 feet. From Copernicus light-streaks
are plentifully extended on all sides, and apparently connect this
object with the many others of similar character which are situated in
this region. Neison says that near Copernicus the light-streaks unite
and form a kind of nimbus or light-cloud about it. The streaks are
most conspicuous towards the N., where they are from 5 to 14 miles in
width. To the N.W. of Copernicus, about halfway in the direction of
the neighbouring ring-plain _Eratosthenes_ (and N. of Stadius), there
is a considerable number of crater-pits. Mädler figured sixty-one of
these, and regarded that number as certainly less than half the total
number visible. They appear to be ranged in rows or streams, and are
so close together in places as to nearly form crater-rills. Schmidt
saw the ground hereabout pierced like a honeycomb, and managed to
count about 300 little craters; but they are so thickly strewn in this
district that exact numbers or places cannot be assigned. They are best
observable when the Sun is rising on the E. wall of Copernicus. The
interior of this fine object shows six or seven peaks, which are often
capped with sunshine, and very brilliant amid the black shadow thrown
from the surrounding wall.
_Theophilus._ Another ring-plain, and one of the deepest visible. Its
terraced lofty wall, 64 miles in diameter, rises in a series of peaks
to heights varying between 14,000 and 18,000 feet. There is a central
mountain, broken by ravines; but from one of the masses a peak ascends
to a height of about 6000 feet.
_Petavius._ A large walled plain, surrounded by a double wall or
rampart, which rises to 11,000 feet on its E. side. There are hills
and ridges in the interior, and a central peak, A, reaching to 5500
feet above the E. part of the floor, which is convex in form. A smaller
peak, of nearly 4000 feet, lies W. of A. Several small craterlets have
been seen in the interior.
_Newton._ The deepest walled plain known upon the Moon’s surface. In
form it is elliptical; its length is 143 miles, while its breadth
is only 69 miles. The walls show the terracing so common in these
objects, and one lofty peak reaches the unusual height of 24,000 feet
above the floor. The interior includes some small craters, mountain
protuberances, and other irregularities. Neison says that, owing to
“the immense height of the wall, a great part of the floor is entirely
lost in shadow, neither Earth nor Sun being ever visible from it.”
[Illustration: Fig. 26.
Petavius and Wrottesley at sunset. 1885, Dec. 23, 9^h to 10^h 30^m.
(T. Gwyn Elger.)]
_Grimaldi._ An immense walled plain, extending over 148 miles from N.
to S. and about 130 miles from E. to W. Its interior is very dark.
_Clavius_ is another grand example of this class of object, and is
rather larger than Grimaldi, but unfavourably placed near the S. pole.
_Schickard_ may also be mentioned as a large formation of similar type,
and situated near the S.E. limb of the Moon.
[Illustration: Fig. 27.
Birt, Birt A, and the Straight Wall. 1883, Feb. 15, 6^h to 8^h 40^m.
(T. Gwyn Elger.)]
_Rill or Cleft of Hyginus._ A conspicuous example of the lunar rills,
and one which yields to very moderate instruments. Neison notes that
it is readily visible in a 2-inch telescope; while Webb remarks that
a power of only 40, in a good instrument, is enough to show it under
any illumination. The rill is about 150 miles long. It cuts through
a number of crater-pits, and Mädler found so many widenings in it
that it appeared like a confluent train of craters. The rill traverses
the large crater-pit Hyginus, which is 3-3/4 miles in diameter and
moderately deep. Other fine examples of rill-systems will be found
between Rheita and Metius and near Triesnecker and Ramsden.
_Straight Wall._ A singular structure on the E. side of the ring-plain
Thebit. It is a ridge or wall, which looks regular enough for a work
of art, according to Webb. Its average height is 450 feet (Schröter),
1004 feet (Mädler), or 880 feet (Schmidt). These several determinations
are given to show the discordances sometimes found in the measures of
good observers. This object is about 60 miles long; at one extremity
lies a small crater, at the other there is a branching mountain nearly
2000 feet high. Elger has drawn this object, under both a rising and a
setting sun, in the Liverpool Astronomical Society’s ‘Journal,’ vol. v.
p. 156, and remarks that it may be well observed at from 20 to 30 hours
after the Moon’s first quarter.
_Valley near Rheita._ South of the ring-plain Rheita, on the S.W. limb,
there is an enormous valley, which extends in its entire length over
187 miles, with a width ranging from 10 to 25 miles. There are several
fine valleys in this particular region.
_Leibnitz Mountains._ These are really situated on the further
hemisphere of the Moon, but libration brings them into view, and they
are sometimes grandly seen in profile on the S. margin. Four of the
peaks ascend to elevations of 26,000 or 27,000 feet, and one mass,
towering far above the others, is fully 30,000 feet in height, and is
unquestionably the most lofty mountain on the Moon.
_Dörfel Mountains._ Visible on the Moon’s S.S.E. limb. They exhibit
three peaks, which, on the authority of Schröter, rise to more than
26,000 feet above the average level of the limb. The loftiest mountains
on the Earth are in the Himalayas—a range of immense extent to the N.
of India. The three highest peaks are Mount Everest (29,002 feet),
Kunchinjinga (28,156 feet), and Dhawalagiri (28,000 feet). The only
lunar mountain more elevated than these is that of the Leibnitz range,
which, as we have already stated, ascends to fully 30,000 feet.
[Illustration: Fig. 28.
Aristarchus and Herodotus at sunrise. 1884, Jan. 9, 8^h 30^m to
10^h 30^m.
(T. Gwyn Elger.)]
_Apennines._ A vast chain of mountains, extending over more than 450
miles of the lunar surface. _Huygens_ is the most elevated peak, rising
to more than 18,000 feet, and on its summit it shows a small crater.
There are several other very lofty peaks in this range. The Sun rises
upon the westerly region of these mountains at the time of first
quarter, and the peaks and ridges, with their contrasting shadows,
create a gorgeous effect just within, and projecting into the darkness
beyond, the terminator. There is an immense amount of detail to be
studied here, and much of it is within the reach of small instruments.
As the lunar mountains and craters are best seen near the terminator,
it may be useful to give a table of objects thus favourably placed
between the times of new and full Moon. The summary may assist the
student, though it does not aim at exactness, only even days being
given.
_Objects near the Terminator._
Moon’s age
in days.
2 Mare Crisium, Messala, Sunrise on the Mare Humboldtianum,
Langrenus, Vendelinus, Condorcet,
Hansen, Gauss(β), Hahn, Berosus.
3(α) Craters in Mare Crisium, Taruntius, Picard, Fraunhofer,
Vega, Pontécoulant, Cleomedes(γ), Furnerius,
Petavius, Endymion, Messier(δ), Vlacq.
4 Mare Nectaris, Macrobius(ε), Proclus, Sunrise on
Fracastorius, Rheita and Metius with the intervening
valley, Guttemberg, Colombo, Santbech,
Mountainous region W. of Mare Serenitatis,
Hercules, Atlas.
5 Palus Somnii, Plana, Capella, Isidorus, Polybius,
Piccolomini, Vitruvius, Littrow, Fabricius, Posidonius,
LeMonnier, Theophilus, Cyrillus, Catharina,
Hommel.
6 Tacitus, Maurolycus, Barocius, Dionysius(ζ), Sosigenes,
Abulfeda, Descartes, Almamon, Gemma Frisius,
Plinius, Ross, Arago, Delambre, Aristoteles,
Eudoxus, Julius Cæsar, Linné, Menelaus.
α The objects for observation when the Moon’s age is from 2 to 4 days
may be suitably re-examined a few days after the full.
β An extensive walled plain, 110 miles in length.
γ A large walled plain containing a small crater, Cleomedes A.
δ A curious double crater, with comet-like rays crossing the Mare
Fœcunditatis.
ε A circular ring-plain, 42 miles in diameter.]
ζ The interior of this crater exhibits some interesting features as
the Sun rises higher above it.
7 Ptolemæus, Albategnius, Manilius[17], Hyginus and
its rill-system, Hipparchus, Autolycus, Aristillus,
Cassini, Alpine Valley, W. C. Bond, Walter,
Miller, LaCaille, Apennines, Triesnecker and the
rills W. of it.
8 Mare Frigoris, Arzachel, Alphonsus, Alpetragius,
Bode, Pallas, Archimedes, Plato, Maginus[18],
Mösting[19], Thebit, Saussure, Moretus, Straight
Wall, Lalande, Kirch.
9 Tycho, Clavius, Eratosthenes[20], Stadius and the craters
running to N.E., Timocharis, Pitatus, Gruemberger,
Teneriffe Mountains, Straight Range[21],
Formation W. of Fontenelle[22], Gambart.
10 Sinus Iridum, Copernicus, Hesiodus and the rill
to E., Wilhelm I., Longomontanus[23], Heinsius,
Pytheas, Lambert, Helicon, Wurzelbauer.
11 Bullialdus, Campanus, Mercator, Reinhold, Riphæan
Mountains, Hippalus, Capuanus, Blancanus, Tobias
Mayer.
12 Mare Imbrium, Gassendi[24], Aristarchus and sinuous
valley to the N.E., Herodotus, Marius, Flamsteed,
Letronne, Schiller, Mersenius, Doppelmayer.
13 Schickhard, Wargentin, Grimaldi, Byrgius, Phocylides,
Hevelius, Seleucus, Crüger, Briggs,
Segner, Sirsalis.
14 Mare Smythii, Bailly, Inghirami, Bouvard, Riccioli,
Olbers, Hercynian Mountains, Cardanus, Krafft,
Cordilleras[25], Pythagoras[26].
_Occultations of Stars._—Among the various phenomena to which the lunar
motions give rise none are more pleasing to the possessors of small
telescopes than occultations of stars. Several of these occurrences
are visible every month. If the amateur has the means of obtaining
accurate time, he will engage himself usefully in noting the moments
of disappearance and reappearance of the stars occulted. This work
is efficiently done, it is true, at some of our observatories, and
therefore little _real_ necessity exists for amateurs to embark in
routine work which can be conveniently undertaken at establishments
where they have better appliances and trained observers to use them.
The mere watching of an occultation, apart from the registry of exact
results, is interesting; and there are features connected with it which
have proved exceedingly difficult to account for. The stars do not
always disappear instantaneously. On coming up to the edge of the Moon
they have not been suddenly blotted out, but have appeared to hang on
the Moon’s limb for several seconds. This must arise from an optical
illusion, from the action of a lunar atmosphere, or the stars must be
observed through fissures on the Moon’s edge. The former explanation
is probably correct; for it has happened that two observers at the
same place have received different impressions of the phenomenon. One
has seen the star apparently projected on the Moon’s limb for about 5
seconds, while the other has witnessed its sudden extinction, in the
usual manner, as it met the Moon’s edge. New observations, made with
good instruments and reliable eyes, and fully described, will doubtless
throw more light on the peculiar effects sometimes recorded.
_Visibility of the new and old Moon._—It is an interesting feature
of observation to note how soon after conjunction the Moon’s thin
crescent is observable with the naked eye. A case has been mentioned
in which the old Moon was seen one morning before sunrise and the new
Moon just after sunset on the next day. At Bristol, on the evening of
March 30, 1881, I saw the new Moon at 7^h 10^m, the horizon being very
clear in the west. She was then only 20^h 38^m old. On June 4, 1875, I
observed the Moon’s crescent at 9^h 10^m, or 22^h 49^m after new Moon.
Dr. Degroupet, of Belgium, saw the old Moon on the morning of Nov. 22,
1889, between 6^h 47^m and 7^h 22^m G.M.T., or within 18^h 22^m of the
time of new Moon.
FOOTNOTES:
[16] In September 1889 Prof. Thury, of Geneva, reported a change in the
centre of the crater Plinius. With a 6-inch refractor he saw, instead
of the usual two hills in the interior, a circular chalk-like disk
“with a dark spot in its centre like the orifice of a mud-volcano.”
[17] A fine ring-plain, 25½ miles in diameter.
[18] Mädler says “the full Moon knows no Maginus,” meaning that this
object is invisible under a vertical Sun.
[19] Mösting, Lalande, and Herschel form a fine triangle when the Sun
has attained a great altitude. Mösting is a ray-centre.
[20] A ring-plain 37½ miles in diameter, with very irregular terraced
walls.
[21] A range of mountains, with intervening valleys.
[22] Mädler describes this as a square enclosure with rampart-like
boundaries, which “throw the observer into the highest astonishment.”
[23] A great walled plain, 91 miles in diameter.
[24] A walled plain, 55 miles in diameter, in which Schröter suspected
changes.
[25] An extensive mountain-range on the E. by S. limb.
[26] A walled plain, 95 miles in diameter, and probably the deepest in
the N.E. quadrant, for the S.E. side of its wall rises to nearly 17,000
feet.
After the full the same objects should be re-examined under the
reversed illumination.
CHAPTER VII.
_MERCURY._
Supposed planet, “Vulcan.”—Visibility of Mercury.—Period
&c.—Elongations.—Amateur’s first view.—Phases.—Atmosphere of
Mercury.—Telescopic observations.—Schiaparelli’s results.—Observations
of Schröter and Sir W. Herschel.—Transits of Mercury.—Occultations of
Mercury.
“Come, let us view the glowing west,
Not far from the fallen Sun;
For Mercury is sparkling there,
And his race will soon be run.
With aspect pale, and wav’ring beam,
He is quick to steal away,
And veils his face in curling mists,—
Let us watch him while we may.”
_Supposed planet “Vulcan.”_—Mercury is the nearest known planet to
the Sun. It is true that a body, provisionally named Vulcan[27],
has been presumed to exist in the space interior to the orbit of
Mercury; but absolute proof is lacking, and every year the idea is
losing strength in the absence of any confirmation of a reliable
kind. Certain planetary spots, observed in motion on the solar disk,
were reported to have been transits of this intra-Mercurial orb. Some
eminent astronomers were thus drawn to take an affirmative view of
the question, and went so far as to compute the orbital elements and
predict a few ensuing transits of the suspected planet. But nothing
was seen at the important times, and some of the earlier observations
have been shown to possess no significance whatever, while grave doubts
are attached to many of the others. Not one of the regular and best
observers of the Sun has recently detected any such body during its
transits (which would be likely to occur pretty frequently), and there
is other evidence of a negative character; so that the ghost of Vulcan
may be said to have been laid, and we may regard it as proven that no
major planet revolves in the interval of 36,000,000 miles separating
Mercury from the Sun.
_Visibility of Mercury._—Copernicus, amid the fogs of the Vistula,
looked for Mercury in vain, and complained in his last hours that he
had never seen it. Tycho Brahe, in the Island of Hueen, appears to
have been far more successful. The planet is extremely fugitive in his
appearances, but is not nearly so difficult to find as many suppose.
Whenever the horizon is very clear, and the planet well placed, a
small sparkling object, looking more like a scintillating star than a
planetary body, will be detected at a low altitude and may be followed
to the horizon.
_Period &c._—Mercury revolves round the Sun in 87^d 23^h 15^m 44^s in
an eccentric orbit, so that his distance from that luminary varies
from 43,350,000 to 28,570,000 miles. When in superior conjunction the
apparent diameter of the planet is 4″·5; at inferior, conjunction it
is 12″·9, and at elongation 7″. His real diameter is 3000 miles.
_Elongations._—Being situated so near to the Sun, it is obvious that
to an observer on the Earth he must always remain in the same general
region of the firmament as that body. His orbital motion enables him to
successively assume positions to the E. and W. of the Sun, and these
are known as his elongations, which vary in distance from 18° to 28°.
He becomes visible at these periods either in the morning or evening
twilight, and under the best circumstances may remain above the horizon
two hours in the absence of the Sun. The best times to observe the
planet are at his E. elongations during the first half of the year, or
at his W. elongations in the last half; for his position at such times
being N. of the Sun’s place, he remains a long while in view. It is
unfortunate that when the elongation approaches its extreme limits of
28° the planet is situated S. of the Sun, and therefore not nearly so
favourably visible as at an elongation of only 18° or 20°, when his
position is N. of the Sun.
I have seen Mercury on about sixty-five occasions with the naked eye.
In May 1876 I noticed the planet on thirteen different evenings, and
between April 22 and May 11, 1890, I succeeded on ten evenings. I
believe that anyone who made it a practice to obtain naked-eye views
of this object would succeed from about twelve to fifteen times in a
year. In a finer climate, of course, Mercury may be distinguished more
frequently. Occasionally he presents quite a conspicuous aspect on the
horizon, as in February 1868, when I thought his lustre vied with that
of Jupiter, and in November 1882, when he shone brighter than Sirius.
The planet is generally most conspicuous _a few mornings after his W.
elongations_ and _a few evenings before his E. elongations_.
_Amateur’s First View._—The first view of Mercury forms quite an event
in the experience of many amateurs. The evasive planet is sought
for with the same keen enthusiasm as though an important discovery
were involved. For a few evenings efforts are vain, until at length
a clearer sky and a closer watch enables the glittering little
stranger to be caught amid the vapours of the horizon. The observer
is delighted, and, proud of his success, he forthwith calls out the
members of his family that they, too, may have a glimpse of the
fugitive orb never seen by the eye of Copernicus.
_Phases._—In the course of his orbital round Mercury exhibits all
the phases of the Moon. Near his elongations the disk is about half
illuminated, and similar in form to that of our satellite when in the
first or third quarter. But the phase is not to be distinctly made out
unless circumstances are propitious. Galilei’s telescope failed to
reveal it, and Hevelius, many years afterwards, found it difficult.
This is explained by the small diameter of the planet and the rarity
with which his disk appears sharply defined. The phase is sometimes
noted to be less than theory indicates; for the planet has been seen
crescented when he should have presented the form of a semicircle.
Several observers have also remarked that his surface displays a rosy
tint, and that the terminator is more deeply shaded and indefinite than
that of Venus.
_Atmosphere._—The atmosphere of Mercury is probably far less dense
than that of Venus. The latter being farthest from the Sun might be
expected to shine relatively more faintly than the former, but the
reverse is the case. Mercury has a dingy aspect in comparison with the
bright white lustre of Venus. On May 12, 1890, when the two planets
were visible as evening stars, and separated from each other by a
distance of only 2°, I examined them in a 10-inch reflector, power
145. The disk of Venus looked like newly-polished silver, while that
of Mercury appeared of a dull leaden hue. A similar observation was
made by Mr. Nasmyth on September 28, 1878. The explanation appears to
be that the atmosphere of Mercury is of great rarity, and incapable of
reflection in the same high degree as the dense atmosphere of Venus.
_Telescopic Observations._—As this planet is comparatively seldom to
be observed under satisfactory conditions, it is scarcely surprising
that our knowledge of his appearance is very meagre, or that amateurs
consider the planet an object practically inaccessible as regards
the observation of physical peculiarities, and one upon which it is
utterly useless to apply the telescope in the hope of effecting new
discoveries. Former attempts have proved the extreme difficulty of
obtaining good images of this planet. The smallness of the disk, and
the fact that it is usually so much affected by the waves of vapour
passing along the horizon as to be constantly flaring and moulding in
a manner which scarcely enables the phase to be made out, are great
drawbacks, which render it impossible to distinguish any delicate
features that may be presented on the surface.
These circumstances are well calculated to lead observers to abandon
this object as one too unpromising for further study; but I think the
view is partly induced by a misconception. The planet’s diminutive
size is a hindrance which cannot be overcome; but the bad definition,
resulting from low altitude, may be obviated by those who will select
more suitable times for their observations and not be dismayed if
their initiatory efforts prove futile. As a naked-eye object, Mercury
must necessarily be looked for when near the horizon; but there is
no such need in regard to telescopic observation, which ought to be
only attempted when the planet surmounts the dense lower vapours and
is placed at a sufficient elevation to give the instrument a fair
chance of producing a steady image. The presence of sunshine need not
seriously impair the definition or make the disk too faint for detail.
I have occasionally seen Mercury, about two or three hours after
his rising, with outlines of extreme sharpness and quite comparable
with the excellent views obtained of Venus at the time of sunrise or
sunset. Those who possess equatoreals should pick up the planet in
the afternoon and follow him until after sunset, when the horizontal
vapours will interfere. Others who work with ordinary alt-azimuth
stands will find it best to examine the planet at his western
elongations during the last half of the year, when he may be found soon
after rising by the naked eye or with an opera-glass, and retained
in the telescope for several hours after sunrise if necessary. He
may sometimes also be brought into the field before sunset (at the
eastern elongations in the spring months), by careful sweeping with
a comet-eyepiece, especially when either the Moon, Venus, or Jupiter
happens to be near, and the observer has found the relative place of
the planet from an ephemeris.
_Schiaparelli’s Results._—Mercury was displayed under several
advantages in the morning twilight of November 1882, and I made a
series of observations with a 10-inch reflector, power 212. Several
dark markings were perceived, and a conspicuous white spot. The general
appearance of the disk was similar to that of Mars, and I forwarded a
summary of my results to Prof. Schiaparelli, of Milan, who favoured me
with the following interesting reply:—
“I have myself been occupied with this planet during the past year
(1882). You are right in saying that Mercury is much easier to
observe than Venus, and that his aspect resembles Mars more than any
other of the planets of the solar system. It has some spots which
become partially obscured and sometimes completely so; it has also
some brilliant white spots in a variable position. As I observe the
planet entirely by day and near the meridian I have been able to see
its spots many times, but not always with the necessary distinctness
to make drawings sufficiently reliable to serve as a base for a
rigorous investigation. It is remarkable that the views taken near
superior conjunction have been more instructive for me than those
taken when the disk is near dichotomy, the defect in diameter being
compensated by the possibility of seeing nearly all the disk, which,
under those conditions, is more strongly illuminated. I believe that
by instrumental means, such as our 8½-inch refractor at Milan gives,
it is possible to prove the rotation-period of Mercury and to gain a
knowledge of the principal spots as regards the generality of their
forms. But these spots are really very complicated, for, besides the
difficulties attending their observation, they are extremely variable.”
Prof. Schiaparelli used an 8½-inch refractor in this work, and was
able, under some favourable conditions, to apply a power of 400. The
outcome of his researches, encouraged since 1882 by the addition of
an 18-inch refractor to the appliances of his Observatory, has been
recently announced in the curious fact that the rotation of Mercury is
performed in the same time that the planet revolves round the Sun! If
this conclusion is just, Mercury constantly presents one and the same
hemisphere to the Sun, and the behaviour of the Moon relatively to the
Earth has found an analogy. But these deductions of the eminent Italian
astronomer require corroboration, and this is not likely to be soon
forthcoming owing to the obstacles which stand in the way.
_Observations of Schröter and Sir W. Herschel._—Schröter observed
Mercury with characteristic diligence between 1780 and 1815. In 1800
he several times remarked that the southern horn of the crescent
was blunted, and fixed the planet’s rotation-period at 24^h 4^m. He
also inferred the existence of a mountain 12 miles in height. But
elements of doubt are attached to some of Schröter’s observations;
and Sir W. Herschel, whose telescopic surveys of both Mercury and
Venus were singularly barren of interesting results, pointed out their
improbability. But the great observer of Slough was not very amicably
disposed towards his rival in Germany. His strictures appear, however,
to have been not without justice if we consider them in the light of
modern observations.
_Surface-markings._—Spots or markings of any kind have rarely been
distinguished on Mercury. On June 11, 1867, Prince recorded a bright
spot, with faint lines diverging from it N.E. and S. The spot was a
little S. of the centre. Birmingham, on March 13, 1870, glimpsed a
large white spot near the planet’s E. limb, and Vögel, at Bothkamp,
observed spots on April 14 and 22, 1871. These instances are quoted by
Webb, and they, in combination with the markings seen by Schiaparelli
at Milan and by the author at Bristol in 1882, sufficiently attest that
this object deserves more attentive study.
[Illustration: Fig. 29.
1882 Nov. 5, 18^h 49^m. 1882, Nov. 6, 18^h 55^m.
Mercury as a morning star. (10-inch Reflector; power 212.)]
Amateurs with moderately large instruments would be usefully employed
in following this planet at the most opportune periods and making
careful drawings under the highest powers that can be successfully
applied. Mercury has been persistently neglected by many in past years,
and no doubt this “swift-winged messenger of the Gods” has eluded some
of his would-be pursuers; but there is every prospect that a patient
observer, careful to utilize all available opportunities, would soon
gather some profitable data relating to his appearance.
_Transits of Mercury._—One of the most interesting phenomena, albeit a
somewhat rare event, in connection with Mercury, is that of a transit
across the Sun. The planet then appears as a black circular spot.
Observers have noticed one or two very small luminous points on the
black disk, and an annulus has been visible round it. These features
are probably optical effects, and it will be worth while to remember
them on the occasion of future transits, of which the subjoined is a
list:—
1891, May 9.
1894, Nov. 10.
1907, Nov. 12.
1914, Nov. 6.
1924, May 7.
1927, Nov. 8.
1937, May 10.
1940, Nov. 12.
1953, Nov. 13.
1960, Nov. 6.
1970, May 9.
1973, Nov. 9.
The first observer of a transit of Mercury appears to have been
Gassendi, at Paris, on Nov. 7, 1631.
_Occultations of Mercury._—There was an occultation of Mercury by the
Moon on April 25, 1838. It occurred on the day of the planet’s greatest
elongation E., and at a time in the evening when it might have been
most suitably witnessed, but cloudy skies appear to have frustrated
the hopes of intending observers. There was a repetition of the event
on the morning of May 2, 1867, and it occurred, curiously enough, less
than 24 hours after an occultation of Venus.
FOOTNOTES:
[27] Chambers, in his ‘Descriptive Astronomy,’ 4th edition, 1889,
devotes a chapter to the discussion of facts having reference to
Vulcan; and the reader desiring full information will find it here.
CHAPTER VIII.
_VENUS._
Beauty of Venus.—Brilliancy.—Period &c.—Venus as a telescopic
object.—Surface-markings on the planet.—Rotation-period.—Faintness
of the markings.—Twilight on Venus.—Alleged Satellite.—Further
observations required.—Transits of Venus.—Occultations of Venus.
“Friend to mankind, she glitters from afar,—
Now the bright evening, now the morning star.”
_Beauty of Venus._—This planet has an expressive name, and it
naturally leads us to expect that the object to which it is applied
is a beautiful one. The observer will not be disappointed in this
anticipation: he will find Venus the most attractive planet of our
system. No such difficulties are encountered in finding Venus as in
detecting Mercury; for the former recedes to a distance of 47° from
the Sun, and sometimes remains visible 4½ hours after sunset, as in
February 1889. But Venus owes her beauty not so much to favourable
position as to surpassing lustre. None of the other planets can compare
with her in respect to brilliancy. The giant planet Jupiter is pale
beside her, and offers no parallel. Ruddy Mars looks faint in her
presence, and does not assume to rivalry.
This planet alternately adorns the morning and evening sky, as she
reaches her W. and E. elongations from the Sun. The ancients styled her
_Lucifer_ (“the harbinger of day”) when a morning star and _Hesperus_
when an evening star.
_Brilliancy._—Her brightness is such as to lead her to occasionally
become a conspicuous object to the naked eye in daytime, and at night
she casts a perceptible shadow. This is specially the case near the
epoch of her maximum brilliancy, which is attained when the planet
is in a crescent form, with an apparent diameter of about 40″, and
situated some 5 weeks from inferior conjunction. Though only a
fourth part of the disk is then illuminated, it emits more lustre
than a greater phase, because the latter occurs at a wider distance
from the Earth and when the diameter is much less. Her appearance is
sometimes so striking that it is not to be wondered at that people,
not well informed as to celestial events, have attributed it to causes
of unusual nature. When the planet was visible as a morning star in
the autumn of 1887, an idea became prevalent in the popular mind that
the “Star of Bethlehem” had returned, and there were many persons who
submitted to the inconvenience of rising before daylight to gaze upon a
spectacle of such phenomenal import. And they were not disappointed in
the expectancy of beholding a star of extreme beauty, though altogether
wrong in surrounding it with a halo of mystery and wonder.
At intervals of eight years the elongations of Venus are repeated on
nearly the same dates as before, and the planet is presented under very
similar conditions. This is because five synodical periods (nearly = 13
sidereal periods) of Venus are equal to eight terrestrial years. Thus
very favourable E. elongations occurred on May 9, 1860, May 7, 1868,
May 5, 1876, and May 2, 1884; and on April 30, 1892, there will be a
similar elongation.
_Period &c._—Venus moves round the Sun in an orbit of slight
eccentricity, and completes a revolution in 224^d 16^h 49^m 8^s. Her
mean distance from that luminary is 67,000,000 miles. The apparent
diameter of the planet varies from 9″·5 at superior to 65″ at
inferior conjunction, and it averages 25″ at elongations. Her real
diameter is 7500 miles. The polar compression is very slight—in fact,
not sufficiently decided for measurement; this is also true of Mercury.
_Venus as a Telescopic Object._—When the telescope is directed to Venus
it must be admitted that the result hardly justifies the anticipation.
Observers are led to believe, from the beauty of her aspect as viewed
with the unaided eye, that instrumental power will greatly enhance
the picture and reveal more striking appearances than are displayed
on less conspicuous planets. But the hope is illusive. The lustre
of Venus is so strong at night that her disk is rarely defined with
satisfactory clearness; there is generally a large amount of glare
surrounding it, and our instruments undergo a severe ordeal when
their capacities are tested upon this planet. Observations should be
undertaken in the daytime, or near the times of sunrise or sunset,
when the refulgence of this object does not exert itself in extreme
degree. But putting aside the question of definition for the moment,
there are other circumstances which conspire to render the view a
somewhat unattractive one. There are no dark spots, of bold outline,
such as we may plainly discern on Mars, visible on her surface. There
is no wonderful arrangement of luminous rings, such as encircle Saturn.
There are no signs of dark variegated belts, similar to those which
gird both Jupiter and Saturn; nor is there any system of attendant
satellites, such as accompany each of the superior planets. But though
Venus is wanting in these respects, she may yet boast an attraction
which the outer planets can never display to us, namely, the beautiful
crescented phase, which, tradition says, was predicted by Copernicus,
and, when afterwards observed in Galilei’s telescope, justly considered
a convincing fact in support of the Copernican system. The phases are
best seen in strong twilight, whenever Venus is favourably situated.
It has been asserted that the crescent of this planet has been
distinguished with the naked eye; but the statement is undoubtedly
erroneous. Any small glass will show it, however, as it is sometimes
well visible when subtending an angle of 50″ or 55″.
_Surface-markings._—In 1666 and the following year J. D. Cassini
observed several bright spots on Venus and also two obscure markings;
but the latter were extremely faint and of irregular extent, so that
little could be gleaned from them. He watched these forms closely and
remarked certain changes in their positions, which finally enabled him
to determine the period of the planet’s rotation. In 1726 and 1727
Bianchini, at Rome, repeatedly observed dark spots, and their outlines
seem to have been so consistent that he depicted them on a chart and
gave them names. But J. Cassini, at Paris, failed to confirm these
results, though he used telescopes of 82-and 114-feet focus; and it
was supposed the climate of Paris was not suitable for such delicate
observations. Schröter reviewed this planet in 1788 and later years,
and succeeded in detecting various markings and irregularities in the
terminator and cusps. He announced that he had seen the S. horn of the
crescent truncated, so that a bright point was apparently isolated at
its extremity. From this he concluded there must be mountains of great
altitude on the planet, and the perpendicular height of one of these
he computed at 22 miles, which is four times the height of the most
lofty mountain on the Earth. If the surface of Venus were uniformly
level, then her cusps would taper gradually away to points, and no such
deformation as that described by Schröter could possibly be produced.
And there is strong negative evidence among modern observations as
to the existence of abnormal features; so that the presence of very
elevated mountains must be regarded as extremely doubtful, if, indeed,
the theory has not to be entirely abandoned. The detached point at the
S. horn shown in Schröter’s telescope was probably a false appearance
due to atmospheric disturbances or instrumental defects. Whenever the
seeing is indifferent, this planet assumes some treacherous features
which are very apt to deceive the observer, especially if his telescope
is faulty. Spurious details are seen, which quite disappear from the
sharp images obtained in steadier air with a good glass. I have never
observed truncation in either of the horns of Venus; but on certain
occasions, when the planet has been ill-defined in passing vapours,
it was most easy to believe that a fragment became detached from the
extremity of the cusp, just in the manner described by Schröter.
But close attention has showed the effect to be false, and revealed
its cause. It was the rippling of the image that gave rise to the
apparently dissevered cusp, in the same way that passing air-waves
and resulting quivers in the image of Saturn’s ring will sometimes
produce displacements, so that the observer momentarily sees several
black divisions, and the edges are multiplied and superimposed one on
another. Refraction, exercised by heated vapours in crossing objects,
is obviously the source of all this.
Sir W. Herschel frequently examined this planet between 1777 and 1793,
but could not discern spots sufficiently definite and durable to enable
him to fix the time of rotation. He dissented from Schröter as to the
alleged mountains, and said, “No eye which is not considerably better
than mine, or assisted by much better telescopes, will ever get a sight
of them.”
Mädler effected some observations of this planet in 1833 and some
subsequent years. He detected spots on two occasions only, but noticed
irregularities in the terminator and cusps. Di Vico and others at Rome,
in 1840-1, devoted much attention to this object, and secured a large
number of observations. They appear to have recovered the spots charted
by Bianchini, and described them as of the last degree of faintness.
The observers who saw the spots most readily were those who had the
most difficulty in detecting the faint companion of a close double
star. In the spring of 1841 Di Vico saw a marking on the northern cusp
involved in an oval luminosity, and he likened it to a crater on the
Moon viewed obliquely. This spot had a diameter of at least 4½″, and
it was seen to advance even into the obscure part of the disk.
_Rotation-Period._—The following are the periods of rotation as given
by the different authorities whose observations we have mentioned:—
1666-7. Cassini 23 hrs. 21 min.
1726-8. Bianchini 24 days 8 hrs.[28]
1811. Schröter 23 hrs. 21 min. 8 sec.[29]
1840-1. Di Vico 23 hrs. 21 min. 22 sec.
Schiaparelli has recently discussed a large number of observations of
this planet, and concludes that, like Mercury, she rotates on her axis
in the same time that she completes a sidereal revolution round the
Sun, viz. in 224·7 days! I merely mention this remarkable deduction,
without quoting any facts in opposition to it.
From observations by Perrotin at Nice in 1890, including 74
observations, the rotation of this planet is very slow, and is made in
such a way that the relative positions of the spots and terminator do
not experience any notable change during many days.
[Illustration: Fig. 30.
1881, Mar. 22, 6^h. 1881, Mar. 26, 7^h. 1881, Mar. 28, 6½^h.
Venus as an evening star. (10-inch Reflector; power 212.)]
_Faintness of the Markings._—Several observers have noticed a slight
blunting of the S. horn of Venus, and in recent years dusky spots
have not unfrequently been seen, notably by Buffham, Langdon, and
others. The only markings distinguishable with my 10-inch reflector
are faint grey areas, without definite boundaries. These are sometimes
so delicate that it is difficult to assign exact form and position
to them, and occasionally I have regarded their very existence as of
doubtful character. They appear to be mere inequalities in brightness
of the surface, and may be due to different reflective power in parts
of the dense atmosphere of this planet. Certainly the spots are nothing
like those seen on the disks of Mars and Jupiter, many of which are
extremely distinct and show sharply terminated outlines. Dawes, an
observer endowed with very keen sight, could never succeed in finding
any markings on Venus, and many others have failed. But the evidence
affirming their reality is too weighty and too numerously attested to
allow them to be set aside. Occasionally the disk appears speckled with
minute shadings, and some observers have noticed crateriform objects
near the terminator; but these are uncertain. Brilliant spots have also
been recorded quite recently at the cusps.
Perhaps it may be advisable here to add a word of caution to observers
not to be hastily drawn to believe the spots are visible in very small
glasses. Accounts are sometimes published of very dark and definite
markings seen with only 2 or 3 inches aperture. Such assertions are
usually unreliable. Could the authors of such statements survey the
planet through a good 10-or 12-inch telescope, they would see at once
they had been deceived. Some years ago I made a number of observations
of Venus with 2-, 3-, and 4-1/4-inch refractors and 4-and 10-inch
reflectors, and could readily detect with the small instruments what
certainly appeared to be spots of a pronounced nature, but on appealing
to the 10-inch reflector, in which the view became immensely improved,
the spots quite disappeared, and there remained scarcely more than
a suspicion of the faint condensations which usually constitute the
only visible markings on the surface. I believe, also, the serrated
terminator is not a real feature of the object, but rather an effect
either of the rippling contour of the image or of an imperfect or
inadequate telescope.
An atmosphere of considerable density probably surrounds this planet,
for at the limb the brightness of the disk is much intensified. A
medium like this, that reflects and refracts light in extreme degree,
is brighter under oblique vision, as at the limb of Venus.
_Twilight on Venus._—When Venus is a slender crescent, near inferior
conjunction, a feeble luminosity pervades the dark part of the disk
similar to the “ashy light” or earthshine observed on the crescented
Moon. On such occasions the unilluminated surface appears to be
involved in a phosphorescence. Several observers have, however,
described the unilluminated limb of Venus as darker than the background
of sky. Zenger, at Prague, has noticed a brownish-red ring surrounding
the planet, and he attributes the appearance to much the same cause as
that which occasions the coppery colour of the Moon in a total eclipse.
_Alleged Satellite._—Cassini, Short, Montaigne, and others, in the 17th
and 18th centuries, observed small crescents near Venus and inferred
the existence of a satellite; but no such object has presented itself
in more recent times. It is extremely probable that the observers
were mistaken. In some cases the duplicate image may have been formed
by reflection in the eyepiece; in others a small star or planet
situated near Venus gave rise to the deception. M. Stroobant has
fully investigated this astronomical myth, and disposed of many of
the observations, without having recourse to the apocryphal satellite
named “Neith” by M. Niesten, who has discussed the question from an
affirmative point of view.
_Further Observations required._—From the foregoing summary amateurs
will notice that several difficult and more or less evanescent features
on this brilliant member of our system stand in need of confirmation.
Certain disputed forms require also to be looked for. The faint dusky
patches, the bright spots at the horns, and the inequalities in the
curve of the terminator will sure to be re-observed in future years;
and it is necessary that such details should be precisely noted in
regard to their positions and outlines as often as possible. A series
of reliable observations of this character might enable a fresh value
of the rotation-period to be deduced from them; and this is desirable,
for though Cassini, Schröter, and Di Vico give periods which are in
close harmony, there are elements of uncertainty attached to their
results. A new determination of the period would be valuable, and
especially so if based on really trustworthy data obtained by one of
the best modern telescopes. With the planet situated near inferior
conjunction, the crescent (reduced at such a time to a mere thread
of light) should be brought into the field, and the observer should
look for the extension of a faint glow over the interior parts of
the surface, and make comparisons between the relative brightness
of the planet’s dark limb and of the sky on which it is projected.
The telescopic images of Venus are often excellent in daylight, and
those who possess means of readily finding the planet at such times
will be very likely to gain some useful materials. As to the presumed
satellite, that may be relegated to the care of observers who have the
leisure and inclination to pursue an _ignis fatuus_; but should any
doubtful object appear in the field with Venus at any time, it ought to
be fully recorded and identified, if possible.
_Transits of Venus._—Those who were prevented by circumstances of
weather or otherwise from witnessing either of the transits of Venus
which occurred in 1874 and 1882 lost a spectacle of great rarity,
and one which they can never have another chance to behold. The next
transit occurs in the year 2004, and its phenomena will doubtless
be watched with avidity by the astronomers of a future generation.
The transit of 1882 was seen by many observers in England, though in
some parts of the country the Sun was obscured by clouds. The planet
was distinctly visible to the naked eye as a black circular spot in
gradual motion across the solar disk. The most important result of the
telescopic observations was of course the re-determination of the Sun’s
distance; but amongst the physical features noted, one of the most
interesting was the appearance of a silver arc of light on that portion
of the planet’s edge which was outside the Sun. This is assumed to have
been caused by the refraction of an atmosphere on Venus. The phenomenon
was seen by several observers, including Prof. Langley in America and
Messrs. Prince and Brodie in England.
_Occultations of Venus._—An occupation of this planet by the Moon
appears to have been recorded by the Chinese on March 19, 361 A.D.
Tycho Brahe witnessed a similar phenomenon on May 23, 1587. Mœstlin
observed Venus occult Regulus on Sept. 16, 1574; and on Oct. 2, 1590,
this planet appears to have passed over Mars. Visible occultations of
Venus are somewhat rare; they usually occur in daylight. A phenomenon
of this kind was witnessed on Dec. 8, 1877, over all the W. part of the
United States; and Prof. Pritchett, of Missouri, says:—“The interest
taken in it was shared alike by the educated and the illiterate, and
even by children.” The evening was cloudless, and many persons noted
the time of disappearance of Venus as seen by the unassisted eye. With
a 12½-inch refractor, power 275, Prof. Pritchett noted that “when the
bright limb of Venus was within 8″ or 10″ of the Moon’s dark limb, a
border of wavering light, several seconds in width, seemed to precede
the planet. Its general effect was such as to place in doubt the moment
of external contact.” A full description of this event, and of the
partial occultation of Venus on Oct. 12, 1879, is given in No. 1 of the
‘Publications’ of the Morrison Observatory, Missouri, U.S.A.
Venus is said, by the Arabian astronomer Ibn-Jounis, to have occulted
Regulus on Sept. 9, 885 A.D.; and Hind has examined the observations,
by means of Le Verrier’s tables of the Sun and planets. He finds
that on Sept. 9 in the year mentioned, at 16h 43m mean time, Venus
approached the star within 1′·7; so that to the naked eye the latter
would appear to be occulted, being overpowered in the glare of the
planet.
FOOTNOTES:
[28] This period was probably derived erroneously by Bianchini. It
includes 25 periods of 23^h 22^m, which corresponds with the times of
rotation by Cassini and others given in the table.
[29] Schröter’s final result. In 1788 he had derived a period of
23^h 28^m from observations of faint dark spots, and in 1789-91
irregularities in the S. horn of Venus gave him a period of 23^h 20^m
59^s.
CHAPTER IX.
_MARS._
Appearance of the planet.—Period &c.—Phase.—Surface
Configuration.—Charts and Nomenclature of Mars.—Discovery
of two Satellites and of Canal-shaped markings.—Summary of
Observations.—Rotation of Mars.—Further Observations required.—Changes
on Mars.—The two Satellites.—Occultations of Mars.
_Appearance of the Planet._—Mars is the fourth planet in the order of
distance from the Sun. He revolves in an orbit outside that of the
Earth, and is the smallest of the superior planets. His brilliancy is
sometimes considerable when he occupies a position near to the Earth,
and he emits an intense red light[30], which renders his appearance
all the more striking. Ordinarily his lustre does not equal that of
Jupiter, though when favourably placed he becomes a worthy rival of
that orb. In 1719 he shone so brightly and with such a fiery aspect as
to cause a panic. The superstitious notions and belief in astrological
influences prevailing at that time no doubt gave rise to the popular
apprehension that the ruddy star was an omen of disaster, and thus
it was regarded with feelings of terror. Fortunately the light of
science has long since removed such ideas from amongst us, and
celestial objects, in all their various forms, are contemplated without
misgiving. They are rather welcomed as affording the means of advancing
our knowledge of God’s wonderful works as displayed in the heavens.
_Period &c._—Mars revolves round the Sun in 686^d 23^h 30^m 41^s, and
his mean distance from that luminary is 141,500,000 miles. The orbit
is one of considerable eccentricity, the distance varying between
154,700,000 and 128,360,000 miles. The apparent diameter of the planet
when in conjunction with the Sun is only 4″; but this may augment
to 30″·4 at an opposition, when the Earth and Mars occupy the least
distant parts of their orbits. The real diameter of Mars is nearly 5000
miles.
_Phase._—At opposition the disk of Mars is round, but when in
quadrature he appears distinctly gibbous and resembles the Moon three
days from full. The phase is so palpable that Galilei glimpsed it at
the end of 1610. In delineations of Mars the disk is generally drawn
circular, the compression being very slight and the phase too trivial
to be regarded.
_Surface Configuration._—This planet being singularly variable in
his position relatively to the Earth, presents at times a diameter
so small that the most powerful instruments are ineffective to deal
with him. But at certain epochs he becomes an excellent object, with
a much expanded disk, on which are displayed a number of bright and
dark markings. This happens, however, with comparative rarity; for only
during two months or so near every opposition, occurring at intervals
of 780 days, can the planet be well seen. Generally the apparent size
of Mars is very inconsiderable, and the disk not sharply defined,
especially when the altitude is low. Reliable observations are seldom
made at a time far removed from the date of opposition. When the planet
was badly placed, in July 1882, an observer secured some observations
of position,, and published them, thinking he had seen Wells’s Comet,
which happened to be in the same quarter of the sky!
Mars, in nearer degree than any other member of our system, shows a
configuration which may be likened to that of the Earth as regards its
permanency; and in some of its outlines a general resemblance also
exists, though in detail there is evidently much that is dissimilar.
It is fortunate that the atmosphere of Mars is so rarefied that
observers can look upon his real surface-lineaments with satisfactory
perspicuity. For more than 250 years now, the telescope has been
engaged in perfecting our knowledge of Martian features, and these have
exhibited no mobility of form or place (apart from that due to rotation
or varying inclination of the planet) so far as may be judged from a
comparison of drawings. Plenty of differences exist in the latter, it
is true, though similar objects are represented; but the explanation
obviously lies in the inaccuracies of amateur artists, and has little
if anything to do with physical changes on the planet.
When the spots were discovered in 1636 by Fontana they were, of
course, very dimly glimpsed in the incompetent appliances available
at that time. Huygens, in 1659, saw them better by means of his long
telescopes, but still very imperfectly. Cassini, in 1666, effected a
further advance in the same field, and gathered data from which he
was able to announce the period of rotation. His value has proved
remarkably correct, considering the means he employed to obtain it
and the very short interval over which his inquiries were conducted.
Huygens had previously, in 1659, witnessed the returns of a certain
spot to the same approximate place on the planet, and was led to infer
rotation in either 12^h or 24^h. But this was little better than a
guess, and not nearly of the same precision as that which marked
Cassini’s subsequent determination.
[Illustration: Fig. 31.
Mars, 1836, April 13, 9^h 50^m; long. 332°.
(10-inch reflector; power 252.)
]
Near the poles of Mars are intensely bright patches, which have been
considered to be vast areas of snow-crowned surface or fields of ice.
These “polar snows” are not situated exactly at the poles, nor are
they opposite to each other. Changes affect their aspect. Occasionally
these or other bright markings, when on the limb, appear to protrude
beyond the disk, and this curious effect of irradiation distorts the
limb in a striking manner.
_Charts and Nomenclature of Mars._—It is not desirable to trace
with any detail the successive labours of those who have chiefly
contributed to our knowledge of areographic features. Maraldi, W.
Herschel, Schröter, Mädler, Schmidt, and Dawes were foremost amongst
the observers of the past; while Schiaparelli and Green are the most
successful observers of to-day. As telescopes improved in effectiveness
the true forms and characteristics of the markings were discerned, and
at the present time some thousands of delineations of this planet must
be in existence. Charts of the leading and best-assured features have
been formed, and the regions of light and shade (supposed to represent
land and sea) have received proper names to distinguish them. Thus
there is “Fontana Land,” “Maraldi Sea,” “Herschel Continent,” and
others of similar import. Schiaparelli has framed a chart in which the
spots are furnished with Latin names taken from classical geography.
Mädler’s plan was to designate the markings by capital letters of the
alphabet, and to divide these by small letters in necessary cases.
But the charts of Proctor, Green, and others, in which the names of
past and present astronomers are applied, seem to find most favour,
though it is admitted that this method of nomenclature is not free
from objections. In some instances the names have not been wisely
selected. A few years ago, when christening celestial formations was
more in fashion than it is now, a man simply had to use a telescope
for an evening or two on Mars or the Moon, and spice the relation of
his seeings with something in the way of novelty, when his name would
be pretty certainly attached to an object and hung in the heavens for
all time! A writer in the ‘Astronomical Register’ for January 1879
humorously suggested that “the matter should be put into the hands of
an advertizing agent” and “made the means of raising a revenue for
astronomical purposes.” Some men would not object to pay handsomely
for the distinction of having their names applied to the seas and
continents of Mars or to the craters on the Moon. But it is all very
well to disparage a system: can a better one be found? Probably not;
but the lavish use of undeserving names is calculated to bring any
system into contempt.
[Illustration: Fig. 32.
Orbits of the Satellites of Mars.]
_Discovery of Satellites and of Canal-shaped markings._—The interest
in this planet has been accentuated in recent years by several
circumstances. The discovery of two satellites in 1877 by Prof. Hall,
with the 25·8-inch Washington refractor, caused the directors of
large instruments to test their capacity upon these minute objects.
Schiaparelli’s observations of the canal-shaped markings have afforded
another attractive feature in connection with this planet. He detected
a network of dark straight lines stretching generally from N. to S.
across the planet; and in the winter of 1881 found these objects
duplicated, _i.e._ the lines ran in pairs so close together that
they were separated with difficulty. The study of the topography of
Mars had never previously revealed structures like these; yet the
Italian astronomer appears to have observed them with “comparative
ease whenever the air was still.” Other observers have not wholly
confirmed the appearances alluded to, but no favourable opposition has
occurred since 1877, and no surprise need be felt that the delicate
features visible in the pellucid sky of Italy should elude detection
in less genial climes. In 1886 M. Perrotin, at Nice, using a 15-inch
equatoreal, saw a number of the “canals,” and some of them were double.
In 1888 the observers having charge of the 36-inch refractor at Mount
Hamilton re-observed the “canals” as broad bands, but none of them
appeared to be duplicated. The conditions were unfavourable, the planet
being more than three months past opposition.
Prof. Schiaparelli re-observed the duple “canals” in June 1890 with
a refractor by Merz of 18 inches aperture, powers 350 and 500. His
observations are supported by Mr. A. S. Williams, of Brighton, who
informs me that he detected forty-three of the “canals,” and seven of
them were “clearly and certainly seen to be double.” Mr. Williams’s
instrument is a 6½-inch reflector by Calver, and powers of 320 and 430
were successfully employed on it; magnifiers under 300 were found of
little use.
_Summary of Observations._—From observations at Bristol I have drawn up
the following summary as to the configuration of Mars:—
1. That the “Hour-glass” or “Kaiser Sea,” and some other markings of
analogous character, present very bold, dark, and clearly defined
outlines, enabling them to be visible in very small telescopes. In
1873 I saw certain spots with a refractor of only 1-3/4-inch aperture.
Mr. Grover, in 1867, “made a set of pencil-drawings, with a 2-inch
telescope, which gave the general markings of the planet very well.”
In ‘Recreative Science’ it is mentioned that on June 7, 1860, a
semi-circular dark spot on the N.W. part of the disk of Mars was
distinctly seen with a 1½-inch telescope, power 120.
2. There is an intricate mass of surface-markings on the planet, which,
in its main features, is capable of being satisfactorily delineated,
and which in its general aspect is similar to the canals depicted by
Schiaparelli, though not nearly so pronounced, straight, and uniform as
he has shown in his charts.
3. The detail is visible in the form of irregular streaks,
condensations, and veins of shading, very faint and delicate in some
parts. The veins apparently connect many of the larger spots, and
here and there show condensations, which have sometimes been drawn as
isolated spots. A night of good definition, however, reveals the feeble
ligaments of shade connecting them.
4. That there exists on the immediate borders of many of the darker
patches and veins a remarkable brightness or shimmering, which reminds
one of the bright spots merging out of the dark belts on Jupiter.
Just contiguous to the “Kaiser Sea,” and on its eastern limits, this
brightness was so striking in March 1886 as to compare with that
exhibited by the N. polar cap. In drawings by many observers these
regions of special luminosity have no place, but there is little doubt
they occupy a leading position in the physical configuration of Mars.
5. That there is no trace of a dense atmosphere on Mars, as some of
the text-books infer. The pronounced aspect of the chief markings,
their durableness and continuity of form, the ease with which they
may be traced up to the limb, the absence of phenomena indicating
dense cloud-bearing air-strata, and other observed facts verify the
conclusion that the planet’s surface is comparatively free of vapours,
and in a totally different condition to that of Jupiter and Saturn.
_Rotation of Mars._—The diurnal period of this planet is known with
far greater certainty and precision than that of any other planet, the
Earth excepted. It will be useful to quote the values derived since
Cassini’s time:—
h m s
1666. J. D. Cassini 24 40
1704. J. P. Maraldi 24 39
1781. W. Herschel 24 39 21·7[31]
1784. W. Herschel 24 37 27
1838. J.H. Mädler 24 37 23·8
1845. O. M. Mitchell 24 37 20·6
1859. A. Secchi 24 37 35
1864. F. Kaiser 24 37 22·62
1866. R. Wolf 24 37 22·9
1869. R. A. Proctor 24 37 22·735
1873. F. Kaiser 24 37 22·591
1873. J. F. J. Schmidt 24 37 22·57
1883. A. Marth 24 37 22·626
1884. W. F. Denning 24 37 22·34[32]
1885. H. G. v. de S. Bakhuyzen 24 37 22·66
The last of these, by Prof. Bakhuyzen of Leyden, is probably the best.
It was based on a large number of observations extending over 220
years, viz. from those of Huygens in 1659 to those of Schiaparelli in
1879.
In a terrestrial day Mars rotates through 350°·8922, according to Mr.
Marth’s period. In one hour the axial motion is 14°·6, whereas on
Jupiter the horary rate of rotation is 36°·7. At intervals of 40 days
(during which Mars completes 39 rotations) the various features on the
disk are presented at very nearly the same times as before. Mr. Marth’s
ephemerides of this planet are extremely useful to those who study
the markings; and these, in combination with the charts and memoirs
of Schiaparelli, Green, Terby, and others, greatly facilitate and
encourage the renewed study of this object.
_Further Observations required._—Favourable oppositions of Mars occur
every 15 years, as in 1877 and 1892. It is at such periods that
this planet should be sedulously interrogated for new features, or
for corroboration of those already known. Rather a high power must
be employed—certainly more than 200; and if the telescope has an
aperture of at least 8 inches, the observer will be sure to discern
a considerable extent of detail. He should compare his views with the
various charts previously alluded to, and note any inconsistencies.
Fresh drawings should also be made; and if the forms are not well
assured on one night, he may confirm them by coming 37 minutes later
to his instrument on the following night. Or the collective issue of
several nights’ work may be included in the same drawing. The bright
spots on the planet should be as attentively studied as the darker
regions, and given a place in every drawing; for it is probably in
connection with these luminous objects that active changes may be
recognized. The “canals” and their duplication form the principal
markings to be looked for; though the successful elucidation of these
appearances can only be expected in a case where a powerful telescope,
a keen eye, and a good atmosphere operate together. Something of them
may be seen under ordinary conditions, and they ought to be very
generally sought for by amateurs; for it is not always that success
is found where the best conditions prevail. The great telescopes at
Mount Hamilton, Nice, and other observatories may be expected to
command some advantages of light, power, and position; but this need
not prevent competition, or induce the idea that common appliances are
practically of no avail. Everyone should strive to achieve as much as
is consistent with his means and opportunities; indeed there is all
the more need for effort and energy in the observer when his tools are
seemingly inadequate to a research, and he should endeavour to find,
in his own eye and understanding, that power which shall compensate
in a great measure for lack of instrumental capacity. Mr. Proctor, in
his ‘Old and New Astronomy,’ has justly remarked:—“The directors of
Government observatories have usually been much less successful in
studying planetary details than those zealous amateurs who take delight
in the study of the heavenly orbs and are ready to wait and watch for
favourable opportunities.”
_Changes on Mars._—Changes have been confidently reported in some of
the Martian spots. Instances have been quoted in which particular
markings, though very plain at certain times, have scarcely been
perceptible at others. Variations in outline as well as in visibility
appear to have been witnessed, and the subject is one which merits
more extended notice. It has been asserted that the origin of such
variations probably lies in the aerial envelope of Mars. In April and
May 1888 M. Perrotin, with the great refractor at Nice, failed to
re-observe the feature known as the continent “Libya” on Schiaparelli’s
chart, and stated that though this formation was plainly visible in
1886, it had ceased to exist in 1888. He suggested that the obscuration
was really produced by clouds or mists circulating in the atmosphere of
Mars. But Prof. Holden reported, from the Lick Observatory, that the
object alluded to was distinctly visible with the 36-inch refractor
there at the end of July, and in the same form in which it was drawn by
Prof. Schiaparelli in 1877-8. It is to be assumed, therefore, that if
any change occurred it was one of transient nature.
There are other questions relating to the physical aspect of this
planet which future observers should be able to answer. Do the
markings retain their distinctness right up to the limb? Is the opaque
crescent of the disk (when Mars is in quadrature) involved in any
phosphorescence or glow indicating an atmosphere? Are the bright spots
and luminous borders to the continents equally as stable as the dark
spots, and do they maintain an equable brilliancy?
The N. hemisphere of Mars needs much further study, as it is not so
familiarly known as the S. hemisphere. This is due to the circumstance
that, at favourable oppositions, the region of the S. pole is suitably
presented for observation. It is only when the planet is comparatively
distant, and small in diameter, that his N. hemisphere comes into view.
The difference of inclination under which the features are seen at
successive oppositions gives rise to many apparent changes of figure.
When the S. hemisphere is exposed to the Earth numerous objects
are seen which are quite invisible when the opposite hemisphere is
displayed to us. These altering conditions have to be considered in
their influences by every student of areography.
_Satellites of Mars._—After evading the keen and searching eyes of
Sir W. Herschel, and the power of his 40-foot telescope—after eluding
the grasp of Lord Rosse’s 6-foot speculum, and the frequent scrutiny
of Lassell with his 2-and 4-foot mirrors, the two satellites of Mars
were ultimately revealed to Prof. Hall in the 25·8-inch refractor
at Washington. These tiny orbs had been enabled to avoid previous
discovery by their minuteness and by their close proximity to Mars.
Yet as soon as they were known to exist many observers saw them, and
in certain cases success was undoubtedly attained with comparatively
small instruments. The late Dr. Erck picked up the outermost satellite
with a 7-1/3-inch objective, and Mr. Pratt saw it with an 8-1/7-inch
mirror by With. But the effect of this eye-straining may just possibly,
in one or two instances, have drawn the imagination out of its normal
repose. Mr. Pratt’s instrument shows stars in the group ε Lyræ which
are invisible in the great Washington telescope and in the 36-inch
mirror formerly used by Mr. Common; so that it may well have produced a
spectral satellite of Mars. But the satellites are certainly within the
occasional reach of moderate means; for they were repeatedly seen with
a 9½-inch refractor at the Observatory of Princeton, U.S.A., in October
and November 1879. They “were decidedly more easy to see than Mimas,”
the innermost satellite of Saturn.
Phobos, the inner satellite, revolves round the planet in 7^h 39^m, in
an orbit 6000 miles from the centre of Mars. At max. elongation the
satellite is about 12″ distant from its primary, and its opposition
magnitude is 11½. Deimos, the outer satellite, revolves in 30^h 18^m,
and its orbit is 15,000 miles distant from Mars. Its elongations extend
to 32″, and its opposition mag. is 13½. These diminutive objects are
probably not more than 10 miles in diameter. They are obviously too
faint for common instruments, nor are they objects on which ordinary
amateurs may occupy themselves with advantage. Of course it forms a
highly interesting spectacle to glimpse, just for once, it may be, the
small bodies which resisted telescopic power for more than two and a
half centuries; but for really useful observations, large aperture and
means of accurate measurement are required.
_Occultations of Mars._—The most ancient account of a planetary
occultation is probably that given by Aristotle, who refers to a lunar
obscuration of Mars that occurred on April 4, 357 B.C., according to
the calculations of Kepler. Another occultation of Mars appears to
have been recorded by the Chinese on Feb. 14, 69 B.C. Tycho Brahe
observed a repetition of the event on Dec. 30, 1595. Mr. Baily
describes a phenomenon of this kind which occurred on Feb. 18, 1837,
when “the planet appeared of a fine yellow colour both at its ingress
and egress. No projection was observed.” Mr. Snow, of Ashhurst, saw
the occultation of March 12, 1854, and he states the planet “was of
almost precisely the same colour as the Moon, and he could not help
comparing it to a spangle on the face of the sky. Whilst it was slowly
and solemnly vanishing, it gave for several seconds the notion of its
being the summit of a lunar mountain, but melted gradually away.” As
Mars emersed, “nothing whatever was to be seen of the two bodies,
clinging together, as it were, by threads of light; nothing of the
pear-shaped appearance often recorded as put on by planets under
similar circumstances.” Mr. J. Tebbutt, of Windsor, N. S. W., watched
an occultation of Mars in full daylight on Aug. 12, 1875, when “the
rapid disappearance of the planet’s disk was an exceedingly interesting
phenomenon, its extinction taking place at a considerable distance from
the Moon’s illuminated disk. The line marked by the Moon’s dark limb
across the disk was well defined.” At the reappearance clouds were
prevalent, and “the planet was observed as a small projection on the
bright limb;” but he found it difficult to fix the exact time of last
contact, owing to the ill-defined character of the planet’s gibbous
limb. An occultation of Mars was also seen by Prof. Grant at Glasgow on
June 3, 1878.
FOOTNOTES:
[30] This was believed by Sir J. Herschel to be due to “an ochrey tinge
in the general soil, like what the Red-Sandstone districts on the Earth
may possibly offer to the inhabitants of Mars, only more decided.”
[31] Herschel’s earlier observations were made in 1777-79, and his
period, like that of his predecessors, is about 2 min. in excess of
the correct value; but Mädler pointed out that, by giving Mars an
additional rotation on his axis, Herschel’s value will agree within
2 sec. of his own. Herschel appears to have adopted 768 rotations
instead of 769, and may have been led to this by the excessive periods
of Cassini and Maraldi and by the want of intermediate data between
his own observations in April 1777 and May-June 1779. His second
determination, made in 1784, is more correct.
[32] Deduced from observations extending over 15 years only, at
Bristol.
CHAPTER X.
_THE PLANETOIDS._
Number.—History of their Discovery.—Dimensions and
Brightness.—Occultation of Vesta.
_Number._—These bodies, also called minor planets, and, formerly,
asteroids, comprise a very numerous class, and they are extremely
small, being quite invisible to the naked eye except in one or two
special cases. They all revolve in orbits situated between Mars and
Jupiter. The total number discovered is about 300, of which Prof. J.
Palisa of Vienna has found more than 70, and the late Dr. C. H. F.
Peters of Clinton, N.Y., 49. I have not given exact numbers in the two
former cases, because these discoveries are still rapidly progressing.
_History of their Discovery._—The first known planetoid (Ceres) was
sighted by Piazzi on Jan. 1, 1801. The following year, on March 28,
Olbers found another (Pallas). In 1804, on Sept. 1, Harding discovered
a third (Juno); and in 1807, March 29, Olbers was a second time
successful (Vesta). Then for thirty-eight years no additions were made
to the number. The host of planetoids circulating between Mars and
Jupiter preserved their incognito without disturbance from the prying
and wakeful eyes of astronomers.
But in 1845 Hencke, of Driessen, after years of watching, at length
broke the spell of tranquillity by finding another small planet; and
his example was emulated by many other observers in subsequent years.
Hind, De Gasparis, and Goldschmidt were amongst the earliest and
most successful of those who gathered new planets from amongst the
stars of the zodiacal constellations. In later years Luther, Watson,
and Borrelly further extended the list; but Palisa and Peters have
distanced all competitors, and shown a zeal in the work which has
yielded an astonishing aggregate of discoveries. Charlois, at Nice, has
latterly earned distinction in the same field.
Since 1845 new planetoids have been found at the rate of more than
six per annum, and a rich harvest yet remains to be gathered by
the planet-seekers of the future. A very large proportion of those
already detected are between the tenth and twelfth magnitudes, and are
therefore only to be discerned in good instruments. They present no
distinction from small star-like points, and are to be identified by
their motions alone. The mythological dictionary has furnished names
for them, and they are numbered in the order of their discovery as well.
_Dimensions and Brightness._—Vesta is the largest and brightest of
the group, while Ceres and Pallas rank as second and third in the
same respect. Vesta is about 214 miles in diameter; but the more
insignificant members of this family are probably not more than about
15 or 20 miles in diameter. Pallas has the most inclined orbit of all,
the inclination amounting to 30° 44′; so that its position is by no
means confined to the planet-zone of the ecliptic. Vesta is sometimes
brighter than a 6th mag. star; while Ceres, Pallas, and Juno vary
between about the 7th and 8th magnitudes, according to their distances
from the Earth. A real variation of light has been assumed to occur,
but this is not fully proved.
In March 1887 Mr. Backhouse, of Sunderland, saw an apparently new,
yellowish-white star near 103 Piscium, and it was just visible to the
naked eye. This proved to be Vesta, though the identity of the object
was not known at first, and it formed the subject of two Dun Echt
circulars.
Formerly, hazy indefinite outlines were attributed to some of the
planetoids; but the appearance probably arose from instrumental defects.
The search for these bodies is not a work likely to engage amateurs.
Professional observers are best able to grapple with the difficulties
attending this kind of observation, where large telescopes, means of
exact measurement, and ample data, such as star-charts and ephemerides
of the planetoids previously discovered, are requisite. The ‘Nautical
Almanack’ annually contains ephemerides of Ceres, Pallas, Juno, and
Vesta; and observers wishing to pick up any one of them may readily
ascertain positions by reference to this work.
_Occultation of Vesta._—An occultation of Vesta occurred on Dec. 30,
1871, and it was observed by Mr. C. G. Talmage at Leyton with a 10-inch
refractor, power 80. He says the planet was exceedingly bright right up
to the Moon’s limb.
CHAPTER XI.
_JUPITER._
An interesting Object.—Brightness and Position.—Period &c.—Belts and
Spots on the Planet.—Observations of Hooke, Cassini, and others.—The
“Ellipse” of 1869-70.—The Red Spot, its appearance, dimensions,
and rotation-period.—Bright equatoreal Spots.—Dark Spots in N.
hemisphere.—Rotation-period.—Nature of the Red Spot and of the bright
and dark equatoreal Spots.—New Belts.—Changes on Jupiter.—Further
Observations required.—Occultations of Jupiter.—The four Satellites,
and their phenomena.—Occultation of a star by Jupiter.
“Beyond the sphere of Mars, in distant skies,
Revolves the mighty magnitude of Jove,
With kingly state, the rival of the Sun.”
Of all the planets, Jupiter is the most interesting for study by the
amateur. It is true that Saturn forms an exquisite object, and that
his wonderful ring-system is well calculated to incite admiration as a
feature unique in the solar system. But when the two planets come to
be repeatedly observed, and the charm of first impressions has worn
away, the observer must admit that Jupiter, with his broad disk and
constantly changing markings, affords the materials for prolonged study
and sustained interest. With Saturn the case is different. His features
are apparently quiescent; usually there are no definite spots upon the
belts or rings. There is a _sameness_ in the telescopic views; and this
ultimately leads to a feeling of monotony, which causes the object to
be neglected in favour of another where active changes are in visible
progress.
_Brightness and Position._—Jupiter is a brilliant object in the
heavens, his lustre exceeding that of Mars or Saturn, though not equal
to that of Venus. I have occasionally seen the planet with the naked
eye in the daytime, about half an hour after sunrise; and it has been
frequently observed by Bond, in America, with the Sun at a considerable
altitude. Humboldt and Bonpland, at Cumana, 10° N. lat., saw Jupiter
distinctly with the naked eye, 18 minutes after the Sun had appeared in
the horizon, on Sept. 26, 1830. The planet is favourably visible for a
considerable time every year, and is only beyond reach near the times
of his conjunctions with the Sun, when he usually evades observation
for about three months. As regards his altitude, Jupiter becomes
exceptionally well placed at intervals of 12 years; thus in 1859,
1870-1, and 1882 his declination was 22° or 23° N., and his height
therefore very great when passing the meridian. In 1894 he will occupy
a similarly auspicious region to observers in the N. hemisphere. In
1865, 1877, and 1889 his declination was 23° S., and he was favourably
presented to southern astronomers.
The image of Jupiter as seen in a telescope is involved in a slight
yellow tinge, and with the naked eye the same colour is often apparent.
But when observed through a very pure transparent atmosphere, his light
nearly approaches the silvery lustre of Venus or the Moon. The planet
shines with unusual splendour, considering his great distance from
the Sun, and his atmosphere must be highly reflective and possibly
intensified by inherent light from the planet himself. The central
parts of Jupiter’s disk are usually the brightest, as there is a faint
shading-off and indefiniteness at the limbs. These and other facts
support the view that Jupiter is still incalescent and sufficiently
self-luminous to emit a small amount of light.
_Period &c._—This planet revolves round the Sun in 4332d 14h 2m, which
is equal to more than 11-3/4 years. His orbit is somewhat eccentric, so
that his distance from the Sun varies from 506,500,000 to 460,000,000
miles, and the mean is 483,300,000 miles. His apparent diameter ranges
from a max. of 50″ at a good opposition to 30″·4 in conjunction. The
planet’s diameter measured along the equator is 88,000 miles, and the
polar compression is very marked, amounting to 1/16, or, more exactly,
to 1/15·82, according to Engelmann, from a mean derived from eleven
observers. When Jupiter is in quadrature there is a slight phase
evident in the shading-off of the limb furthest from the Sun.
_Belts and Spots on the Planet._—From the time that the telescope
became available as a means of astronomical research, it may be readily
surmised that an object coming so well within the reach of ordinary
appliances, and one displaying so many prominent and variable features,
should absorb a large share of attention, and that many facts of
interest should have been gleaned as to his physical peculiarities. But
it must be confessed that, though something has been learned as to the
visible behaviour of the markings, there is much that is perplexing
in their curious vagaries. No doubt the vast changes affecting the
Jovian envelope, the diversity of the markings, and their proper
motions result from the operations of a peculiarly variable atmosphere,
affected probably by a heated and active globe beneath it, and by the
very rapid movement of rotation to which it is subject.
The telescope, on being turned towards Jupiter, reveals at once an
array of dark and light stripes or belts stretching across the disk in
a direction parallel to one another and to the equator of the planet.
These belts are supposed to have been first detected by Zucchi in 1630.
Usually there are two broad and prominent dark belts, one on either
side of the equator; while towards the poles other belts appear, some
of them very narrow, partly by the effects of foreshortening. The
equatoreal zone of the planet is of a lighter tint, and variegated
with white and dark spots and streaks, liable to rapid changes, and
indicating that this region is in a highly disturbed condition.
_Observations of Hooke, Cassini, and others._—Hooke and Cassini were
amongst the first to find definite spots on the surface of Jupiter.
From 1664 to 1667 a particularly large and distinct spot was frequently
seen in the planet’s S. hemisphere. This object disappeared in the
latter year, but returned in 1672, and was seen until the close of
1674, when it again temporarily vanished, to reappear at subsequent
epochs. Cassini was enabled to determine the rotation-period from
this spot. He found that the markings in the immediate vicinity
of the equator moved with greater celerity than those in higher
latitudes, the difference in their rotation-periods being nearly 6
minutes. A century later Sir W. Herschel confirmed these results: he
saw a bright spot which completed a rotation in nearly 5 minutes less
time than several dark spots. Schröter also made many observations,
and noted frequent changes in the spots and differences in their
rotation-periods. He watched a bright object near the equator which had
a period more than 5 minutes less than some dark spots. In later years
Mädler and others followed up the investigation of these markings,
and with nearly similar results. The various spots were undoubtedly
affected by proper motions, enabling them to yield discordant
rotation-periods. Bright forms near the equator moved with great
rapidity and effected a rotation in about 9^h 50^m, while dark spots on
either side of it occupied between 9^h 55^m and 9^h 56^m. The markings
were evidently controlled by currents of different velocities in the
planet’s atmosphere.
Dawes, in 1849 and following years, noted luminous spots, like
satellites in transit, on a belt in the planet’s S. hemisphere. In
October 1857 he observed a group of eleven of these objects; and in
1858 Lassell saw many similar appearances in a bright belt near the
equator.
_The Ellipse of 1869-70._—In 1869 and 1870 Gledhill, of Halifax, and
Prof. Mayer, of the Lehigh University, saw a remarkable formation just
south of the great belt lying on the S. side of the equator. It was
in the form of a perfect ellipse, ruddy in colour, and very distinct
in outline. Its major axis was parallel with the belts. It was first
observed on Nov. 14, 1869, and had disappeared in July 1870, though
on Dec. 1, 1871, a similar elliptic ring was seen resting on the S.
equatoreal dark band.
_The Red Spot._—In July 1878 a large spot, of oval form and intense
red colour, appeared in about the same latitude as the ellipse seen by
Gledhill and Mayer in 1869-70. It was first announced by Dennett of
Southampton, though it appears to have been seen a few weeks earlier
by Prof. Pritchett, of Missouri, U.S.A. The object alluded to soon
attracted general notice; and as it continued visible during the
oppositions of 1879, 1880, and 1881 under the same striking aspect, it
created a considerable stir among telescopists, and the “great red spot
on Jupiter” became familiarly known both in appearance and in title.
No planetary marking in modern times has enlisted half the amount of
attention that has been devoted to this object. It has endured amid
all the turmoils of the Jovian atmosphere for twelve years, and has
preserved an integrity of form and size which prove it to have been
singularly capable of withstanding disruption. But its tint has varied
greatly; so that at times the oval outline of the spot could hardly
be discerned amongst the contiguous belts. In the winter of 1881 the
interior of the ellipse began to lose tone, and in 1882 it faded
rapidly, so that the central region of the spot assumed nearly the same
light tint as the outlying bright belts. Apparently the spot had either
been filled up with luminous cloudy material or had been partially
obscured by the interposition of matter situated higher in the Jovian
atmosphere. The elliptical contour of the object was still intact,
however, though it had quite lost its bold and prominent character.
Only the skeleton of its former self remained, and its entire
disappearance seemed imminent. But further decadence was fortunately
averted by influences unknown to us, and the spot has continued visible
to this day, though shorn of the attributes which roused so much
enthusiasm amongst observers more than ten years ago.
From measures at Chicago, in the years from 1879 to 1884, Prof. Hough
found the mean dimensions of the spot to be:—Length 11″·75, breadth
3″·71. These figures represent a real length of 25,900 miles and a
diameter of 8200 miles. The latitude of the spot was 6″·97 S.
This object has served an important end in attracting wide-spread
observation, not only to itself, but to the general phenomena occurring
on the surface of Jupiter. Observers, in studying the red spot, were
also led to study the bright equatoreal spots and other features so
plentifully distributed over the disk. It was most important this
should be done; for since the time of Herschel and Schröter not much
progress had been made in elucidating the proper motions of the spots
and finding an accurate rotation-period for the planet. Dawes, Lassell,
and many others had, it is true, secured some interesting observations
and drawings, but not of the special kind required, and thus no fresh
light had been thrown upon the vagaries in the behaviour of the spots,
as described by the old observers. But a mass of new facts were now
to be realized. Schmidt at Athens, Prof. Hough at Chicago, A. Stanley
Williams at Brighton, and many others, including myself at Bristol,
began systematic observations of Jupiter, with a view to learn
something more of the periods, changes, and general characteristics
of the spots and other features. The results were of an interesting
nature, though too extensive for more than bare mention here. In 1879
the red spot gave a rotation-period of 9^h 55^m 34^s·2, but this
increased to 9^h 55^m 35^s·6 in 1880-1 and to 9^h 55^m 38^s·2 in
1881-2. During the ensuing three years the period was almost stationary
at 9^h 55^m 39^s·1, but in 1885-6 it further augmented to 9^h 55^m
41^s·1, since which year it has ranged between 9^h 55^m 40^s and 41^s.
From ten years’ observations, the mean period of the red spot is as
nearly as possible 9^h 55^m 39^s.
_Bright Equatoreal Spots._—The bright spots near the equator rotated
in 9^h 50^m 6^s in 1880; but in subsequent years the time slightly
increased, for in 1882 I found it 9^h 50^m 8^s·8, and in 1883 9^h 50^m
11^s·4. The bright spots therefore perform a rotation in 5½ minutes
less time than the red spot. The former move so much more swiftly than
the latter that they pass it at the rate of 260 miles per hour, and in
44½ days have effected a complete circuit of Jupiter relatively to it.
Thus a brilliant white spot, if noticed in the same longitude as the
red spot on one night, will, on subsequent nights, be observed to the
W. of it, and, after an interval of about 44½ days, the same objects
will again occupy coincident longitude.
_Dark Spots in N. hemisphere._—In the autumn of 1880 there was a
confluent outbreak of dark spots from a belt in about 25° N. latitude,
and these exhibited a rotation-period of only 9^h 48^m, so that they
travelled more rapidly than the white spots on the equator. Some short
dusky belts were also remarked slightly S. of the latitude of the red
spot, and these indicated a period of 9^h 55^m 18^s. It is clear from
these various results that the motion of the Jovian markings does not
decrease according to their distance from the equator.
_Rotation-Period._—Below are given the times of rotation ascertained by
some previous observers:—
h m s
1665. J. D. Cassini 9 55 58
1672. ” 9 55 50
1692. ” (A)9 50
1708. J. P. Maraldi 9 56 48
1713. ” 9 56
1773. Sylvabelle 9 56
1779. (B)W. Herschel 9 54 53}
to 9 55 40}
1779. ” (A)9 50 48 }
to 9 51 45·6}
1786. (C)J. H. Schröter 9 55 33·6
” ” 9 55 17·6
” ” (A)9 50 27
1835. J. H. Mädler 9 55 26·5
” G. B. Airy 9 55 21·3
1836. J. H. Mädler 9 55 23·5
1862. J. F. J. Schmidt 9 55 25·7
1866. ” 9 55 46·3
1873. O. Lohse 9 55 19·6
1880. J. F. J. Schmidt 9 55 34·4
” ” (A)9 50
1881. W. F. Denning 9 48
” ” 9 55 17·9
1883. ” (A)9 50 8·7
1885. G. W. Hough 9 55 37·4
” ” (A)9 50 9
1886. A. Marth 9 55 40·6
1887. A. S. Williams:—
Spots in 12° N. lat. 9 55 36·5
” 4° N. lat. (A)9 50 40·1
” 8° S. lat. (A)9 50 22·4
” 30° S. lat. 9 55 17·1
1890. (D)W. F. Denning 9 55 39
A: Bright spots near the equator of Jupiter.
B: Herschel’s observations embraced few rotations, and the periods
he derived differed considerably.
C: Schröter also alleges he saw spots return to the same part of
the disk in 7^h 7^m, 7^h 36^m, and 8^h 1^m!
D: From ten years’ observations of the red spot.
The foregoing list is by no means complete, for, owing to the large
number of recent determinations, I have thought it advisable to omit
some of them.
It should be mentioned here that the above times of rotation are
derived from atmospheric features more or less volatile in nature, and
that therefore the actual sphere of Jupiter rotates in a period which
we have not precisely discovered. No doubt the motion of the real
surface is not very different from that of the atmospheric markings
above it. There is reason to think that, whatever the character of the
planet’s crust may be, we have never yet obtained a glimpse of it. A
dense veil of impenetrable vapours appears to surround the globe on all
sides, and this is subject to violent derangement from the evolution
of heated material or gaseous fluids from the surface below. These
disturbances seem to be very durable in some instances as to their
observed effects. The atmosphere would, in fact, appear to possess a
singular capacity for retaining the impressions of its changes. The
permanency of certain spots can hardly be due to continued action from
those parts of the disk immediately underlying them; for their variable
motions soon transport them far from the places at which they were
first seen, and prove their existence to be quite independent of their
longitude.
_Nature of the Red Spot._—There is much in connection with the red
spot that remains in mystery. Its dimensions, form, and motion have
severally been ascertained within small limits of error, and the
alterations in its tint and degree of visibility have been recorded
with every care. But we can only conjecture as to the origin,
character, and end of this remarkable formation. What agency produced
it, and moulded the definite elliptical outline it has always
preserved—what forces control its oscillations of speed, and keep it
suspended so long in the aerial envelope of Jupiter—are matters of
pure theory. When, in July 1878, it first came under notice it was
a well-developed object, and though Russell in 1876, Lord Rosse and
others in 1873, and Gledhill and Mayer in 1869-70 had delineated forms
suspiciously like the red spot and situated in the same latitude,
yet the several features may not have been absolutely identical, for
nothing was seen of the spot in 1877 or in some other years. But there
is a strong probability that the red ellipse of 1869-70 must have been
the red spot in an incipient stage of its formation. The object may
have undergone temporary obscuration, similarly to Cassini’s spot two
centuries ago.
[Illustration: Fig. 33.
I. 1857, Nov. 27. (Dawes.) | II. 1859, Dec. 29. (Huggins.)
III. 1858, Mar. 2. (Huggins.) | IV. 1870, Jan. 23. (Gledhill.)
V. 1872, Feb. 2. (Gledhill.) | VI. 1885, Feb. 25. (Denning.)
]
My own opinion of the spot is that it represents an opening in the
atmosphere of Jupiter, through which, in 1878-82, we saw the dense red
vapours of his lower strata, if not his actual surface itself. Its
lighter tint in recent years is probably due to the filling-in of the
cavity by the encroachment of durable clouds in the vicinity. Parts of
some of the more prominent belts display an intense red hue like that
formerly shown by the red spot, and they may be due to the same causes.
Extensive fissures are probably formed in the atmosphere, and quickly
distended in longitude by the natural effect of the planet’s tremendous
velocity of rotation. It is curious, however, that these rents, after
a certain distention, assume a durable outline until they lose their
colouring and are temporarily if not finally obliterated.
When the red spot was visible under its best conditions I frequently
examined it, hoping to detect some mark well in its interior which
might serve as a clue to the true rotation-period of the sphere of
Jupiter. For if the spot consisted of a clear patch in the planet’s
atmosphere, I thought it possible some real object on the surface might
be discerned through it, in which case the difference in its motion
and that of the red spot would enable the rate of motion of the globe
to be found. If the spot moves more slowly than the planet, then a
surface-marking must appear to pass from the E. to the W. side of the
spot; but no such evidence could be obtained, owing to the absence of
suitable markings. The red tint of the great spot seemed very general
over the entire area of the ellipse until its central regions paled in
1882. There were two dark specks, one at the E. and another at the W.
extremity of the spot; but these were unchangeable as regards position.
[Illustration: Fig. 34.
Jupiter, 1886, April 9^d 10^h 12^m. (10-inch reflector; power 252.)
]
The spot, though placed very near the border of the great S. belt, has
never been connected with it, though in Jan.-Feb. 1884, May 1885, and
March-April 1886 the spot became temporarily attached to a belt on its
S. side. There was some controversy as to this feature, Prof. Hough,
from observations with the 18½-inch refractor at Chicago, alleging that
at no time had the spot coalesced with or been joined to any belt in
its vicinity. But in 1886 many observers succeeded in detecting the
junction of the markings alluded to, and Prof. Young gave a drawing
of the appearance as seen with power 790 on the 23-inch objective at
Princeton (see ‘Sidereal Messenger,’ vol. v. p. 292). The spot and belt
were probably at different heights in the Jovian atmosphere, so that
there was no commingling of material, one object being simply projected
on the other, for the elliptical form of the red spot remained visible
all the time. The latter moves more slowly than the connecting belts,
and, when clear of them, is often seen with a white aureola fringing
its environs.
_Bright Equatoreal Spots._—These are affected by rapid changes of form,
brightness, and motion. Sometimes they are exceedingly bright; at other
periods they are quite invisible. This intermittency is not occasioned
(as I assured myself by many observations) by the total extinction
of spots and appearance of new ones, but is due to the temporary
obscuration of the same objects. The variations are irregular, and
probably depend upon phenomena also irregular. The motion of these
objects often shows great deviations from their average rate; they
are sometimes much in advance of or behind their computed positions.
One fine spot of this class was closely watched in 1880 and following
years. It was usually in the shape of a brilliant oval, well defined,
and occasionally quite as large as the third satellite of Jupiter; but
it was sometimes seen as a diffused white patch, apparently emerging
from the N. edge of the belt. Whenever the spot was very bright, there
was a trail of light or luminous matter running eastwards from it, as
though there were an eruption of shining material from the spot, which
the rapid rotation of the planet from W. to E. caused to drift in an
opposite direction.
_Dark Equatoreal Spots._—Closely contiguous to the white spots there
are almost invariably seen very dark spots, much deeper in tone than
the dark belt upon which they appear to be projected. It has been
suggested that these dark spots are shadows from the white spots,
which may be elevated formations protruding through the envelope of
Jupiter. This idea seems to me untenable; for the dark spots have been
distinguished under a vertical Sun, and sometimes they are found one
on each side of a white spot. Again, an intensely brilliant spot is
occasionally seen without any outlying condensation of dark matter.
But though they are not shadows, the dark equatoreal spots certainly
have an intimate relation with the brighter markings near them and move
with the same velocity.
It is proved from many observations that the longer an object is
observed the slower becomes its rate of rotation. Sir W. Herschel found
the converse. In discussing his results of 1778 and 1779, he said:—“By
a comparison of the different periods it appears that a spot gradually
performs its revolutions in less time than at first” (Phil. Trans.
1781, p. 126). But his periods were each based on less than fifty
rotations, so that no certain conclusions could be derived from them.
In recent years the rapidly moving bright spots have usually appeared
in the equatoreal side of the great S. dark belt. The polar side of the
great N. belt also exhibits bright spots, but these rotate in a period
only a few seconds less than that of the red spot. Bright spots are
also observed to the S. of the latter object and on other portions of
the disk.
As to the belts, they are usually straight; but cases are recorded
of slant-belts, in which the direction has been very oblique. One of
these was noticed in the planet’s N. hemisphere in Mar.-April 1860, and
another was seen in the S. hemisphere in Jan. 1872. I observed one near
the N. polar shading in Dec. 1881.
_New Belts._—The formation of the dark belts seems to be brought
about gradually, and they appear to be sustained in certain cases by
eruptions of dark matter, which gradually spread out into streams. On
Oct. 17, 1880, two dark spots, separated by 20° of longitude, broke
out on a belt some 25° N. of the equator. Other spots quickly formed
on each side of the pair alluded to, and distended themselves along
the belt so that by Dec. 30 they covered three fourths of its entire
circumference. At the middle of January the spots formed a complete
girdle round the planet; but they became much fainter, and were soon
eradicated by combination with the belt on which they had appeared.
_Changes on Jupiter._—Prof. Hough, of Chicago, is adverse to the
opinion that rapid changes occur on Jupiter, and mentions the
stability of the red spot and other markings in support of his views.
He believes that the erroneous statements about sudden changes made by
both ancient and modern astronomers are largely due to differences in
the telescopic images due to atmospheric variations. No doubt such an
explanation will suffice to meet some instances, and the swift rotation
of the planet may also have been the unsuspected cause of some of the
extraordinary changes described; but there are real variations as well.
These are very frequent in the planet’s equatoreal zone.
_Further Observations required._—Drawings of Jupiter obtained under
the highest powers that may be employed with advantage, and with a
cautious regard to faithful delineation, will probably throw much
light on the phenomena occurring in this planet’s atmosphere. And it
is most desirable to pursue the various markings year after year with
unflagging perseverance; for it is only by such means that we can hope
to unravel the extraordinary problem which their visible behaviour
offers for solution. Too much stress cannot possibly be laid on the
necessity of observers being as precise as possible in their records.
The times when an object comes to the central meridian should be
invariably noted; for this affords a clue to its longitude, and a means
of determining its velocity. Its position, N. or S. of the equator,
should be either measured or estimated; and alterations in tone,
figure, or tint described, with a view to ascertain its real character.
The climate of England is very ill-adapted to an investigation of
this sort, where the most needful point consists in frequency of
observation. If the markings on Jupiter could be re-examined every
night, and traced through their changes, an explanation of certain
phenomena exhibited by them would soon be forthcoming. The interrupted
character of previous observations destroys much of their value.
Closely consecutive results are necessary to remove doubts as to the
identity of the objects observed; so that, in such a research, natural
advantages of position are more desirable than instrumental advantages,
for the latter are impotent in a cloudy atmosphere.
The red spot must be watched as long as any vestiges of it remain.
Its variations of speed may ultimately yield indications of
periodicity[33]; so may its alterations of tint. The belts in the
vicinity of the spot demand an equal share of attention; for it may be
possible to divine from their changes whether there are any links of
association between them and the red spot. In recent years the latter
has apparently repulsed the belts on its N. side, though suffering
encroachments from those on its S. side.
The equatoreal spots also deserve continued vigilance on the part
of observers. It has already been stated that the bright spots vary
rapidly; their motions are not uniform in rate, and what is now wanted
is a large number of new observations. Does accelerated velocity occur
with increased brilliancy of these objects? Are their alternating
disappearances and revivals uniform in period? and are they really due
to transitory obscurations of the same durable forms? Are the dark
spots which frequently border the white spots implicated in effacing
the latter? Many other questions like these are suggested by the
curious behaviour of the markings, and the discriminating observer will
know how to gather the materials likely to aid in answering them. The
rotation-period has been already found in regard to many features; but
this element may be re-investigated with profit, for the velocity of
the spots offers a very complex problem for solution. Do the markings
generally exhibit a retardation of speed as long as they subsist?
Abnormal spots, such as those which made their apparition in the autumn
of 1880, should be traced through any vagaries they may present; and
peculiar shape or direction in the belts will also merit study, as
possibly supplying facts of consequence. It will be important to learn
whether objects in a certain latitude have a common rotation-period, or
whether different forms give different times. The rate of motion shown
by certain features may depend upon their character, and not so much
upon their position in latitude.
The altitudes of the various markings affords another promising line of
research. The appearances and changes of closely contiguous features
may be expected to furnish useful data in this connection. Owing to
their proper motions they apparently overlap each other at times, and
in their alterations of aspect the observer may discover the clue to
their relative heights. The subject is discussed in a practical and
interesting way by Mr. Green (Memoirs R. A. S. vol. xlix. p. 264) and
by Mr. Stanley Williams (‘Zenographic Fragments,’ i. p. 112), and these
works should be consulted by everyone engaged in the study of Jovian
phenomena.
It is unfortunate that the observer, in delineating this object, must
perforce adopt an extremely hurried method of representing what he sees
at the telescope. The planet turns so quickly upon his axis that forms
near the central meridian become sensibly displaced in a few minutes;
indeed, it has been stated that an interval of two minutes only is
sufficient to introduce a change obvious to simple eye-estimation.
In order, therefore, to complete a sketch, the utmost dispatch is
requisite; for this object cannot be depicted from the combined outcome
of several evenings of observation. The proper motions of the different
features prevent this. With Mars, or any orb exhibiting markings
relatively constant, collective results are extremely valuable, and
more trustworthy than pictures depending upon an isolated observation.
Amateurs, in entering upon these observations, should be prepared for
rapid changes in the apparent aspect of Jupiter caused by his rotation,
and not hastily infer them to be real. They should also hesitate before
placing confidence in any anomalous results obtained under indifferent
seeing; for bad images have been directly responsible for many
misleading announcements.
[Illustration: Fig. 35.
Occultation of Jupiter, Aug. 7, 1889.]
_Occultations of Jupiter by the Moon._—Phenomena of this kind are
always awaited with keen interest by the possessors of telescopes;
but it is rarely that all the circumstances are favourable. The first
recorded instance appears to have been in A.D. 847. In 1792, on April
7, Schröter observed an occultation of this planet, with a desire to
verify his suspicion of a lunar atmosphere. He saw that “some of the
satellites became indistinct at the limb of the Moon, while others
did not suffer any change of colour. The belts and spots of Jupiter
appeared perfectly distinct when close to the limb of the Moon.” On
Jan. 2, 1857, an occultation took place under conditions which rendered
it visible to many observers in this country, and the most interesting
fact elicited was that at emersion a dark border was seen attached to
the arc of the Moon projected on the planet. Mr. Lassell described
this dark border as “a shadowy line, in character, magnitude, and
intensity extremely like Saturn’s obscure ring projected on the ball.”
During the thirty years following 1859 only two occultations visible
in England occurred, and the last of these, on August 7, 1889, was
widely observed. On this occasion Capt. Noble and others redetected the
shadowed edge of the Moon seen by Lassell in 1857. “It was a strongly
marked shading, following the outline of the Moon’s limb.” At Bristol I
recorded that, at the disappearance, the outer margin of our satellite
was fringed with light where it crossed the planet; but at the
reappearance this effect had vanished, and the appearance was perfectly
normal. The disk of Jupiter, where it met the edge of the lunar disk,
looked dusky by the effects of contrast; but I saw no marked shading
with a sharply terminating boundary, such as appears to have been
remarked elsewhere. As the planet emerged definition was superb, the
belts were lividly distinct, and the spectacle was one of the prettiest
that could be imagined. The red spot was going off the W. limb, and the
disk was covered with belts; many of them near the poles were extremely
narrow, like fine lines drawn with a sharp lead pencil. I used a 4-inch
refractor, powers 65 and 145, with this instrument the foregoing sketch
was made. The exceptional distinctness of the Jovian markings on this
occasion shows that the proximity of the Moon has certainly no tendency
to efface planetary details, but rather to intensify them[34].
On Sept. 3, 1889, an occultation of Jupiter was visible in America, and
observed by Mr. Brooks at Geneva, N.Y., with a 10-1/8-inch equatoreal.
His drawing, made from a photograph and eye-observations, shows nothing
of a dark fringe bordering the Moon’s limb.
[Illustration: Fig. 36.
Jupiter and satellites seen in a small glass.]
_The four Satellites._—When Galilei directed his telescope to Jupiter
on the evening of Jan. 7, 1610, he saw three small star-like points
near the planet; so:—
[Illustration]
On Jan. 13 he discovered a fourth; thus:—
[Illustration]
and ascertaining that these bodies followed Jupiter in his course,
concluded them to be moons in attendance upon him. At first the
discovery was discredited by others; but it soon had to be accepted
as an incontestable fact of observation. These satellites are usually
among the very first objects which the amateur views in his telescope,
and they form, in combination with their primary, an exquisite picture,
the impression of which is not soon forgotten. The periods, distances,
&c. of the satellites are as follows:—
+—————————————+—————————————————————+————————+————————-+————————-+
| | Mean Distance. | | | |
|No. and Name.+———————————+————————-+Sidereal|Mean |Real |
| |Diameters | Miles. |Period. |Apparent |Diameter,|
| |of Jupiter.| | |Diameter.|in miles.|
+—————————————+—————————————————————+————————+————————-+————————-+
| | | | h m s| ″ | |
| I. Io | 3·03 | 267,000| 1 18 29| 1·02 | 2390 |
| II. Europa | 4·72 | 425,000| 3 13 18| 0·91 | 2120 |
|III. Ganymede| 7·71 | 678,000| 7 4 0| 1·49 | 3480 |
| IV. Callisto| 13·55 |1,193,000|16 18 5| 1·27 | 2970 |
+—————————————+—————————————————————+————————+————————-+————————-+
The third satellite is much the largest, and its brightness is about
equal to that of a star of the 6th mag. The other three may be rated
as generally 7th mag., though their brightness is variable, especially
that of the fourth satellite, which has been seen exceedingly faint.
It is customary to distinguish these objects, not by their names, as
in the case of the moons of Mars, Saturn, and Uranus, but by the Roman
numbers affixed to them progressively according to their distances from
Jupiter.
The satellites are just visible to the naked eye when the conditions
favour their detection; but they are so much involved in the rays
of the planet, and often so near to him, that it may be regarded as
an exceptional feat to discern them without telescopic aid. When
III. and IV. are near their max. elongation and on the same side of
the planet, they have been occasionally observed separately. I. and
II., though much closer to Jupiter and more within the influence of
his glare and rays, have been similarly seen. When attempting such
observations it is best to hide the bare disk of the planet behind
some terrestrial object, as this will cut off the obnoxious rays and
prevent the brilliant light from dazzling the eye. An opera-glass, or
any small portable telescope, reveals the whole retinue of satellites,
and enables them to be traced through their revolutions. The ‘Nautical
Almanack’ gives diagrams of their diurnal positions, and with this work
as a reference observers will find no difficulty in identifying them
apart.
Sir W. Herschel, in the years 1794 to 1796, found that the satellites
revolve on their axes in the same time that they revolve about the
planet. He was led to this conclusion by a study of the variations in
the light emitted by the satellites in different parts of their orbits,
and described I. as “of a very intense bright, white, and shining
light,—brighter than II. or IV. (not larger). IV. inclines to red, and
nearly as bright as II. The latter is of a dull ash-colour. III. is
very white.” Modern observers have selected II. as relatively the most
highly reflective, while IV. is the least. Spots exist on the surfaces
of these objects, and probably occasion many of the differences
observed.
The eclipses, occultations, and transits of the satellites afford a
very fertile and attractive series of phenomena for telescopic review.
The exact times of occurrence are tabulated in the ‘Nautical Almanack’
and asterisks are affixed to such as are visible in this country. Prior
to the date of opposition of Jupiter the eclipses occur of course on
the W. side of the disk, while after opposition they take place on the
E. side. The durations are as follow for the several satellites:—I. =
2^h 20^m, II. = 2^h 56^m, III. = 3^h 43^m, IV. = 4^h 56^m. In reference
to III. and IV. the entire phenomenon may be generally observed; but
this is not so in regard to II., as the emersions are frequently
effected behind the planet. Only the immersions of I. are visible
before opposition, from the same cause; for the satellite enters the
cone of shadow close to the planet’s limb, and only comes out of it
when the globe of the planet is interposed in the line of sight. In
such cases the satellite emerges soon after from the limb of Jupiter;
so that its obscuration has been compounded of two separate phenomena,
viz. an eclipse and an occultation. After opposition this satellite
is first occulted and then eclipsed. IV. sometimes escapes eclipse
altogether, by passing above or below the shadow.
The motion of light was discovered, and its velocity determined, by
means of the eclipses of Jupiter’s satellites. These phenomena are also
useful in ascertaining longitudes. A spectator on Jupiter himself would
see a vast number of solar and lunar eclipses—about 4500 of each—during
the Jovian year of 4332·6 days, because the three inner satellites
exhibit these phenomena at every revolution, their orbits being very
slightly inclined to Jupiter’s equator, and the latter being but little
inclined to the plane of the ecliptic.
The occultations of the satellites are comparatively frequent, and
may be well observed in a good telescope. A tolerably high magnifier
is required to witness these occurrences with the best effect, the
disks of the satellites being small and not clearly traceable through
the various stages of their disappearances unless much amplified.
With considerable telescopic power the disks are well seen, and it
then becomes feasible to watch the satellites, first as they come
into contact with the limb, then as the globe of the planet overlaps
more and more of their diminutive forms, and finally as they reach
last contact and withdraw their narrow unobscured segments behind the
expansive sphere of their primary. Both the beginning and end phase
of these occultations is generally observable in regard to Sat. IV.,
and frequently also in the case of III. But with reference to II. and
I. it often happens that only the disappearance or reappearance can
be witnessed. These occultations have furnished some singular and
unexplained facts of observation. On meeting the limb of Jupiter Sats.
I. and II. have not always disappeared in a normal way. On April 26,
1863, Wray, with an 8-inch objective, saw II. distinctly projected
within the limb for nearly 20^s. Other similar cases are recorded. The
satellites have been seen apparently “through the edge of the disk.”
One observer mentions that II. appeared and disappeared several times
before occultation. The explanation appears to be that there is so much
irradiation round the disk of Jupiter that it produces a false limb,
and it is through this the satellites have been seen. A very tremulous
image, in bad air, may also be responsible for some of the anomalies
recorded.
The transits of the Jovian moons offer the most attractive phenomena
of all, and they come well within the reach of small telescopes.
On entering upon the planet they are visible as bright round spots
projected on the dusky limb, and subsequently present some eccentric
features. II. is invisible, except on the limbs; I. is often seen
as a grey spot threading along the belts; III. appears as a large
dark spot[35], nearly as black as its shadow; IV. seems to be black,
and scarcely to be recognized from its shadow. The appearances are
certainly to some extent variable. Mr. Stanley Williams has seen
III. as a _brilliant_ disk at mid-transit. I. sometimes crosses the
whole disk as a white spot; at certain other times it is invisible;
at others, again, it is seen as a faint grey spot. IV. is not always
black, its aspect depending upon the chord it traverses. Thus, on the
evening of Sept. 12, 1889, Mr. Williams, Mr. G. T. Davis of Reading,
and myself were observing Jupiter when IV. was in transit on a belt in
the N. hemisphere, but not a vestige of the satellite was seen by any
of us. On the morning of May 23, 1890, at 3^h 30^m A.M., however, while
observing the red spot on Jupiter, I noticed a black circular spot on
the great N. equatoreal belt; and this proved to be IV. in transit.
These peculiarities have been accounted for as partly due to contrast
and partly to dusky spots on the surfaces of the satellites. Dr.
Spitta has made a number of experiments to elucidate this subject, and
concludes that “the perpetual whiteness of the second satellite, and
the darkened tints of the others during transit, are due to differences
in their relative albedo [reflective power] as compared with that of
Jupiter, and are not dependent upon the relative quantity of light
reflected by one or the other, or upon any physical peculiarities of
the Jovian system.”
The shadows of the satellites transit the disk as dark spots larger
than the satellites themselves, owing to the penumbral fringes. Before
opposition these shadows precede the satellites; after opposition the
latter come first. The shadow of II. appears to be much lighter than
the others, and is usually of a pale chocolate-colour; and I saw it
thus at the opening of the year 1885:—
[Illustration: Fig. 37.
Shadows of Jupiter’s Satellites II. and III. near an equatoreal white
spot (Jan. 1, 1885, 7^h 20^m A.M.).]
Sat. II. is probably involved in an atmosphere sufficiently dense to
enable it to present undue luminosity relatively to the others; and if
so, the feeble shadow it transmits on Jupiter may be partly explained
by the effects of refraction. On the day of opposition both satellites
and shadows are projected on the same part of the disk, and the latter
are occulted by the former. On Jan. 14, 1872, Mr. F. M. Newton saw I.
centrally placed on its shadow; so that the satellite was apparently
surrounded with a ring of shade. On May 13, 1876, Mr. G. D. Hirst saw
Sat. I. partly occulting its own shadow; a black crescent was seen in
the bright zone N. of the equator. On Feb. 18, 1885, Dr. R. Copeland,
at Dun Echt, saw the shadow of I. “almost totally occulted by the
satellite itself; as the satellite approached Jupiter’s limb it came
out quite bright and large, with a mere crescent of the shadow showing
on its southern edge.” This phenomenon was also observed at Bristol.
Occasionally all the satellites become invisible at the same time,
being either eclipsed, occulted, or in transit. An instance of this
kind was recorded by Molyneux on Nov. 2, 1681 (O.S.). Sir W. Herschel
observed a similar occurrence on May 23, 1802; also Wallis on April
15, 1826, and Dawes and others on Sept. 27, 1843, and Aug. 21, 1867.
A visible repetition of the event was narrowly avoided on the morning
of Oct. 15, 1883. On this occasion the planet should, according to
the ‘Nautical Almanack,’ have been denuded of his satellites for a
period of 19 minutes; but this disappearance did not occur, for at no
time were all the satellites included within the margin or shadow of
Jupiter. I observed that Sat. III. entered upon the disk just as IV.
released itself, and the two formed a curious configuration at 4^h
A.M., hanging close upon the planet’s limbs.
Spots have been seen on the satellites both in transit and while
shining on the dark sky. This particularly refers to III. and IV. II.
has never given indications of such markings on its bright uniformly
clear surface. Dawes, Lassell, and Secchi frequently observed and drew
the spots. Secchi described III. as similar in aspect to the mottled
disk of Mars as seen in a small telescope; his drawings exhibit no
analogy, however, to those by Dawes of the same object. III. has been
remarked of a curious shape, as if dark spots obliterated part of the
limbs. Sat. I. was observed in transit on Sept. 8, 1890 by Barnard and
Burnham, and it appeared to be _double_, being divided by a bright
interval or belt. They used a 12-inch refractor, powers 500 and 700,
and the seeing was very fine.
Many other curious points have been noticed in the various aspects and
phenomena of the Jovian satellites. Further observations will doubtless
throw new light on some of the puzzling records of the past.
_Occultation of a Star._—An occultation of the 7th mag. star 4
Geminorum by Jupiter took place on Nov. 7, 1882, and it was observed by
Prof. Pritchett, of Glasgow, Missouri, with a 12-1/4-inch equatoreal,
power 200. “The images of both planet and star were very steady. The
margin of Jupiter’s disk was very sharply defined. The immersion was
very near the N. border of the broad S. equatoreal belt. At 11^h 28^m
10^s·65 local mean time the star was _apparently_ within the dark
outline of the disk, apparent geometric contact having occurred at
11^h 20^m 24^s·49. For a moment the star seemed to disappear, but a
moment later was plainly seen, as if through a well-defined notch in
the otherwise _continuously even_ margin. This notch lasted 46^s·26,
and at 11^h 28^m 56^s·91 it _vanished_, and the light of the star
was _entirely extinguished_.” The emersion of the star could not be
observed, as clouds supervened.
FOOTNOTES:
[33] The question of periodicity is an extremely interesting one
as affecting the disposition, form, and colours of the markings
on Jupiter. Certain features visible in 1869-70 were unmistakably
reproduced in 1880, and it has been suspected that the cycle of these
changes accords with the length of the Jovian year. Future observations
must be compared with old drawings and records for the identification
of similar features if they are recurrent.
[34] On the morning of Dec. 5, 1887, I made a drawing of Saturn, the
image of the planet being remarkably well defined, though the Moon was
only 1° distant.
[35] Amongst the first observers of these dark transits were Cassini
(Sept. 2, 1665), Romer (1677), and Maraldi (1707).
CHAPTER XII.
_SATURN._
Apparent lustre.—Grand spectacle afforded by the Rings.—Period
&c.—“Square-shouldered” aspect.—Early Observations.—Belts and
Spots on the Planet.—Rotation-Period.—The Rings.—Divisions
in the outer Ring.—The transparent or Crape-ring.—Discordant
Observations.—Eccentric position of the Rings.—Aspect.—Further
Observations required.—Occultations of Saturn.—The
Satellites.—Occultations of Stars by Saturn.
“Muse, raise thy voice, mysterious truth to sing,
How o’er the copious orb a lucid ring,
Opaque and broad, is seen its arch to spread
Round the big globe, at stated periods led.”
This planet shines brighter than an ordinary first-magnitude star, and
is a pretty conspicuous object, though less luminous than either Venus,
Jupiter, or Mars. He emits a dull yellowish light, steadier than the
sparkling lustre of Mercury or Venus.
The globe of Saturn is surrounded by a system of highly reflective
rings, giving to the planet a character of form which finds no parallel
among the other orbs of our system. His peculiar construction is well
calculated to be attractive in the highest degree to all those who
take delight in viewing the wonders of the heavens. Saturn is justly
considered one of the most charming pictures which the telescope
unfolds. A person who for the first time beholds the planet, encircled
in his rings and surrounded by his moons, can hardly subdue an
exclamation of surprise and wonder at a spectacle as unique as it is
magnificent. Even old observers, who again and again return to the
contemplation of this remarkable orb, confess they do so unwearyingly,
because they find no parallel elsewhere; the beautifully curving
outline of the symmetrical image always retains its interest, and
refreshes them with thoughts of the Divine Architect who framed it!
The luminous system of rings attending this planet not only gratifies
the eye but gives rise to entertaining speculations as to its origin,
character, and purposes with regard to the globe of Saturn. Why, it has
been asked, was this planet alone endowed with so novel an appendage?
and what particular design does it fulfil in the economy of Saturn? It
cannot be regarded as simply an ornament in the firmament, but must
subserve important ends, though these may not yet have been revealed to
the eye of our understanding.
_Period &c._—Saturn revolves round the Sun in 10,759 days 5 hrs. 16
min., which is equal to nearly 29½ years. His mean distance from the
Sun is 886,000,000 miles, but this interval varies from 841 to 931
millions, owing to the eccentricity of his orbit. When in opposition
his apparent diameter reaches 20″·7, and declines to 15″ at the time
of conjunction. The planet’s actual diameter is 75,000 miles, and his
polar compression very considerable, viz. about 1/10, which exceeds
that of any other planet. His synodic period is equal to 378 days; so
that he comes into opposition with the Sun thirteen days later every
year. The oblate figure of his disk is very noticeable when the rings
are turned edgeways to the Earth and practically invisible; but when
they are inclined the complete contour of the globe is lost, and the
polar flattening becomes scarcely obvious.
_“Square-shouldered” Aspect._—Sir W. Herschel, from observations in
April 1805, said:—“There is a singularity which distinguishes the
figure of Saturn from that of all the other planets.” On April 19 of
the year named he described the planet as “like a parallelogram with
the four corners rounded off deeply, but not so much as to bring it to
a spheroid.” This gave the globe a “square-shouldered” aspect. But this
curious figure appears to have been very rarely observed in subsequent
years; and accurate measures with the micrometer were adduced in
1833-48 in proof that no such anomaly had a real existence. Dr.
Kitchiner, commenting on Herschel’s remarks, said:—“I have occasionally
observed this planet during thirty years, and I do not remember to have
seen the body of it of this singular form except for a few months about
September 1818.” But there is no doubt that occasionally the planet
_does_ assume an _apparent_ form similar to that attributed to it by
Herschel. In the autumn of 1880 I studied the visible appearance of
Saturn by means of a 10-inch reflector, and recorded as follows:—“The
S. pole, over which the dark belts lay, seemed compressed in the most
remarkable manner; but where a bright belt intervened, in about lat.
45°, the contrary effect was produced. Here the limbs were apparently
raised (by irradiation) above the spherical contour; so that the
distorted image gave the planet that distinctly ‘square-shouldered’
aspect sometimes mentioned in text-books.” The explanation appears to
me very simple. The singular figure is due to the contrasting effects
of the belts. While the bright belt in lat. 45° causes a very evident
shouldering-out of the limbs at its extremities, the dark belts nearer
the pole and the equator act with opposite effect, for they apparently
compress the disk where they meet the limbs, and thus the eye discerns
a figure to all appearance distorted into the “square-shouldered” form.
Mr. J. L. McCance confirmed these remarks by independent observations
at the same period with a 10-inch reflector by Calver (‘Monthly
Notices,’ vol. xli. pp. 84, 282).
_Early Observations._—The appearance of Saturn offered a considerable
difficulty to observers soon after the invention of the telescope.
Galilei became greatly perplexed. He saw the planet, not as a circular
globe like Jupiter, but distinctly elongated in shape, and conceived
the appearance to be due to a central globe with smaller spheres
hanging on the sides! He continued his observations, without, however,
arriving at the solution of the mystery, until the malformation began
to disappear; and in 1612 he was astonished to find the disk spherical.
In his surprise, he asked—“Were the appearances indeed illusion and
fraud, with which the glasses have so long deceived me, as well as many
others to whom I have shown them?... The shortness of the time, the
unexpected nature of the event, the weakness of my understanding, and
the fear of being mistaken, have greatly confounded me.” Gassendi, in
1633, also announced that Saturn appeared to him to be closely attended
by two globes of the same colour as the planet. Riccioli alleged
that the planet was surrounded by a thin, plain, elliptic ring,
connected with the sphere by two arms. None of Galilei’s contemporaries
possessed the instrumental means to extricate him from his doubts;
and it remained for Huygens, in 1654 (twelve years after the death
of Galilei), to discover that Saturn “is surrounded by a slender
flat ring, which in no part coheres with the body of the planet, and
is inclined to the ecliptic”[36]. The same observer showed that the
disappearance which had so puzzled Galilei arose from the varying
inclination in the ring: at times it would become invisible, when
presenting its narrow edge to the Earth, and this actually occurred
again in 1671, as Huygens had predicted. In 1676 Cassini detected a
belt upon the planet, and also a dark division in the ring. Dr. Smith’s
‘Optics’ (1738) thus alludes to these discoveries:—
[Illustration: Fig. 38.
Saturn, as observed by Cassini in August 1676.]
“In the year 1676, after Saturn had emerged from the Sun’s rays, Sig.
Cassini saw him in the morning twilight with a darkish belt upon his
globe, parallel to the long axis of his ring as usual. But what was
most remarkable, the broad side of the ring was bisected right round
by a dark elliptical line, dividing it, as it were, into two rings, of
which the inner ring appeared brighter than the outer one, with nearly
the like difference in brightness as between that of silver polished
and unpolished—which, though never observed before, was seen many times
after with tubes of 34 and 20 feet, and more evidently in the twilight
or moonlight than in a darker sky.”
From the time when Galilei’s inadequate glass revealed the “threefold”
aspect of Saturn, and led up to Huygens’s solution of the mystery in
1654, this planet has been successively interrogated with the improved
telescopes which every generation has produced. Cassini, W. Herschel,
Encke, Bond, Lassell, Dawes, and Hall are names familiar to us as
having materially advanced our knowledge of this unique orb, both as to
his surface-configuration and as to his numerous retinue of satellites.
_Belts and Spots on the Planet._—Parallel belts are seen on the surface
of Saturn, but they are much fainter than those on Jupiter, and they
seldom display the spots and other irregularities interspersed with
the belts of the latter planet. Well-bounded spots have rarely been
distinguished on the disk of Saturn; the belts normally appear equal
in tone, without breaks, condensations, abrupt curves, or branches,
so that the rotation-period has only been accurately determined by
Herschel and Hall. And in these cases the markings were certainly
atmospheric, and probably affected by proper motions similar to those
operating on Jupiter.
Cassini and Fatio remarked two bright streaks on the planet as early
as 1683. Sir W. Hershel, in 1790, observed a very dark spot near the
margin of the limb, and a few modern observers have been successful
in distinguishing either bright or dark spots or patches, though no
continuous and useful observations appear to have been secured. In the
winter of 1793 Herschel noticed a very distinct quintuple belt, which
consisted of three dusky and two intervening light zones. The dark
belts presented a dusky yellow hue, while the spaces separating them
were white. He recognized the evidences of rotation in the quintuple
belt; for on the same nights, after a few hours’ interval, it exhibited
considerable variation. Though seen with great precision at first, it
became indistinct at a later hour, and the individual belts were placed
at unequal distances.
_Rotation-Period._—Prof. A. Hall, at Washington, discovered on Dec. 7,
1876, a well-defined white spot, 2″ or 3″ in diameter, and situated
just below the ring of Saturn. He watched this object till Jan. 2
following, when it had become faint and indistinct, and the planet
being low and the weather unfavourable no further observations were
made. The spot had fortunately been seen at four other observatories
in the United States, Prof. Hall having notified its existence to
them; and on discussing the results, a rotation-period was found not
differing largely from Herschel’s value derived from the quintuple belt
in 1793. These are, in fact, the only two determinations on which we
may place confidence. They are as below:—
h m s Probable error.
1793. Sir W. Herschel 10 16 0·4 2 min.
1877. Prof. A. Hall 10 14 23·8 2·3 sec.
Schröter, from different spots, computed periods of 11^h 40^m 30^s,
11^h 51^m, and more than 12^h; but these are probably excessive. The
difference of 1^m 37^s between the values of Herschel and Hall is
relatively a trivial one, as the markings observed were doubtless
atmospheric and subject to irregularities of motion. As to the rotation
of the ring, Herschel, in 1789, detected some bright marks on it, and
deduced the period as 10^h 32^m 15^s·4[37]. Many astronomical works
give the rotation-period of Saturn as 10^h 29^m 16^s·8; and this is
adopted in Chambers’s ‘Descriptive Astronomy,’ 4th edit. vol. i. p.
653. The mistake has its origin in Laplace’s _Système du Monde_, where
it is stated that Saturn rotates in 0·428 of a day, and the ring in
0·437, which, reduced to hours, minutes, and seconds, give 10^h 16^m
17^s·2 and 10^h 29^m 16^s·8.
The equator of Saturn is usually the brightest part of the disk. On its
S. side, in recent years, it has been bounded by a very dark narrow
belt. Further S. the whole disk seems involved in a faint shading, of
a decidedly yellowish hue. Sometimes a considerable number of belts
are visible; but they are evidently liable to changes, so that the same
number and arrangement are not preserved from year to year.
_The Rings._—As to the luminous rings, the extreme diameter of the
outer one is about 40″, or more than 170,000 miles; and the black
division, separating it from the inner one, is 0″·4, or 1700 miles.
The outer ring has a breadth of 2″·4, or 10,000 miles; while the inner
one measures 3″·9, or 17,000 miles. The outer ring is less luminous
than the inner; the latter, round its outer edges, is extremely
brilliant, and has sometimes been described as the brightest part of
the Saturnian system. The inner part of this ring is much shaded-off,
and offers a strong contrast to the silvery whiteness of the other
portion.
[Illustration: Fig. 39.
Saturn, 1885, Dec. 23, 7^h 54^m. (10-inch reflector, power 252.)
]
_Divisions in the Outer Ring._—In the middle of the eighteenth century
Short, the optician, using one of his excellent reflectors, thought he
saw the outer ring divided by several dark lines; but no other observer
confirmed his suspicion. In the third decade of the present century
Quetelet and Capt. Kater appear to have observed Short’s divisions, but
Sir J. Herschel and Struve looked for them in vain. In 1837 Encke fully
satisfied himself, by several observations and measurements, as to the
objective existence of the divided outer ring. The division was not
central, cutting the ring into equal parts, but situated in the inner
part of the ring, so that the wider part was outermost. In subsequent
years this division has been sometimes seen and placed nearest the
outer edge of the ring. Certain observers, provided with ample means,
have seen nothing of it; others regard the division as variable. It is
sometimes described as a narrow black line; while others refer to it
as a faint pencil~like shading, and not a real division at all. One
observer occasionally sees it with considerable distinctness at the
very same time that another observer, with a more powerful telescope,
cannot glimpse it though looking specially for such an appearance! It
is difficult to reconcile such discordant experiences, and unsafe to
accept results of such a contradictory nature.
_The “Crape”-Ring._—A far more certain feature was discovered in the
autumn of 1850[38], and one in reference to which there is unanimity of
testimony. On Nov. 11 G. P. Bond, in America, and Dawes, in England, on
Nov. 25, saw a nebulosity or faint luminous appearance like twilight,
fringing the interior margin of the inner ring. Later observations
showed this to be occasioned by a transparent ring situated immediately
within the inner luminous ring. Dawes considered the new ring to
be divided into two parts; but Lassell, with his large reflector,
subsequently negatived this supposition. Both limbs of Saturn may be
readily perceived through the transparent ring where it crosses the
globe of the planet. Some irregularities have been suspected in it at
different times by various observers. In 1887 dark condensations were
reported to disturb its normal aspect; but these were not seen at many
observatories where such features, if real, could hardly have escaped
detection.
[Illustration: Fig. 40.
Saturn, as observed by F. Terby, February 1887.]
It is strange to reflect that this transparent ring avoided discovery
for so long a period. It forms a feature distinctly to be recognized
in relatively small telescopes—in fact, Grover has seen it, where it
crosses the globe of Saturn, with only 2 inches of aperture. Yet,
though ever on the alert to detect new formations, and exercising
constant vigilance in their pursuit, Sir W. Herschel, Schröter, and
many others allowed this ring to escape them! There is no reason to
suppose that it is variable, and that it was not so plain a century ago
as now. It affords another instance of how easily an unknown object
may elude recognition, though everyone sees it readily enough when
attention is called to it.
In March 1889 a white spot was detected on the rings by Dr. Terby, at
Louvain, and it was seen by other observers with comparatively small
instruments. The spot was stationary, and placed near the apparent
junction of the globe and rings, in the E. ansa. But with large
telescopes nothing of this object could be detected: it was shown to be
an optical effect.
_Discordant Observations._—It is curious that the details of Saturn
have occasioned more dissension amongst observers than those of any
other planet. This may have partly arisen from the great distance of
Saturn, the comparative feebleness of his light, and complexity of his
structure. The planet is usually better defined than either Mars or
Jupiter; but with tolerably high powers on small instruments the image
is faint, and the features so diluted that the impressions received
cannot always be depended upon, especially when the air is unsteady.
A fluttering condition of the object is sufficient in itself to cause
deception. Prof. Hall, in speaking of the work done by the 25·8-inch
refractor at Washington in 1883, says:—“Saturn’s ring has been
observed, but many of the strange phenomena noted by other observers
have not been seen even on the best nights.” The evidence afforded by
this large instrument may not always be conclusive, but in this case
there can be no doubt it properly failed to show “phenomena” which had
no existence.
_Eccentric Position of the Rings._—The rings are slightly eccentric
with regard to the ball; in other words, the ball is not situated in
the centre of the rings. Differences have been observed denoting this,
though the observations are not altogether satisfactory. It has been
shown theoretically that the eccentricity referred to is necessary
to maintain the stable equilibrium of the system; for were the rings
perfectly concentric with the planet, they must coalesce with the ball.
The preservation of so complicated a structure must evidently require
judicious and nicely balanced conditions.
With the great 23-inch refractor at Princeton, U.S.A., the ball of
Saturn was seen through the division in the ring in November 1883—an
observation which had previously been made by Lassell in 1852.
_Aspect of the Rings._—In different years the rings present a varying
outline, owing to the fact of their inclination (28° 10′) and to
changes in the relative positions of the Earth and Saturn. At intervals
of about fifteen years the rings are widely open, as they were in 1855,
1869, and 1885, and will be in 1899. At similar intervals they are
rendered invisible, being turned edgeways to the Earth, as in 1848,
1862, 1877, and 1891. Since 1877 the S. side of the rings has been
presented to terrestrial observers; but in 1893 the N. side will come
under inspection, and remain in view until 1907. The S. side of the
rings is obviously more favourably visible to observers in England and
other N. latitudes, because the planet is always above the equator
and attains a fair altitude when it is presented. The N. side of the
rings is exposed when Saturn is in S. declination, and therefore more
liable to our atmospheric disturbances owing to his comparatively
low altitude. The extreme narrowness of the rings is apparent at the
periods when the planet crosses the node and they are situated in the
plane of the line of sight. In small telescopes they become invisible,
and the finest instruments only exhibit them as thread-like extensions
from the equator of the planet. Sir J. Herschel says that on April 29,
1833, the disappearance of the ring was _complete_ when observed with a
reflector of 18 inches aperture and 20 feet focal length. It remained
visible in 1862 as a broken line of light. At such times the satellites
are seen as bright beads, threading their way along the narrow wavering
line of the belts. Inequalities have been observed at such times; for
the line of light into which the rings are then resolved is not uniform
in breadth, but appears broken and undulatory, as though indicating a
very rugged character of surface.
Sir J. Herschel estimated the thickness of the rings as 250 miles, but
Bond thought it far less—about 40 miles. There are great obstacles in
the way of ascertaining the exact proportions of a structure so distant
and offering such an extremely slender form to our view.
_Further Observations required._—The globe and rings of Saturn
offer an encouraging prospect for additional discoveries. Though
the more prominent details have already been descried, there remain
other features, probably of more delicate outline and intermittent
visibility, which will be glimpsed in future years. Small instruments
will scarcely be competent to deal efficiently with this object:
observers who can command at least a moderate grasp of light may,
however, enter upon the work with every assurance of interesting
results. In this, as in other sections of observational astronomy, the
student will realize that in oft-repeated observation and comparison
of records and drawings he acquires a familiarity with the appearance
of the object which will enable him to discern more and more of its
configuration, until ultimately he feels confident he has progressed as
far as the utmost capacity of his instrument will permit. It is in the
sedulous application of his powers that the observer will find the key
to success. Partial devotion to a subject offers a prospect far less
encouraging; for observations of a disconnected character are seldom
valuable.
Changes are unquestionably occurring both in connection with the
ball and rings of Saturn[39]. Some of the discrepancies between the
observations published from time to time are only to be explained on
this assumption. It should therefore be the aim of observers to obtain
further evidence of such variations, and this may be best accomplished
by assiduously watching the lineaments of the planet during the most
favourable periods of each opposition. The collection of a number of
reliable materials through a series of years would undoubtedly possess
weight in removing some of the anomalies of past observation, and
afford us a more thorough knowledge of the delicate markings.
The rotation-period of Saturn is probably not much different from
that given by the atmospheric markings seen by Herschel and Hall.
But additional determinations are very desirable for many reasons.
The spots which are so plentiful on Mars and Jupiter have furnished
observers with a valid and concise means of ascertaining the rate
of axial motion of those planets. Saturn, however, has far more
sparingly provided the data for such an investigation; for if we
disregard Schröter’s uncertain figures, we have but two values for
the rotation-period. These were fortunately effected by observers
of exceptional ability, and the periods may be accepted without
reservation; but other independent determinations are much required.
By multiplying results of this nature, we have a prolific source of
comparison; and comparisons, apart from being interesting, are of
importance in denoting erratic results and indicating those entitled
to credence. Moreover, a reliable mean value may be sometimes deduced
from multiple records; hence it becomes advisable to secure as many as
possible.
The planet should be frequently examined during every opposition with
the highest powers that are consistent with a perfectly distinct image;
and the observer should closely scan the various parts of the disk,
with an endeavour to trace spots, breaks, or other irregularities
in the belts. Certain inequalities of tone have been occasionally
apparent in past years, and they will doubtless reappear. The recovery
of these features will form a welcome addition to our knowledge, and,
if adequately observed, will enable the rotation-period of the planet
to be rediscussed. In an enquiry of this kind many observations are
needful, and the longer the interval over which they extend the more
accurate the results derived from them are likely to be. If a broken
belt should appear on Saturn, the time of its passing the planet’s
central meridian should be recorded, either by measurement or careful
estimation, and an ephemeris computed based on a rotation-period of
10-1/4^h, which is equal to a daily rate of nearly 843°. Then it should
be carefully looked for on subsequent evenings at the times given in
the ephemeris, and on every occasion when re-observed its time of
transit should be noted as at first. As long as the break continues
visible, so long ought it to be kept in view and the times of its
central passages tabulated. It would be advisable in such a case to
secure cooperation from other observers, as more numerous observations
would be sure to accrue, so that, on the appearance of a marking
such as that alluded to, the discoverer will do well to announce it
immediately to other amateurs who are engaged upon planetary work and
most likely to assist him. A white or dark spot, or any condensation
on the belts, would of course serve the same purpose as a broken belt.
The nature of the object is not necessarily to be considered, the main
requirement being that it is one of which the longitude admits of
determination. Markings on the belts, if they are ever discernible,
must be watched with corresponding assiduity for traces of motion; and
if such motion should betray itself, the object of the observer will be
to ascertain its rate.
With reference to the narrow division in the outer ring, usually
termed “Encke’s division,” astronomers would regard it as a gratifying
advance could the doubts overhanging this feature be removed. Is it
a real division in the ring, or simply a pencil-line of shading on
the flat surface? Is it constant in place and appearance, or does it
frequently exhibit changes both as to intensity and position? Judging
from prior experiences, this particular object would appear to be
extremely fugitive, and incapable of being assigned either a definite
place or aspect. Yet the more pronounced and well-attested details of
Saturn show no such vagaries: Cassini’s division seems invariable. Are
we therefore to surmise that the curious behaviour of Encke’s division
is to be referred to errors of observation arising from the effects of
unsteady air upon a very delicate object? It is for future observers
to answer these questions, and this will entail no ordinary effort,
for the same impediments will be encountered in the future as in the
past. But fortunately our science is rapidly progressive, and there is
no doubt the mystery of Encke’s division will find-its solution before
long. A powerful telescope, and a keen and continuous study of the
outer ring, will enable some discriminating observer to tell us the
true story of its phenomena.
Many other points in the Saturnian system require renewed attention,
but some of them appear to be so doubtful as to scarcely deserve
mention. Possibly the student had better commence his review of the
planet without any of the bias or prejudice which former observations
might occasion. But it is as well to know the true state of the case;
for the judgment of a careful observer is not likely to be warped by
preconception, and of course some of the doubtful observations may be
amply verified at a future time. Several of these have already been
briefly referred to, and a few others may here be noted. The form
of the shadow thrown on the rings from the ball has been observed
of a curious shape, and M. Trouvelot supposes it to be variable and
occasioned by changes on the level surface of the rings. The same
observer has noticed transverse notches in the edges of the inner
bright ring. Evidence of variation is not entirely wanting in regard
to the chief division, and observers should notice whether it appears
uniformly black, as it has been suggested that a gauze ring fills
the interval. Exterior to the outer ring a faint luminosity has
also been suspected, as though the phenomenon of the inner ring had
its counterpart here. The colour of the belts on the ball should be
ascribed by careful estimates, as many such observations may give an
insight into the variations occurring. Some observers have alleged that
the transparent ring of Bond and Dawes is subject to very perceptible
alterations. It must be remembered, however, that the visible aspect of
this exceedingly delicate structure is much affected by the condition
of the atmosphere, and that the inclination of the Saturnian system
must obviously introduce changes. When the inclination is considerable,
the globe of the planet may be discerned through this ring with greater
effect than at other times, because we have to look through a thinner
stratum of its material.
The observer, in seeking to elucidate some of the anomalies of former
researches, will possibly himself gain a knowledge of features not
hitherto recognized. Of the real existence of these he should assure
himself by many critical observations before venturing to announce them.
We have hinted that further discoveries upon Saturn may be considered
as practically beyond the reach of small telescopes; but the gratifying
fact remains that some of the more noteworthy of the known features are
visible in glasses of little pretention as regards size. With a 2-inch
refractor, power about 90, not only are the rings splendidly visible,
but Cassini’s division is readily glimpsed, as well as the narrow dark
belt on the body of the planet. This sufficiently proves that a very
small and portable instrument is capable of affording some excellent
views of one of the most wonderful objects in the heavens. Grover has
seen, with an aperture similar to that named, not only the belts and
the shadow of the ball on the rings, but two of the satellites as well;
and others may be equally successful.
_Occultations of Saturn by the Moon._—Phenomena of this kind were well
observed in England on May 8, 1859, April 20 and Sept. 30, 1870. Those
of 1859 and Sept. 30, 1870, were observed by the Rev. S. J. Johnson,
who noted that “the dull hue of the planet contrasted strikingly with
the brilliant yellow of the Moon.” Dawes witnessed the occultation
in 1859, and saw the opaque edge of our satellite sharply defined on
the ball and rings of Saturn, without the slightest distortion of
form. No dark shading was remarked by him contiguous to the Moon’s
bright edge at the reappearance, such as he and others had observed on
Jupiter on the occasion of his occultation, Jan. 2, 1857. Saturn was
described as of a pale greenish hue, and offered a strong contrast to
the brilliant yellow lustre of the Moon. On the early morning of April
20, 1870, several observers were on the _qui vive_ for this interesting
occurrence; and their experiences are reported in the ‘Monthly Notices
R. A. S.’ vol. xxx. p. 175 _et seq._, from which the following are
brief extracts:—
Mr. Ellis:—“The light of the planet, by contrast with the Moon, was
very faint.” Mr. Carpenter:—“There was not the least alteration in the
planet’s form.” Capt. Noble:—“Saturn appeared of a richly-greenish
yellow when compared with the brilliant white light of the Moon.”
Mr. G. C. Talmage:—“The difference in colour between Saturn and the
Moon was most marked, the planet appearing of a yellow tint.” Mr. J.
Carpenter:—“At disappearance the planet was a very dull object when
in contact with the Moon; its light probably a twentieth as bright.
At reappearance the planet was rather tremulous; no distortion was
noticed.” On June 13, 1870, the Rev. J. Spear, of Bengal, watched the
Moon pass “steadily over the planet without causing any change of form
or giving any indication of the planet’s light passing through an
atmospheric medium. When near the Moon’s limb Saturn assumed a sickly
green hue.”
I observed the occultation of Sept. 30, 1870, at Bristol, with a
4-1/4-inch refractor; but the event offered no novel traits, the most
prominent feature being the difference of brightness in the Moon and
Saturn. Mr. C. L. Prince observed this event with a Tulley refractor of
6·8 inches aperture, power 250. He says there was not the slightest
distortion of either body, but he noticed that “the edge of the ring
lingered somewhat upon the Moon’s limb about the time of disappearance.”
Another occultation occurred soon after new Moon on April 9, 1883, and
one of the observers, Mr. Loomis, described the disappearance of the
rings as a spectacle of great interest, and said the impression was
forcibly conveyed to his mind that the Moon was very much nearer to the
eye than Saturn.
_The Satellites._—The discovery of the eight moons of this planet
ranged over the long period of 193 years. Five different observers
share the honours between them. Our knowledge of the Saturnian
satellites may almost be said to furnish us with a history of
improvements in the telescope; for they were severally detected at
epochs corresponding to instrumental advances. The following are the
periods, distances, &c. of the satellites:—
+——————————————+————————————————————+————————+——————+
| | Mean Distance. | | |
| +————————————————————+Sidereal| Real |
|No. and Name. |Diameters | |Period. | Diam.|
| |of Saturn.| Miles. | | |
+——————————————+————————————————————+————————+——————+
| | | | d h m|miles.|
|7th. Mimas | 1·53 | 115,000| 0 22 37| 1000 |
|6th. Enceladus| 1·97 | 148,000| 1 8 53| ... |
|5th. Tethys | 2·44 | 183,000| 1 21 18| 500 |
|4th. Dione | 3·12 | 234,000| 2 17 41| 500 |
|3rd. Rhea | 4·36 | 327,000| 4 12 25| 1200 |
|1st. Titan | 10·12 | 759,000|15 22 41| 3300 |
|8th. Hyperion | 12·23 | 917,000|21 7 7| ... |
|2nd. Iapetus | 29·61 |2,221,000|79 7 53| 1800 |
+——————————————+————————————————————+————————+——————+
+——————————————+——————————————-+———————————————-+
| | | |
| | Date of | |
|No. and Name. | Discovery. | Discoverer. |
| | | |
+——————————————+——————————————-+———————————————-+
| | | |
|7th. Mimas |1789, Sept. 17.|W. Herschel. |
|6th. Enceladus|1789, Aug. 28.|W. Herschel[40].|
|5th. Tethys |1684, Mar. 21.|J. D. Cassini. |
|4th. Dione |1684, Mar. 21.|J. D. Cassini. |
|3rd. Rhea |1672, Dec. 23.|J. D. Cassini. |
|1st. Titan |1655, Mar. 25.|C. Huygens. |
|8th. Hyperion |1848, Sept. 19.|Bond & Lassell. |
|2nd. Iapetus |1671, Oct. 25.|J. D. Cassini. |
+——————————————+——————————————-+———————————————-+
The numbers in the first column refer to the order of discovery.
[Illustration: Fig. 41.
Apparent Orbits of the Five Inner Satellites of Saturn, as seen in an
Inverting Telescope.
(The arrows in the diagram show the direction of the motion of the
satellites. The figures indicate the interval, in hours, from the time
of last East elongation.)]
Titan is by far the largest satellite, being equal to a star of the
8th mag. and visible in any small telescope. Iapetus ranks next,
ordinarily about 9th mag., but there are variations at different parts
of the orbit similar to the variations which affect the satellites of
Jupiter; a variegated surface, and the effects of rotation, originate
the changes observed and give strong support to the inference that
this satellite rotates in the same period that it revolves round its
primary. Tethys, Dione, and Rhea are fainter, and the difficulty of
seeing them is intensified by their proximity to the planet; but a good
4-inch refractor will reveal them on a clear dark night. The others are
objects for powerful instruments and pellucid skies; but Enceladus is
sometimes seen with moderate aperture. The planet being usually much
inclined, his satellites are dispersed round about the rings, and are
not easy of identification. Minute stars lying near the path of Saturn
are very liable to be mistaken for them. But the ephemerides drawn up
by Mr. Marth, and published annually by the Royal Astronomical Society,
are of the utmost service to amateurs engaged in these observations. By
simple reference they may readily identify the individual satellites
on any night; and these ephemerides are additionally useful as giving
the times of conjunctions of some of the satellites with the ends of
the ring and N. and S. points of the ball.
When the thin side of the rings is presented to the Earth, transits and
other phenomena may be observed in connection with the Saturnian moons;
but they appear to have been rarely recorded. Sir W. Herschel describes
a “beautiful observation of the transit of the shadow of Titan over
the disk in 1789, November 2.” It was also seen in 1833 and 1862. The
late Mr. Capron re-observed it on Dec. 10, 1877, with a 8-1/4-inch
reflector, power 144, and made the following sketch:—
[Illustration: Fig. 42.]
These shadow-transits admit of easy observation with appliances of very
moderate capacity. Mr. Banks witnessed a phenomenon of the kind with a
refractor of only 2-7/8 inches, and says it was watched with the same
facility and ease as the shadow of Sat. I. on Jupiter.
In looking for Iapetus it must be remembered that it is commonly
situated at a great distance from the planet. Titan is relatively much
nearer, and will always be recognized without trouble. Enceladus,
Tethys, Dione, and Rhea hover near the outskirts of the ring; while
Mimas is extremely close to it.
Prof. Hall, with the great Washington refractor, has effected many
valuable measures of this system in recent years. He finds the orbits
of the five inner satellites are sensibly circular, and that they
are situated in the plane of the rings. Hyperion revolves in a very
eccentric orbit, and this satellite may approach very near to Titan. He
obtained an observation on March 25, 1885, which seems pertinent to the
question of variation in the light of the satellites. He says:—“Mimas
was remarkably bright, and could not be missed even when the full light
of the planet was admitted to the eye. Generally this satellite is a
difficult object, and from the ease with which it is occasionally seen
one might think it variable; but I think the difference is due to the
quality of the image.” There is no doubt that this is the main cause of
many assumed changes in celestial objects, and especially in regard to
those of a minute and delicate character.
_Occultations of Stars._—Stars are rarely observed to be occulted by
Saturn. Webb mentions that, in 1707 or 1708, Dr. Clark noticed a star
in the interval between the ball and rings; and Dawes once remarked a
star of 8·5 mag. disappear behind the outer edge of the exterior ring.
It would be extremely interesting to watch a tolerably conspicuous star
pass centrally behind the Saturnian system, and to trace it through
Cassini’s division and the transparent ring, noting any changes in
magnitude or appearance as they occurred.
FOOTNOTES:
[36] Huygens appears to have used a refractor of 2-1/3-inch aperture
and 23-feet focal length, with a power of 100, in effecting this
discovery.
[37] Schröter, Harding, Schwabe, and others have observed luminous
points on the rings, but they have remained stationary, so that the
period of rotation announced by Herschel has never been confirmed,
but rather disproved by counter-evidence. Herschel wrote, in November
1789:—“I formerly supposed the surface of the ring to be rough, owing
to luminous points like mountains seen on the ring, till one of these
supposed luminous points was kind enough to venture off the edge of the
ring and appear as a satellite. I have always found these appearances
to be due to satellites.”
[38] Galle, at Berlin, had, twelve years previously, made an
observation which, if it had been interpreted correctly, would have
given him priority. In June 1838 he remarked, on several nights, that
the inner boundary of the inner ring was very indistinct and “gradually
lost itself towards the body of the planet.” The space between the ring
and Saturn was half filled with a dim veil, extending inwards from the
ring. These observations failed to attract the notice their importance
deserved, and Galle himself did not appreciate their full significance
until the announcements of Bond and Dawes in 1850.
[39] Struve wrote, in 1883:—“That changes do take place in the
ring-system is sufficiently proved.” Trouvelot, Schiaparelli, and
others have also remarked variations of a sufficiently decided
character to be placed on record.
[40] Herschel remarks that he saw this satellite in his 20-foot
speculum two years before, viz. on Aug. 19, 1787, but he was then much
engaged in observations of the satellites of Uranus.
CHAPTER XIII.
_URANUS AND NEPTUNE._
Discovery of Uranus.—Mistaken for a Comet.—True character
revealed.—Period &c.—Observations.—Belts on Uranus.—Further
Observations required.—The Satellites.—Discovery of Neptune.—The
planet observed in 1795.—Period &c.—Observations.—Supposed
Ring.—Satellite.—A trans-Neptunian Planet.—Planetary Conjunctions.
_Discovery._—While Sir W. Herschel was a musician at Bath he formed
the design of making a telescopic survey of the heavens. When engaged
in this he accidentally effected a discovery of great importance, for
on the night of March 13, 1781, an object entered the field of his
6·3-inch reflector which ultimately proved to be a new major planet
of our system. The acute eye of Herschel, directly it alighted upon
the strange body, recognized it as one of unusual character, for it
had a perceptible disk, and could be neither fixed star nor nebula. He
afterwards found the object to be in motion, and its appearance being
“hazy and ill-defined” with very high powers he was led to regard it
as a comet, and communicated his discovery to the Royal Society at its
meeting on April 26, 1781. His paper begins as follows:—
“On Tuesday, March 13, 1781, between 10 and 11 in the evening, while I
was examining the small stars in the neighbourhood of H Geminorum, I
perceived one that appeared visibly larger than the rest. Being struck
with its uncommon magnitude, I compared it to H Geminorum and the small
star in the quartile between Auriga and Gemini, and finding it so much
larger than either of them suspected it to be a comet.... The power I
had on when I first saw the comet was 227.”
The supposed “comet” soon came under the observation of others,
including Maskelyne the Astronomer Royal, and Messier, the “Comet
Ferret” of Paris. The latter, in a letter to Herschel, said:—“Nothing
was more difficult than to catch it, and I cannot conceive how you
could have hit this star or comet several times, for it was absolutely
necessary for me to observe it for several days in succession before I
could perceive that it was in motion.”
_True character revealed._—As observations began to accumulate it was
seen that a parabolic orbit failed to accommodate them. Ultimately the
secret was revealed. The only orbit to represent the motion of the
new body was found to be an approximately circular one situated far
outside the path of Saturn, and the inference became irresistible that
the supposed “comet” must in reality be a new primary planet revolving
on the outskirts of the solar system. This conclusion was justified by
facts of a convincing nature, and its announcement created no small
excitement in the scientific world. Every telescope was directed to
that part of the firmament which contained the new orb, and its pale
blue disk, wrapped in tiny proportions, was viewed again and again
with all the delight that so great a novelty could inspire. From the
earliest period of ancient history, no discovery of the same kind had
been effected. The Chaldæans were acquainted with five major planets,
in addition to the Earth, and the number had remained constant until
the vigilant eye of Herschel enlarged our knowledge, and Saturn was
relieved as the sentinel planet going his rounds on the distant
frontiers of our system.
When the elements of the new body had been computed a search was
instituted amongst the records of previous observers, and it was
found that Herschel’s planet had been seen on many occasions, but it
had invariably been mistaken for a fixed star. Flamsteed observed it
on six occasions between 1690 and 1715, while Le Monnier saw it on
12 nights in the years from 1750 to 1771, and it seems to have been
pure carelessness on the part of the latter which prevented him from
anticipating Herschel in one of the greatest discoveries of modern
times.
The name Uranus was applied to the new planet, though the discoverer
himself called it the _Georgium Sidus_, and there were others who
termed it “Herschel,” in honour of the man through whose sagacity it
had been revealed.
_Period &c._—Uranus revolves round the Sun in 30,687 days, which very
slightly exceeds 84 terrestrial years. His mean distance from the Sun
is 1,782,000,000 miles, but the interval varies between 1,699 and 1,865
millions of miles. The apparent diameter of the planet undergoes little
variation; the mean is 3″·6, but observers differ. His real diameter
is approximately 31,000 miles, and the polar compression about 1/13,
though this value is not that found by all authorities.
_Observations._—The planet near opposition shines like a star of
the 6th magnitude, and is observable with the naked eye. He emits a
bluish light. While engaged in meteoric observations, I have sometimes
followed the planet with the naked eye during several months, and noted
the changes in his position relatively to the stars near. It is clear
from this that Uranus admitted of detection before the invention of the
telescope.
A luminous ring, similar to that of Saturn, was at first supposed to
surround Uranus, and Herschel suspected the existence of such a feature
on several occasions; but it scarcely survived his later researches,
and modern observations have finally disposed of it.
Lassell, when working with his 2-foot speculum at Malta, thought he
saw a spot near the centre of the planet’s disk, but he considered
this might possibly be due to an optical illusion. In 1862, Jan. 29,
he said:—“I received an impression which I am unable to render certain
of an equatoreal dark belt.” In the early months of 1870, Mr. Buffham,
using a 9-inch “With” mirror, powers 212 and 320, saw bright spots and
zones on the planet, and inferred a rotation-period of about 12 hours.
On Jan. 16, 1873, when definition was very good, no traces of any
markings were visible in Lord Rosse’s 6-foot reflector. In May and June
1883 Prof. Young, having the advantage of the fine 23-inch refractor
at the Princeton Observatory, observed two faint belts, one on each
side of the equator, and much like the belts of Saturn. On March 18,
1884, Messrs. Thollon and Perrotin, with the 14-inch equatoreal at
Nice, remarked dark spots similar to those on Mars, towards the centre
of the disk, and a white spot was seen on the limb. Two different
tints were perceived, the colour of the N.W. hemisphere being dark,
and that of the S.E. a bluish-white colour. In April observations were
continued, and the white spot was seen “rather as a luminous band
than a simple spot,” but it was most conspicuous near the limb. The
observers thought the appearances indicated a rotation-period of about
10 hours. The brothers Henry at Paris, in 1884, invariably noticed two
belts lying parallel to each other, and including between them the
brighter equatoreal zone of the planet. Their results apparently show
that the angle between the plane of the Uranian equator and that of the
satellite-orbits is about 41°.
[Illustration: Fig. 43.
Uranus and his Belts. 1884.]
M. Perrotin, with the great 30-inch equatoreal at Nice, re-observed
the belts in May and June 1889. He wrote that dark parallel bands
were noticed several times, and they were very similar to the belts
of Jupiter. On May 31 and June 1 and 7 the direction of the Uranian
belts was measured, and the mean result showed that the plane of the
equator of Uranus differs little (about 10°) from the common plane
of the orbits of the satellites. This deduction is not, it will be
observed, consistent with that of the Brothers Henry at Paris, who
found a difference of 41°. M. Perrotin notes that the bands of Uranus
do not always present the same aspect. They vary in size and number
in different parts of their circumference. This unequal distribution
raises the hope that by an attentive study of these bands it will be
possible to determine the duration of the planet’s rotation.
_Further Observations required._—In the case of an object so faint
and diminutive as Uranus, a powerful telescope is absolutely required
to deal with it effectively. A small instrument will readily show the
disk, and present the picture that caught the eye of Herschel more than
a century ago, but considerable light and power must be at command if
the observer would enter upon a study of the planet’s surface-markings.
With my 10-inch reflector I have suspected the existence of the belts,
but under high powers the image is too feeble to exhibit delicate forms
of this character. It is to be hoped that with the large telescopes now
available at various observatories, some attention will be given to
this planet, more especially with regard to the study of the belts and
determination of the rotation-period. Amateurs will have little trouble
in picking up Uranus; his position can be learnt from an ephemeris
and marked upon a star-map. A little careful sweeping with a low
power in the region indicated will soon reveal the object sought for,
and a higher power may then be applied to expand the disk and render
identification certain.
It may be mentioned as an interesting point that some fifty years after
the discovery of Uranus by Sir W. Herschel the planet was accidentally
rediscovered by his son Sir John Herschel, who mentioned the fact as
follows in a letter to Admiral Smyth, written on Aug. 8, 1830:—“I have
just completed two 20-foot reflectors, and have got some interesting
observations of the satellites of Uranus. The first sweep I made with
my new mirror I _rediscovered_ this planet by its _disk_, having
blundered upon it by the merest accident for 19 Capricorni.” Had the
father failed to detect this planet in 1781, the discovery might
therefore have been made by the son half a century later.
Some spectroscopic observations of Uranus made in 1889 with Mr.
Common’s 5-foot reflector, appear to show that the planet “is to a
large extent self-luminous.” But Mr. Huggins on June 3 seems to have
obtained a different result (see ‘Monthly Notices,’ xlix. p. 404 _et
seq._).
_The Satellites._—For many years it was supposed that Uranus possessed
six satellites, all of which were discovered by Sir W. Herschel, but
later observations proved that four of these had no existence. They
were small stars near the planet. But two of Herschel’s satellites were
fully corroborated, and two new ones were discovered by Lassell and
Struve. The number of known satellites attending Uranus is four, and it
is probable that many others exist, though they are too minute to be
distinguished in the most powerful instruments hitherto constructed.
The following are the periods, distances, &c., of the known satellites:—
+——————————————+————————————————————+———————+————————————————+
| | Mean distance. | | |
| Number and +——————————+————————-+ Max. | Date of |
| name. | Diameters| Miles. |Elonga-| Discovery. |
| |of Uranus.| | tion. | |
+——————————————+————————————————————+———————+————————————————+
| | | | ″ | |
| 3rd. Ariel | 4·03 | 125,000 | 12 | 1847, Sept. 14.|
| 4th. Umbriel | 5·61 | 174,000 | 15 | 1847, Oct. 8.|
| 1st. Titania | 9·19 | 285,000 | 33 | 1787, Jan. 11.|
| 2nd. Oberon | 12·32 | 382,000 | 44 | 1787, Jan. 11.|
+——————————————+————————————————————+———————+————————————————+
————————————-+——————————+
| |
Discoverer. | d h m |
| |
| |
————————————-+——————————+
| |
W. Lassell. | 2 12 29 |
O. Struve. | 4 3 27 |
W. Herschel.| 8 16 57 |
W. Herschel.| 13 11 7 |
————————————-+——————————+
Titania and Oberon are the two brightest satellites, but none of
them can be seen except in large instruments. The two outer ones are
said to have been glimpsed in a 4·3-inch refractor, but this feat is
phenomenal, and certainly no criterion of ordinary capacity. Sir J.
Herschel found them tolerably conspicuous in a reflector of 18 or 20
inches aperture, and mentioned a test-object by which observers might
determine whether their telescopes were adequate to reveal them.
This test is a minute double star lying between the stars β′ and β^2
Capricorni. The magnitudes are 15 and 16, and distance 3″. Relatively
to the satellites of Uranus this faint double is a “splendid object.”
From observations with large modern instruments it appears highly
probable that the four known satellites, must be considerably larger
than any others which may be revolving round the planet. A curious
fact in connection with these satellites is that their motions are
retrograde.
[Illustration: Fig. 44.
Apparent Orbits of the Satellites of Uranus, as seen in an Inverting
Telescope.
(The small circle in the above diagram represents the planet and is
on the same scale as the orbits. The arrows show the direction of the
motion of the satellites, and the figures indicate the number of days
from the time of the last North elongation.)]
_Discovery of Neptune._—The leading incidents in the narrative of the
discovery of Uranus and Neptune present a great dissimilarity—Uranus
was discovered by accident, Neptune by design. Telescopic power
revealed the former, while theory disclosed the latter. In one case
optical appliances afforded the direct means of success, while in the
other the unerring precision of mathematical analysis attained it. The
telescope played but a secondary part in the discovery of Neptune, for
this instrument was employed simply to realize or confirm what theory
had proven.
Certain irregularities in the motion of Uranus could not be explained
but on the assumption of an undetected planet situated outside the
known boundaries of the system. Two able geometers applied themselves
to study the problem of these irregularities, and to deduce from them
the place of the disturbing body. This was effected independently
by Messrs. Le Verrier and Adams; and Dr. Galle, of Berlin, having
received from Le Verrier the leading results of his computations, and
the intimation that the longitude of the suspected planet was then
326°, found it with his telescope on the night of Sept. 23, 1846, in
longitude 326° 52′. The calculated place by Prof. Adams was 329° 19′
for the same date and less accurate than the prediction of Le Verrier.
The former had priority both in attacking the problem and resolving
it, though unfortunately his efforts were not backed up in a practical
way. But for the supineness of certain officials, there is little doubt
that the planet would have been telescopically discovered in the autumn
of 1845, when it was within 1° 49′ of the place attributed to it by
Prof. Adams. Delays occurred owing to the doubts prevailing, and in the
meantime the planet was found elsewhere. This circumstance does not rob
Prof. Adams of his hard-earned laurels, though it shows how seriously
official negligence can mar the character of a discovery.
_Observations in 1795._—The name given to the new planet was Neptune.
When the elements were computed it was found that they presented
rather large differences with those theoretically computed by Messrs.
Le Verrier and Adams. It was also found that the planet had been
previously observed by Lalande on May 8 and 10, 1795, but its true
character escaped detection. This astronomer had observed a star of
the 8th mag. on May 8; but on May 10, not finding the same star in
the exact place noted on the former evening, he rejected the first
observation as inaccurate and adopted the second, marking it doubtful.
Had Lalande exercised a little discretion, and confided in his work,
he would hardly have allowed the matter to rest here. A subsequent
observation would at once have exhibited the cause of the discrepancy,
and the mathematical triumph of Le Verrier and Adams, half a century
later, would have been forestalled. Lalande, like Le Monnier, the
unsuspecting observer of Uranus, let a valuable discovery slip through
his hands.
_Period &c._—Neptune revolves round the Sun in 60,126 days, which is
equal to rather more than 164½ of our years. His mean distance from the
Sun is 2,792,000,000 miles, and his usual diameter 2″·7. He exceeds
Uranus in dimensions, his real diameter being 37,000 miles.
_Observations._—Our knowledge of this distant orb is extremely limited,
owing to his apparently diminutive size and feebleness. No markings
have ever been sighted on his miniature disk, and we can expect to
learn nothing until one of the large telescopes is employed in the
work. No doubt this planet exhibits the same belted appearance as that
of Uranus, and there is every probability that he possesses a numerous
retinue of satellites. In dealing with an object like this small
instruments are useless; they will display the disk, and enable us to
identify the object and determine its position if necessary, but beyond
this their powers are restricted by want of light.
_Supposed Ring._—Directly the new planet was discovered, Mr. Lassell
turned his large reflector upon it and sought to learn something of
its appearance, and possibly detect one or more of its satellites. On
October 3 and 10, 1846, he was struck with the appearance of the disk,
which was obviously not perfectly spherical. He subsequently confirmed
this impression, and concluded that a ring, inclined about 70°,
surrounded the planet. Prof. Challis supported this view, but later
observations in a purer sky led Mr. Lassell to abandon the idea. Thus
the ring of Neptune, like the ring of Uranus, though apparently obvious
at first, vanished in the light of more modern researches.
_The Satellite._—But if Mr. Lassell quite failed to demonstrate
the existence of a ring, he nevertheless succeeded in discovering a
satellite belonging to the planet. This was on Oct. 10, 1846. The
new satellite was found to have a period of 5^d 21^h 3^m, and to be
situated about 220,000 miles distant from the planet. Its apparent star
mag. is 14, and at max. elongation it extends its excursions to 18″ on
either side of its primary. Compared with the other satellites of our
system the one attending Neptune must be excessive in regard to size,
or it could not be discerned at the vast distance separating it from
the Earth.
[Illustration: Fig. 45.
Apparent Orbit of the Satellite of Neptune, as seen in an Inverting
Telescope.
(The small circle in the above diagram represents the planet, the
arrows show the direction of motion, and the figures indicate the
interval from the time of last North-east elongation.)]
_A trans-Neptunian Planet._—Is there a planet beyond Neptune? Prof.
Forbes wrote a memoir in 1880 tending to prove that two such planets
exist. From the influences exerted by these bodies on certain comets of
long period, he approximately deduced the positions of the former, and
they were searched for with the great Washington refractor, but without
success. Flammarion and Todd have also arrived at conclusions affirming
the existence of a planet outside Neptune; but the idea has not yet
been realized by its telescopic discovery.
_Planetary Conjunctions._—Before concluding this chapter, an allusion
should be made to a noteworthy class of events, viz., planetary
conjunctions. These include some of the most attractive aspects
displayed by the heavenly bodies, and they are sometimes witnessed
by ordinary persons with the same amount of gratification as by the
astronomical amateur. In almanacks the times of such conjunctions are
given, so that intending observers may always be prepared for these
events. In a strict sense a conjunction occurs at the instant when
two or more bodies have the same right ascension, but the term is
here intended to have a more general reference, _i. e._, to denote
the assembling together of two or more planets in the same region
of the firmament. Historical records furnish us with a considerable
number of planetary conjunctions, and some of them were attentively
observed long before the telescope came into use. Thus in 2012 B.C.,
Feb. 26, the Moon, Mercury, Venus, Jupiter, and Saturn were in the same
constellation, and within 14° of one another. In 1186 A.D., Sep. 14,
the Sun, Moon, and all the known planets are said to have been situated
in Libra. In 1524 Venus, Mars, Jupiter, and Saturn were near together.
Many similar instances might be quoted, but this is unnecessary.
Occasionally the conjunctions were so close that one planet appeared
to occult another. Kepler refers to an occultation of Jupiter by Mars
which he saw on January 9, 1591; but this would really be a transit of
Mars across the disk of Jupiter, if contact actually occurred, for the
apparent diameter of Jupiter always exceeds that of Mars. Mœstlin seems
to have witnessed an occultation of Mars by Venus on Oct. 3, 1590. It
is probable, however, that these were near approaches only. A genuine
occultation of Mercury by Venus was telescopically observed on May 17,
1737.
On the evening of March 3, 1881, the new Moon, Venus, Jupiter, and
Saturn formed a brilliant quartet in Pisces. On the morning of July
21, 1881, I saw the Moon, Venus, Mars, Jupiter, Saturn, and Aldebaran
in the same region above the eastern horizon. There was a very close
conjunction of Mars and Saturn on the morning of Sept. 20, 1889. Mr.
Marth computed that the nearest approach would occur at 8^h 7^m A.M.,
when the distance between the centres would be 54″·8 and less than
that (74″) observed at the time of the close conjunction of the same
planets on June 30, 1879.
The interest centred in the conjunction of Sept. 20, 1889, was enhanced
by the fact that Regulus was only 47′ distant, while Venus was also in
the same region. I observed this phenomenon in my 10-inch reflector,
and with the help of a comet-eyepiece made the above sketch of the
positions of the objects as they were presented in the field.
[Illustration: Fig. 46.
Mars, Saturn, and Regulus in same field, Sept. 20 1889, 4^h 45^m A.M.]
Perhaps there is not much scientific importance attached to the
observation of these conjunctions, though comparisons of colour and
surface-brilliancy are feasible at such epochs, and are not wholly
without value. As spectacles merely, they possess a high degree of
interest to everyone who “considers the heavens.”
CHAPTER XIV.
_COMETS AND COMET-SEEKING._
Ideas concerning Comets.—Appearance.—Large number
visible.—Nature of Apparition.—Tenuity of Comets.—Differences
of Orbit.—Discoveries of Comets.—Large Comets.—Periodical
Comets.—The Comets of Halley, Encke, Biela, Brorsen, Faye,
D’Arrest, Pons-Winnecke, and Tuttle.—Grouping.—Further Observations
required.—Nomenclature of Comets.—Curiosities of Comets.—Naked-eye
Comets.—Comet-seeking.—English weather.—Aperture and Power
required.—Annual rate of Discovery.—Telescopic Comets and
Nebulæ.—Ascertaining Positions.—Dr. Doberck’s hints.—Prizes.
Superstitious ideas with regard to comets as the harbingers of disaster
have long since been discarded for more rational opinions. They are no
longer looked upon as ill-omened presages of evil, or as
“From Saturnius sent,
To fright the nations with a dire portent.”
Many references are to be found among old writings to the supposed evil
influence of these bodies, and to the dread which their appearance
formerly incited in the popular mind. Shakespeare makes an allusion to
the common belief:—
“Hung be the heavens with black, yield day to night!
Comets, importing change of time and states,
Brandish your crystal tresses in the sky;”
and in relation to the habit of connecting historical events with their
apparition, he further says:—
“When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.”
But, happily, the notions prevalent in former times have been
superseded by the more enlightened views naturally resulting from
the acquirement and diffusion of knowledge; so that comets, though
still surrounded by a good deal of mystery, are now regarded with
considerable interest, and welcomed, not only as objects devoid of
malevolent character, but as furnishing many useful materials for
study. Mere superstition has been put aside as an impediment to real
progress, and a more intelligent age has recognized the necessity of
dealing only with _facts_ and explaining them according to the laws of
nature; for it is on facts, and their just interpretation, that all
true searchers after knowledge must rely. Comets are properly regarded
as bodies which, though far from being thoroughly understood in all the
details of their physical structure and behaviour, have yet a wonderful
history, and one which, could it be clearly elucidated, would unfold
some new and marvellous facts. Under these circumstances we need evince
no surprise that these visitors are invariably hailed with enthusiasm,
not only by scientific men, who make them the special subjects of close
observation, but by everyone who regards celestial “sights and signs”
with occasional attention.
_Appearance._—From whatever point of view a large comet is considered,
it deserves all the interest manifested in it and all the labour
expended in its investigation. Whilst its grand appearance in the
firmament arrests the notice of all classes alike, and is the subject
of much curious speculation amongst the uninformed, it merits, apart
from other considerations, the most assiduous observation on account
of the singular features it displays and the striking variations they
undergo. Indeed, the visible deportment of a comet during its rapid
career near perihelion is so extraordinary as to form a problem, the
solution of which continues to defy the most ingenious theories. The
remarkable changes in progress, the quickness and apparent irregularity
of their development, are the immediate result of a combination of
forces, the operations of which can neither be defined nor foreseen.
Jets of flame and wreaths of vapour start from the brilliant nucleus;
while, streaming away from the latter, in a direction opposite to the
Sun, is the fan-shaped tail, often traceable over a large span of the
heavens and commingling its extreme fainter limits with the star-dust
in the background.
_Large number visible._—The orbits of 400 comets have now been
computed, and more than 500 others have been observed; so that these
bodies are extremely plentiful. Kepler described them to be as numerous
as the fishes in the sea, and no doubt the allegory is justly applied.
Their vagaries of form, size, and place are equally noteworthy; and
those who enter upon the discussion of facts relating to these objects
will find an endless store of interesting materials, opening up a wide
field for conjecture.
_Nature of Apparition._—The apparition of a comet may be either gradual
or sudden. Usually the telescope gives us the earliest intimation
that one of these bodies is approaching us[41]. It is first seen as
a small round nebulosity, with probably a central condensation or
stellar nucleus of the 10th or 11th mag. The whole object brightens and
expands as its distance grows less, and it assumes an elongated form
preparatory to the formation of a tail. The latter varies greatly in
different instances: it may either be a narrow ray, as shown in the
southern comet of January 1887, or a fan-shaped extension like that of
the great comet of 1744. Barnard’s Comet of December 1886 exhibited
a duple tail. Occasionally a fine comet bursts upon us suddenly,
like that of 1843 or 1861. The former was sufficiently bright to be
discovered when only 4° from the Sun, and the latter presented itself
quite unexpectedly as a magnificent object even in the strong twilight
of a June sky.
_Tenuity of Comets._—Comets are noteworthy for the extreme thinness of
their material. The smallest stars may be discerned through the denser
portions of the head, without suffering any apparent diminution of
light. Yet such stars would be quite obscured by the interposition of
a minute speck of cloud or by a little fog or any vapour of trifling
density. Comets are visible in the form of transparent nebulosities;
and their mass must be inconceivably small relatively to the enormous
space over which they frequently extend. Sir J. Herschel has described
the “all but spiritual texture” of comets; and other authorities have
referred to them as feeble wreaths of vapour, which, though obeying
the laws of gravitation and suffering much perturbation, are yet
themselves incapable of exercising any disturbing influence upon the
other bodies near which they pass. It has been asserted that comets
would show phases were they rendered luminous by reflected sunlight,
and that, such features being absent, these bodies must possess a
phosphorescence of their own sufficient to cause the glow observed.
This idea, however, is hardly consistent with our present knowledge.
Comets are not compact and coherent masses of matter; they more
likely represent vast groups of planetary atoms, more or less loosely
dispersed and sometimes forming streams. The effect of sunshine
upon such assemblages will be that the whole mass becomes illumined
according to density, and that no phase will be apparent, inasmuch as
the light is enabled to penetrate through its entirety.
_Differences of Orbit._—When three trustworthy observations of a
comet’s place have been made, its orbit may be computed. This may be
either an ellipse, a parabola, or hyperbola. If an _ellipse_ the comet
is periodical, and the period depends upon the degree of eccentricity.
If a _parabola_ the comet will not be seen again, because this form of
orbit does not reunite; it consists of branches equally divergent and
uniting at perihelion, but extending outwards indefinitely in nearly
parallel lines and without convergence. If a _hyperbola_, the comet is
also not returnable; the branches of the orbit are widely divergent,
and show no tendency to parallelism. These several forms of orbit are
somewhat different as applied to various comets, but they are the same
in effect. Thus Tempel’s Comet of 1867 revolves in an ellipse having
an eccentricity of about 0·4630, while that of Halley’s Comet is
0·9674. No doubt some of the parabolic orbits applied to comets really
represent very eccentric ellipses; but the parabola is a convenient
form of orbit for computation, and unless ellipticity is very decided
it indicates the path with sufficient accuracy.
_Discoveries of Comets._—In the latter part of the last century
Messier, Mechain, and Miss Herschel shared nearly all the cometary
discoveries between them. Then Pons entered the field, and he may be
said to have monopolized this branch during the period from 1802 to
1827, for he was the first to announce thirty comets. Pons died in
1831, but the search was actively continued by others. In about 1843
a great rise became apparent in the rate of these discoveries; and we
find Di Vico, Mauvais, and Brorsen very successful at this period.
Later on, the work was sustained with the same prolific results by
Klinkerfues, Bruhns, and Donati, and subsequently by Winnecke, Tempel,
and Coggia. Swift and Borrelly also assisted materially to swell our
knowledge; while during the last few years Barnard and Brooks have
exhibited a surprising amount of zeal in this department. Since 1881 no
less than twenty-six comets are to be enumerated as the fruits of their
endeavours, and they are still engaged in nightly explorations of the
sky with similar ends in view. Their diligent pursuit of these fugitive
bodies will doubtless result in many further additions during ensuing
years.
It is a curious circumstance that Sir W. Herschel, during all his
star-gaugings and sweeps for nebulæ, never discovered a comet. He found
a nebula on Dec. 18, 1783, near δ Ceti, which he described as “small
and cometic.” In Sir J. Herschel’s ‘General Catalogue of Nebulæ,’
1864, p. 17, this object is presumed to have been a comet, as it could
not be identified; but at p. 45 the doubts are cleared up, and Sir W.
Herschel’s nebula, the position of which was only roughly given, is
shown to be the same as another very near; it is No. 1055 of the new
‘General Catalogue’ published by the Royal Astronomical Society in
January 1888. Quite possibly Sir W. Herschel’s lists of nebulæ contain
several comets, as some of his objects are missing; but errors of
observation in ascribing positions may explain this. Herschel himself,
in speaking of a comet visible in the winter of 1807-8, says:—“If I had
met the comet in one of my sweeps, as it appeared between Dec. 6 and
Feb. 21, I should have put it down as a nebula. Perhaps my lists of
nebulæ, then, contain some comets.”
_Large Comets._—The most widely observed and attractive class of
comets includes those of large proportions, as they are not only
visible to the naked eye, but exhibit features having the lustre
necessary to permit of their examination with high magnifying powers.
A brief summary of some of the finest comets of modern times is
subjoined; but, to save space, a few only of the more salient facts
concerning them are given:—
1577, Nov. and Dec.—Observed by Tycho Brahe. At the end of November it
had a double tail; the longest of the two branches was about 20°. This
comet was visible in the daytime.
1618 II., Nov.—“The length of its tail equalled in extent one sixth
part of the zodiac.” On Nov. 18 it was estimated as 40°. Longomontanus,
however, described it as 104° long, and Cysatus estimated it as 75°.
Kepler referred to it as the largest comet that had appeared for a
hundred and fifty years.
1680, Dec.—A fine comet, which on Dec. 12 had a narrow tail about 80°
long. The nucleus was equal to a 1st mag. star. Hooke remarked jets of
flame issuing from the nucleus. At perihelion the comet approached very
near the Sun’s surface, similarly to the fine comets of 1843, 1880, and
1882.
1744, Jan.-Feb.—Probably the largest comet of the 18th century. At
one time it displayed six tails, each of which was 4° in breadth. The
head was so bright that it was perceived with the naked eye in full
sunshine. At the middle of February the tail was 24° long, and it was
divided into two branches.
1769, Sept.—Discovered on Aug. 8 by Messier. On Aug. 30 the comet had a
trifid tail; there was a central ray of 24° and two outlying ones of 4°
each. On Sept. 19 the tail had increased to 75°, and a few nights later
Pingre estimated it as 90° and 97°.
1811 I., Sept.-Oct.—A very fine comet. The tail was branched; it did
not, however, exceed 25° in length and about 6° in breadth. Sir W.
Herschel found the nucleus to be 428 miles in diameter. This remarkable
comet remained visible during a period extending over seventeen months.
Its period is approximately 3000 years.
1843, Mar.—Visible in the daytime. On Mar. 4 its tail was 69° in
length; it was very narrow, being only 1-1/4° in breadth throughout.
At perihelion this object passed very near to the Sun, like the great
comet of 1680. it revolves in an elliptical orbit; period about 376
years. This comet swept past perihelion with a velocity of 366 miles
per second! The real length of its tail was 200 millions of miles!
1858 VI., Sept.-Oct.—Donati’s Comet: one of the most brilliant comets
of the 19th century. Early in October it displayed a tail about 40°
long, and on the 5th it passed over the star Arcturus. Its period of
revolution appears to be about 2000 years.
1861 II., June-July.—Became suddenly visible at the end of June. In
the opinion of Sir John Herschel this comet surpassed in grandeur
the comets of 1811 and 1858. On June 30 the nucleus was equal to the
brightness of Venus, and the tail was 80° long; but early in July it
increased to 90°. One observer estimated its length as 100° on July 2.
This comet remained visible during twelve months. It appears to have an
elliptical orbit, with a period of 409 years.
1874, July.—Coggia’s Comet: a fine object in the northern sky. On July
14 the tail was 35° long, and it remained visible several days after
the nucleus had disappeared below the horizon. The nucleus was about
equal to a star of the 1st mag. Orbit probably elliptical, with a
period of about 5711 years.
1880 I., Jan.-Feb.—A southern comet, with a long narrow tail, variously
estimated from 30° to 40° in length. It passed very near to the Sun,
and presents an orbital resemblance to the fine comets of 1680 and 1843.
1881 III., June-July.—This large comet appeared in the northern heavens
on June 22, and became generally visible to observers in England. On
the 27th it had a tail 15° long. Its period of visibility extended over
nine months.
1881 IV., Aug.—This comet is scarcely entitled to rank as one of
exceptional character; but it was a conspicuous object for several
weeks in August, and had a tail 6° long on the 19th.
1882 III., Oct.—Well visible in the morning sky, with a tail 22° long.
The nucleus underwent remarkable changes, and on Oct. 23 it showed four
or five bright points or nuclei, looking like “a string of beads.” The
comet threw off several small condensations, which were observed as
separate comets near the parent mass. At perihelion this comet passed
very close to the Sun, like the comets of 1680, 1843, and 1880; and
these bodies were suspected to have an intimate relation, if not an
absolute identity. But subsequent inquiries disproved this startling
supposition; for the comet of 1882 was shown to have a period of about
718 years.
1887 I., Jan.—A fine southern comet, presenting many points of
resemblance to that of 1880 I. On Jan. 22, as observed at Adelaide,
the comet had a long narrow tail of about 30°, but no well-defined
nucleus. On the same date, at the Cape of Good Hope, the tail appeared
as a narrow ribbon of light, quite straight, and of nearly uniform
brightness throughout its length. It was visible in the same region of
the sky as the comet 1880 I., and came into view with equal suddenness.
_Periodical Comets._—On page 235 is a list of the periodical comets as
at present known. Some of these, marked with an asterisk, have only
been observed at one return, and therefore await complete confirmation.
Many other comets have shown indications of pursuing elliptical orbits.
Amongst those of short period may be mentioned 1743 I., 1766 II., 1783
I., 1819 IV., 1844 I., and 1873 VII. The following are examples of
longer periods:—
Comet. Period.
1862 III. 121 years.
1857 IV. 234 ”
1861 I. 415 ”
1860 III. 1089 ”
1889 IV. 5100 ”
1877 II. 8393 ”
1847 III. 13918 ”
1877 III. 28,000 ”
1850 I. 29,000 ”
1780 I. 75,314 ”
1844 II. 102,050 ”
1744 122,683 ”
1849 I. 382,801 ”
1882 I. 400,000 ”
These figures are to be regarded as approximations only.
+——————————————+——————-+——————————————-+——————————-+————————-+
| |Period,| Perihelion | Long. of | Long. of|
| Name. | in | Passage. |Perihelion.|Ascending|
| |years. | | | Node. |
+——————————————+——————-+——————————————-+——————————-+————————-+
| | | | ° ′ | ° ′ |
| Encke | 3·29 |1888, June 28 | 158 36 | 334 39 |
| Tempel (1873)| 5·20 |1878, Sept. 7 | 306 8 | 121 1 |
|*Barnard | 5·40 |1884, Aug. 16 | 301 2 | 5 9 |
| Brorsen | 5·46 |1879, Mar. 30 | 116 15 | 101 19 |
| Pons-Winnecke| 5·73 |1886, Sept. 16 | 276 4 | 101 56 |
| Tempel(1867) | 5·98 |1879, May 7 | 238 11 | 78 46 |
| Tempel-Swift | 5·99 |1880, Nov. 8 | 43 0 | 296 42 |
|*Brooks (1886)| 6·30 |1886, June 7 | 229 46 | 53 3 |
|*Spitaler | 6·40 |1890, Oct. 26 | 58 24 | 45 8 |
| Biela | 6·62 |1852, Sept. 23 | 109 8 | 245 52 |
| D’Arrest | 6·64 |1890, September| 319 9 | 146 9 |
|*Finlay | 6·67 |1886, Nov. 22 | 7 34 | 52 30 |
|*Wolf | 6·78 |1884, Sept. 27 | 352 31 | 206 18 |
|*Swift | 6·91 |1889, Nov. 29 | 69 29 | 331 27 |
|*Brooks (1889)| 7·07 |1889, Sept. 30 | 1 26 | 17 59 |
| Faye | 7·57 |1888, Aug. 19 | 50 56 | 209 42 |
|*Denning | 8·69 |1881, Sept. 13 | 312 31 | 65 57 |
|*Peters | 12·80 |1846, June 1 | 240 7 | 260 28 |
| Tuttle | 13·66 |1885, Sept. 11 | 116 28 | 269 42 |
|*Tempel (1866)| 33·18 |1866, Jan. 11 | 60 28 | 231 26 |
|*Stephan | 33·62 |1867, Jan. 20 | 75 52 | 78 36 |
|*Westphal | 67·77 |1852, Oct. 13 | 43 12 | 346 13 |
| Pons | 71·48 |1884, Jan. 25 | 93 21 | 254 6 |
| Olbers | 72·45 |1887, Oct. 8 | 149 45 | 84 30 |
|*Di Vico | 73·25 |1846, Mar. 6 | 90 35 | 77 36 |
|*Brorsen | 74·97 |1847, Sept. 10 | 79 13 | 309 49 |
| Halley | 76·37 |1835, Nov. 15 | 304 32 | 55 10 |
+——————————————+——————-+——————————————-+——————————-+————————-+
+——————————————+——————-+————-+——————-+
| |Incli- | Mo- | Next |
| Name. |nation.|tion.|Return.|
| | | | |
+——————————————+——————-+————-+——————-+
| | ° ′ | | |
| Encke | 12 53 | D | 1891 |
| Tempel (1873)| 12 46 | D | 1894 |
|*Barnard | 5 28 | D | 1895 |
| Brorsen | 29 23 | D | 1895 |
| Pons-Winnecke| 14 27 | D | 1892 |
| Tempel(1867) | 9 47 | D | 1891 |
| Tempel-Swift | 5 31 | D | 1892 |
|*Brooks (1886)| 12 56 | D | 1892 |
|*Spitaler | 12 52 | D | 1897 |
| Biela | 12 33 | D | ? |
| D’Arrest | 15 43 | D | 1897 |
|*Finlay | 3 2 | D | 1893 |
|*Wolf | 25 16 | D | 1891 |
|*Swift | 10 3 | D | 1896 |
|*Brooks (1889)| 6 4 | D | 1896 |
| Faye | 11 20 | D | 1896 |
|*Denning | 6 51 | D | 1899 |
|*Peters | 30 24 | R | ? |
| Tuttle | 54 19 | D | 1899 |
|*Tempel (1866)| 17 18 | R | 1899 |
|*Stephan | 18 13 | D | 1900 |
|*Westphal | 40 59 | D | 1920 |
| Pons | 74 3 | D | 1955 |
| Olbers | 44 35 | D | 1960 |
|*Di Vico | 84 57 | D | 1919 |
|*Brorsen | 19 8 | D | 1922 |
| Halley | 17 45 | R | 1912 |
+——————————————+——————-+————-+——————-+
_Halley’s Comet._—A fine comet with a tail about 15° long appeared in
the summer of 1682, and Halley computed the orbit according to the
method explained by Newton. He then consulted observations of previous
comets, and discovered a great similarity in the paths of large comets
seen in 1531 and 1607 to that of the one he himself had observed in
1682. He thereupon suspected the three bodies to be one and the same,
and advised posterity to maintain a strict watch for the comet’s return
in about 1758 or 1759. On pursuing his investigations still further, he
alighted upon records of comets in 1305, 1380, and 1456, which greatly
strengthened his opinion that the comet of 1682 moved in an elliptical
path with a period of about 75½ years. He termed this body “the
Mercury[42] of comets, revolving round the Sun in the smallest orbit,”
and said that, should it reappear according to his prediction in about
the year 1758, “impartial posterity must needs allow this to be the
discovery of an Englishman.”
As the time drew near for the return of the comet, interest became
intensified, and computations were made by Clairaut with a view to
determine the precise epoch when it would arrive at perihelion. He
found that the comet would be retarded by the action of Jupiter and
Saturn, but that perihelion would be reached at the middle of April
1759, subject to an uncertainty of 30 days. The comet was rediscovered
on Dec. 25, 1758, by Palitzch, an amateur astronomer at Politz, near
Dresden, who employed a telescope of 8 feet focal length, and appears
to have anticipated Messier and others who were on the alert for it.
It arrived at perihelion on March 12, 1759, and within a month of the
date announced by Clairaut. Early in May it had a tail nearly 50° long,
and presented a fine aspect in the heavens. Thus the sagacity of Halley
had revealed a periodical comet—the first known. It duly returned again
in 1835, and received all the attention which a body so replete with
historical associations deserved.
[Illustration: Fig. 47.
Comet 1862 III. (Aug. 19, 1862).]
[Illustration: Fig. 48.
Sawerthal’s Comet, 1888 I. (March 25, Brooks).]
_Encke’s Comet._—Until the year 1819 Halley’s Comet was the only one
certainly known to be periodical. Then the able deductions of Encke
presented us with a veritable “Mercury[43] of comets.” He showed that
a small comet, discovered by the unwearying Pons of Marseilles on Nov.
26, 1818, was really identical with three previous comets—viz. 1786 I.
(Mechain), 1795 (C. Herschel), and 1805 (Thulis),—and that its period
was a very short one of about 3-1/3 years. Its return to perihelion
was predicted to occur on May 24, 1822, and this was observed in the
southern hemisphere. It returned again on Sept. 16, 1825, and on this
occasion the circumstances were more favourable. Since 1825 this
object has effected nineteen returns to perihelion, and one of the
most singular facts noticed in connection with it is that its period
is gradually shortening. In 1795 it was 1212 days, while in 1858 it
was 1210. In order to explain this contraction of orbit, it has been
necessary to assume the existence of a thin medium in space capable
of affording a slight resistance to the tenuous materials of a comet,
though not dense enough to appreciably affect the motions of planets.
If this closing-up of the orbit and shortening of period continue
to operate through a vast interval of time, Encke’s Comet must be
ultimately precipitated upon the Sun![44]
_Biela’s Comet._—This comet was discovered on Feb. 27, 1826, by Wilhelm
von Biela, an Austrian officer, at Josephstadt in Bohemia, and ten
days later by Gambart at Marseilles. It was found to be revolving in
an orbit of short period, and its elements presented an agreement with
those of the comet of 1772 (Montaigne) and 1806 I. (Pons). Identity
was inferred, and the next return was fixed for Nov. 27, 1832, when
the object reappeared with great punctuality. At the end of 1845 this
comet displayed some startling phenomena; for it divided into two
portions, apparently quite disconnected, and which travelled side by
side, separated by an interval of more than 150,000 miles! The double
comet was observed again in 1852, when the interval separating them
had, however, increased eightfold, for the dark space between measured
1,250,000 miles. This instance of a divided comet is by no means
unique. The great comet of 1882 underwent a process of disruption, by
throwing off small masses of nebulosity, which, however, survived
the separation only a few days. Brooks’s Comet (1889 V.) was found by
Barnard, on Aug. 1, 1889, to be divided into four parts! Two of these
had a brief existence; but one of the minor fragments retained a very
distinct appearance near the parent mass during the ensuing months
of September and October. The phenomena of Biela’s Comet found an
excellent counterpart here.
[Illustration: Fig. 49.
Brooks’s double Comet, Sept. 17, 1889.
(10-inch reflector, power 60. W. F. Denning.) ]
Since 1852 Biela’s Comet has been lost. The most assiduous observations
have failed to recover it, and the conclusion seems irresistible that
further disintegrations have occurred and that its material has been
dispersed beyond recognition. The great meteoric storms of Nov. 27,
1872 and 1885, were derived from this comet, and there is little reason
to hope that _as a comet_ it will ever be seen again.
_Brorsen’s Comet._—A small comet was discovered in Pisces by Theodor
Brorsen, at Kiel, on the evening of Feb. 26, 1846, Its observed path
soon gave traces of an elliptical orbit; and the period was found to be
about 5·58 years. The comet was re-observed at its return to perihelion
in 1857, 1868, 1873, and 1879; but in 1884 it was looked for in vain.
This comet was expected in February 1890, and several observers swept
for it diligently, but to no purpose. Are we, therefore, to regard
this as another lost member of our system? Has the comet of Brorsen,
like that of Biela, suffered dispersion in such degree as to be no
longer within the reach of our powerful telescopes? Should negative
results again attend observers in 1895, when the comet ought to return,
there will be no reason to doubt its actual disappearance. It may be
mentioned that owing to planetary perturbations, the period of this
body has rapidly become shorter since 1846. It was then 2034 days, but
in 1879 was reduced to 1995 days.
_Faye’s Comet._—First seen at the Paris Observatory on the night of
Nov. 22, 1843, when it was near the star Bellatrix in Orion. The
observations clearly proved the comet to be moving in an elliptical
path, and Dr. Goldschmidt of Göttingen determined its period as 7½
years. It was re-observed in 1851, and also during each of its five
subsequent returns, the last of which occurred in August 1888. The
orbit of this body approaches nearer to the circular form than that of
any other known comet, except Tempel’s of 1867. Its perihelion distance
is considerable, for it never comes within the orbit of Mars. Prof.
Möller, of Lund, has investigated the path with all the critical acumen
of a profound mathematician, and, chiefly owing to his labours, it is
now regarded as one of the best known members of our system.
_D’Arrest’s Comet._—Discovered at the Leipsic Observatory on June
27, 1851. M. Villarceau discussed the orbit, and announced it as an
elliptic one with a periodic time of about 6½ years. The comet was
redetected at its return in 1857-8, 1870, 1877, and 1890. It is a very
faint object.
_Pons-Winnecke’s Comet._—Discovered at Bonn on March 8, 1858, and on
the elements being computed they were found nearly coincident with
those of Pons’s Comet, 1819 III. Encke had assigned a period of 5·62
years for the latter, but it managed to escape observation during the
six returns that occurred in the 39 years between 1819 and 1858. Its
identity was fully established in 1869, when it was again observed.
_Tuttle’s Comet._—A faint, diffused comet was discovered in the
northern part of Hercules by H. P. Tuttle, of Cambridge, U.S.A., on
Jan. 4, 1858. Its elements on being calculated were found by Pape to
be similar to those of a comet discovered by Mechain on Jan. 9, 1790,
and an elliptic orbit with a period of 13·66 years was derived from the
new observations. On the assumption that the two bodies were one and
the same there must have occurred four unobserved returns to perihelion
between 1790 and 1858. The year 1871 was awaited in settlement of the
question. When it came the comet returned, and the predictions received
exact verification. Thus the comets of Mechain and Tuttle were placed
in the inseverable bonds of identity.
Of the other periodical comets it will be unnecessary to give details.
Some of them are still without full corroboration, only one return
to perihelion having been observed. The reappearance of Pons’s Comet
(1812) in 1883-4, and of Olbers’s Comet (1815) in 1887 furnished two
excellent examples of well-determined comets belonging to the same
class as that of Halley. Tempel’s discoveries in 1867, 1869, and 1873
afforded some interesting additions to the family of short-period
comets, and the list of these is continually extending owing to the
assiduity of observers, though the lost comets of Biela and Brorsen
will have to be removed from it. Peters’s Comet of 1846 is also
doubtful, as it escaped rediscovery in about 1859, 1872, and 1884; but
this object may yet be captured at one of its succeeding apparitions.
These bodies often evade redetection when their periods and paths are
not accurately known. This has been fully exemplified in the case of
the comets of Pons-Winnecke and Tuttle, which were unseen at several
consecutive returns. It has been supposed, and not without reason, that
the periodical comets are in process of wearing away. They apparently
grow fainter at each return. Halley’s Comet in 1835 was only moderately
bright, whereas in ancient times its appearance was magnificent.
[Illustration: Fig. 50.
Pons’s Comet (1812). Telescopic view. 1884, January 6, 5^h 50^m.
(10-inch reflector, power 60. W. F. Denning.)]
[Illustration: Fig. 51.
Pons’s Comet (1812). Telescopic view. 1884, January 21, 13^h 18^m.
(5-inch refractor; comet-eyepiece, field 1-1/4°. E. E. Barnard.)]
_Grouping of Periodical Comets._—It is a curious circumstance that
these bodies are assorted into groups having their aphelia near the
orbits of major planets. The short-period comets comprised within
the orbits of Encke’s (3·29 years) and Denning’s (8·69 years)
have aphelia in the region of the path of Jupiter, hence they
are occasionally referred to as Jovian comets. The next group is
represented by the comets of Peters and Tuttle, with aphelia near
Saturn. The third group includes the comets of Tempel and Stephan,
with aphelia just outside the orbit of Uranus. The fourth group is
shown by the comets of Halley, Pons, and Olbers, with three others
less certainly ascertained, with aphelia exterior to Neptune. There
are unmistakable indications of other groups far outside the known
boundaries of the solar system, but these are not so well defined.
This clustering of cometary orbits has been ascribed to the attractive
influences of the large superior planets, which are so capable of
disturbing the paths of comets passing near that the orbits become
transformed, and the aphelia henceforth lie near the points of extreme
perturbation. This has been called the “capture theory;” and there is
also an “ejection theory,” which supposes the periodical comets to have
had their birth in planetary ejections.
M. Hoek of Utrecht has found cases in which the orbits of two or more
comets exhibit a common point of intersection in distant space, and
infers their derivation from the same origin.
_Further Observations required._—One of the chief and essential
features in cometary work is the accurate determination of positions.
But this entails the possession of expensive instruments, and a
knowledge which amateurs have not always acquired. This department of
labour can well be left to the trained hands at large observatories,
where, fortunately, it meets with every attention. Ordinary observers
will merely require to know the approximate place, and this is to be
found by estimating the difference in R.A. and Dec. between a comet
and a known star. The position of the latter may be found in a good
catalogue and corrected for precession; then, allowing for the observed
differences, the comet’s place may be assigned to within very small
limits of error. A low power, embracing a field of 1° or more, is best
adapted for these observations, as it is more likely to include a
catalogued star, and will exhibit the comet, especially if a large one,
to the best effect.
The announcement of a new comet is always read with avidity by
amateurs, and their first desire is to see it for themselves. This
they may readily do by marking its place on a star-map or globe, and
noting its relative place amongst the stars near. The telescope should
then be directed towards the point indicated, and if the comet is not
presented in the field, the instrument should be moved a little so
that the surrounding region may be examined. If failure still attends
the effort, the observer should point the telescope a few degrees E.
or W. of the suspected point, and then carefully sweep over the place
of the comet. It will then be picked up, unless it is too faint for
his aperture. The first announcement of a comet generally gives the
position at discovery, and the daily rate and direction of motion. The
latter must of course be allowed for when the search is instituted.
The physical aspects of comets are as diversified as they are variable.
No two comets are exactly alike, nor does the same comet exhibit a
permanency of detail. Of course, when these objects are enormously
distant, and barely visible, many of them appear to present similar
characteristics; but under the closer and more expanded views
obtainable near perihelion the resemblance vanishes, and every comet is
seen to possess features peculiar to itself. To trace these features,
and to record them by delineation and description, forms one of the
most interesting branches in which amateurs may engage. Much has been
learnt of previous comets by successively noting their transitions of
form and brightness, and the same scrupulous attention should be given
to future comets.
The tails of comets are not always turned away from the Sun. Indeed,
the contrary effect is sometimes produced. Occasionally there is a
duple tail, the largest branch of which follows the normal direction,
while the other is turned towards the Sun. Forms of this character
require close watching from night to night. Is the sunward tail
developed suddenly? and has it a fairly durable existence? Instances of
singular curvature should also be noted. The tails are seldom perfectly
straight, especially those attached to naked-eye comets, and decided
changes affect their visible outlines at very short intervals. In large
comets the space over which the tail extends should be sketched upon a
star-map on successive evenings; its changes of position and curve will
then be manifested by comparisons, and its increasing or decreasing
length will also be apparent. Dark rifts, like shadows, often run
lengthwise through the tail, and occasion a fan-like appearance
analogous to that which distinguished the great comet of 1744 and gave
it a sextuple tail.
The light of comets sometimes fluctuates in a very extraordinary
manner, and too rapidly and irregularly to be consistent with theory.
In this respect, Pons’s Comet, at its last return in 1883-4 presented
an eccentric behaviour. Bigourdan found that during the nineteen days
from Sept. 5 to 24, 1883, the increase in the comet’s brilliancy
exceeded by thirty or forty times that resulting from reflected light
alone! This increase appears to have been due to a sudden outburst on
Sept. 22, which occurred some time within the four hours preceding
midnight. Dr. Müller, of Potsdam, witnessed a further outburst on Jan.
1, 1884, within 1-3/4 hour; and the extent of this was accurately
determined by means of a photometer. He found an augmentation of
seven tenths of a magnitude in the brightness of the comet, and an
equally sudden fall to its previous lustre. While these fluctuations
were in progress, he noticed variations in the shape of the nucleus
not less remarkable than its variations in light. Those who observe
future comets will do well, therefore, to be on the alert for similar
phenomena. The apparent brightness of the nuclei and alterations of
shape or size should be recorded on every night when observations are
feasible.
As a comet approaches the Sun its material apparently contracts, while
with increasing distance from that luminary it expands. Usually the
nucleus is extremely small and bright, and it often looks like a star
shining through nebulosity. High powers must therefore be applied
in its examination. Jets, aigrettes, luminous sectors, and other
appendages are often involved with the nucleus and outlying coma, and
they form a complicated structure well deserving further study. A good
deal of mystery still surrounds these appearances; their curious forms
and vagaries have yet to be explained.
Stars are frequently observed through the head of a comet, which
apparently, however, exercises no influence in dimming their lustre.
But the stars are commonly seen behind the envelopes or comæ, and very
rarely through the nucleus. Nothing is better calculated to exhibit
the transparent and tenuous character of comets than observations of
this kind, and observers should seek for further opportunities of
making them. If the motion of a comet is obviously carrying it in the
direction of one of the stars in the field, the observer may determine
for himself the approximate time of conjunction by noting the distance
between the star and comet and allowing for the motion of the latter.
He will then know when to come to his telescope and witness the
phenomenon. Should it appear probable that the comet’s nucleus will
pass over the star, he should commence his watch some time before it
occurs; he may then make comparisons before the star is involved in
the outlying nebulosity, and trace the whole event from beginning to
end. Any changes in the light or aspect of either star or comet would
then be manifested. The comet of 1847 is said to have passed centrally
over a 5th mag. star, but the latter was unaffected. Encke’s Comet on
one occasion interposed itself directly over one of a pair of 10th
mag. stars, but their relatively equal brilliancy suffered no change.
Encke’s Comet, however, has no stellar nucleus. The latter feature
is so bright and compact as displayed in many other comets, that its
transit over a small star must have some effect either in obliterating
it altogether, or in detracting from its lustre.
Visible evidences of rotation seem to have been suspected in certain
comets, but this has never been substantiated on sufficient grounds.
The circumstance is one, however, which should be remembered. During a
series of observations the observer who notes the details of structure
with particular regard to position may discover similar traces, and
possibly learn something of the cause. The nucleus of a bright comet
should always be examined with a moderately high power, so that any
variations or peculiarities of form may be detected.
_Nomenclature of Comets._—It must be confessed that no perfectly
satisfactory method has yet been devised as regards the naming of
comets. The plan of affixing Roman numbers progressively for each
year, according to date of perihelion passage, answers pretty well,
though a little confusion is sometimes caused by prematurely affixing
the number, especially when two comets are discovered successively,
the first of which is a long time before perihelion, and the second
considerably after it. Until a comet can be safely assigned its
catalogue place, it is preferable to refer to it by the name of the
discoverer and date of discovery. This is more distinctive than the
common method of lettering comets according to the epochs of their
detection. As to periodical comets, it is not difficult to find some
inconsistencies in their names. In the case of Halley’s Comet (1682,
discovered by Flamsteed) and Encke’s Comet (1819 I., discovered by
Pons), it was most fitting that they should be known to posterity by
the names of the two able computers whose investigations first revealed
to us comets of long and short period respectively. Under ordinary
circumstances the name of the discoverer is applied to a comet as a
means of convenient reference, and perhaps as a suitable recognition
of the patient labours of the man who first announced it to the world.
The plan seems to have been to name comets after those fortunate
persons who sighted them at the particular apparition during which
periodicity was determined. Thus Tuttle’s Comet (1858 I.) had been seen
as long before as 1790 by Mechain, and Biela’s (1826 I.) was previously
observed in 1772 by Montaigne, and in 1805 by Pons. It is, however,
strange that a comet found by Pons in 1819 (III. of that year), and
which Encke showed to be revolving in an ellipse with a periodic time
of 5½ years, should be called after Winnecke, who rediscovered it in
1858. To Pons the real priority belongs, though Winnecke deserves
much praise for redetecting and identifying this body after it had
effected six unobserved returns to perihelion. It is also curious to
find that the comet of short period discovered by Swift in 1880 is
called “Tempel’s 3rd Comet” in Galle’s catalogue (1885), from the fact
that Tempel found it at a previous return (1869), when, however, its
period was not ascertained. There is little doubt that the title justly
belongs to Swift. Tempel himself called it “Swift’s Comet.” One plan
should in fairness to observers be consistently adhered to. If comets
are to be called after their original discoverers, then Biela’s Comet
should be known as “Montaigne’s,” Tuttle’s as “Mechain’s,” &c.
_Curiosities of Comets._—The comet of 1729, which was hardly visible to
the naked eye, has far the greatest perihelion distance (4·0435) of any
comet known. Barnard’s Comet (1885 II.) comes next with a perihelion
distance of 2·5068.
Pons’s Comet at its return in 1883-4 remained visible for nine months.
When last seen, on June 2, 1884, it was 470 millions of miles from the
Earth, and more remote in the depths of space than any other observed
comet since that of 1729. Barnard’s Comet (1889 I.), though never
visible to the naked eye, was followed from Sept. 2, 1888, to Aug. 18,
1890. Its distance from the Sun was then 6·25 (Earth’s distance = 1),
or about 580 millions of miles, which is greater than that of many of
the short-period comets at aphelia. The most prolonged visibility of
any previous comet was that of 1811 I. (510 days). But this comet of
Barnard has been retained in view 715 days.
The great comet of 1882 was watched right up to the Sun’s limb by
Messrs. Finlay and Elkin at the Cape of Good Hope on Sept. 16, 1882.
The comet was actually seen to disappear at the margin, and not a
vestige of it could be traced during its transit across the solar
disk. The nucleus of the comet was 4” in diameter, and before transit
it looked as bright as a part of the Sun’s surface; but it was quite
invisible when projected on the disk. The alleged observations by
Pastorff and Stark, which were construed into visible transits of
comets, are therefore thoroughly disproved, and will require another
interpretation.
At the time of the total solar eclipse of May 17, 1882, a bright comet
was observed near the Sun. It was a striking object visible to the
naked eye. In the photographs which were taken of the eclipse the comet
is well shown, but this body escaped subsequent observation, so that
its orbit could not be determined.
_Naked-eye Comets._—Arago mentions that twelve comets were visible to
the naked eye during the period from 1800 to 1853, but there appear
to have been certainly thirty comets fulfilling this condition, and I
believe a careful search amongst cometary records would further augment
the number. During the ten years from 1880 to 1889 inclusive there
were no less than sixteen comets perceptible to unaided vision, and a
considerable proportion of these were fine comets. It is very rarely
that two naked-eye comets are to be seen at the same epoch, as in
August 1881 and at the end of April 1886.
_Comet-seeking._—For a long time after the invention of the telescope
comet-seeking does not appear to have been undertaken in a methodical
way, and to have formed the habitual work of certain observers. But
the expected return of Halley’s Comet in 1759 roused observers to
take the initiative in a branch of practical research which in after
years was destined to prove remarkably productive. Messier, Palitzch,
and others began a system of sweeping the heavens for the predicted
comet; and it had a successful issue, for Palitzch, who did not relax
his labours even on Christmas day, alighted upon the coveted prize on
Dec. 25, 1758. Since that time a regular search after comets has been
maintained. Messier pursued it with indomitable energy through a long
period of years, and achieved many successes. It is said of him that
on one occasion he was anticipated in a discovery by Montaigne, and
he appears to have deplored the loss of the comet more than the loss
of his wife, who was lying dead at the time. A friend visited him,
and spoke a few words of sympathy in reference to his bereavement,
but Messier, in despair about the comet, exclaimed: “I had discovered
twelve—alas! that I should be robbed of the thirteenth by Montaigne!”
and his eyes filled with tears. Recollecting himself, and appreciating
the loss he had sustained in his wife, he added, “Ah, this poor woman!”
Messier encountered some serious obstacles to his favourite pursuit.
Breen, in his ‘Planetary Worlds,’ mentions that Messier, while walking
in President Saron’s garden, fell into an ice-house, and was disabled
for a time. Later on “the revolution deprived him of his little income
and every evening he was wont to repair to the house of Lalande to
replenish the supply of oil for his midnight lamp. The political storm
necessitated his removal to another neighbourhood, where he no longer
heard the clocks of forty-two churches sounding the hours during his
night-watchings.” Messier discovered all his comets with a small 2-foot
telescope of 2½ inches aperture magnifying 5 times and with a field of
4°.
Dr. Olbers, of Bremen, was another diligent student in this field. He
did not effect many discoveries, but, from an upper apartment of his
house, he observed nearly all the comets which appeared during half a
century.
During the first twenty-seven years of the present century, Pons
discovered the majority of the comets that were seen. He was a
door-keeper at the Observatory at Marseilles, and owing to the teaching
and encouragement he received from Thulis, the director, he achieved
phenomenal success as a comet-hunter.
Discoveries of comets have rarely been effected in England. This
is chiefly to be assigned to two circumstances. First, because the
labour involved in seeking for these bodies has never perhaps been
pursued to an equal degree and with the same tenacity as it formerly
was in France, and as it has recently been in the United States; and
second, because the cloud-laden skies of England oppose the successful
prosecution of a research in which a clear atmosphere is eminently
desirable.
Though comet-seeking does not always produce new discoveries, it is
certainly entertaining to those engaged in it; for one of the most
agreeable diversions of telescopic work is to scan the firmament with a
large-field comet-eyepiece, which exhibits the most pleasing views of
star-groups, coloured stars, nebulæ, and telescopic meteors.
The operation of sweeping for comets is attractive from other aspects,
though it undoubtedly needs close application, patience, and much
caution. The possibility of seeing a comet in the field at any time
proves a constant source of allurement to the observer, and sustains
his enthusiasm. The glimpsing of a nebulous object, and the expectation
(before it has been identified) that it _may_ prove a comet, induces
a little excitement which pleasantly relieves the monotony that might
otherwise be attached to a sedulous research of this nature; and it
is one in which amateurs may suitably engage with a fair prospect of
success. Instruments of great power, refinement, and expense are not
required. It is rather a work calling for the exercise of patience and
acute perception, and for that tireless servitude which those only who
have an inborn love for it can maintain.
_English Weather and Comet-seeking._—Only two new comets having been
discovered in England during the last forty years some people regard
our climate as in a great measure responsible for this. But the opinion
seems to be erroneous. The lack of discoveries has arisen from want
of effort as much as from want of opportunity. The best weather for
comet-seeking is when the atmosphere is very transparent, and the
stars are lucid and sparkling. Haze, fog, or cloud of any kind offers
a serious hindrance. A thoroughly good night for planetary work is
not usually good for cometary observation, because sharp definition
is not so requisite as a very clear sky. A little fog or thin cloud,
which will often improve planetary images, utterly obliterates a small
telescopic comet. The air is sometimes very pure and dark after storms,
and the stars remarkably bright; it is then that the best opportunities
are afforded for comet-hunting. Any systematic and regular work like
this may be pursued in this country with every prospect of success by
an observer who will persevere in it. From some statistics printed in
the ‘Science Observer,’ Boston, it appears that during the seven months
from May to November, 1882, Lewis Swift was comet-seeking during 300
hours. I have no English results of the same kind, but my meteoric
observations will supply a means of comparison. From June to November,
1887 (six months), I was observing during 217 hours, and for a nearly
similar period during the last half of 1877, though in each year work
was only attempted with the Moon absent. My result for 1887 averages
36 hours per month, which is little less than the average derived
from the comet-seeking records above quoted. It is therefore fair to
suppose that as much may be done here as in some regions of the United
States. Mr. W. R. Brooks wrote me in 1889, saying: “We have much cloudy
weather in this part of America. While in other portions of the country
clear weather abounds, it is not so in this section, where much of
my work has been done. This is a most fertile section—the beautiful
lake region of N.Y.,—but it is for this reason a cloudy belt. It is
far different in Colorado and California. In the latter place, at the
Lick Observatory, I hear they have 300 clear nights in the year—a
paradise for the astronomical observer! My former site, the Red House
Observatory, Phelps, N.Y., is only six miles from Geneva, and hence
in the same cloudy region.” Prof. Swift also referred to the subject
of weather in a letter to me dated July 30, 1889, where he says: “I
arrived home, after a five weeks’ visit to the Lick Observatory, on
March 1, and have not had half a dozen first-class nights since—not
in thirty years have I seen such prolonged rainy and cloudy weather.”
Now Mr. Brooks has discovered 13 comets in 7 years, Prof. Swift has
found 8 comets (1862-1890), and in addition to these has detected
more than 700 new nebulæ, all of the latter since 1883. From this it
appears conclusively that if such extensive and valuable results can
be obtained, notwithstanding frequently bad weather, then English
observers may prove equally successful, the important factor being that
similar energy and ability direct their labours.
_Aperture and Power required._—Opinions are divided as to the most
suitable aperture and power for this work. Any telescope of from
4-to 10-inches aperture may be employed in it. A low power (30 to
50) and large field (50′ to 90′) eyepiece are imperative; and the
instrument, to be really effective, should be mounted to facilitate
sweeping either in a vertical or horizontal direction. A reflector on
an alt-azimuth stand is a most convenient form for vertical sweeps.
The defining-capacity of the telescope need not necessarily be perfect
to be thoroughly serviceable, the purpose being to distinguish faint
nebulous bodies, and not details of form. Far more will depend upon
the observer’s aptitude and persistency than upon his instrumental
means, which ought to be regarded as a mere adjunct to his powers
and not a controlling influence in success, for the latter lies in
himself. Very large instruments are not often used, because of their
necessarily restricted fields. Moreover, a small instrument, apart
from its advantage in this respect, is worked with greater facility
and expedition. This is important, especially when the observer is
to examine the region in the immediate neighbourhood of the Sun. He
has then a very brief interval for the attainment of his purpose,
and a small telescope must be used on account of its large field,
its ready manipulation, and its general effectiveness on objects at
low altitudes. The case is somewhat different when the search is to
be conducted in regions far removed from the Sun’s place; for here
the comets are in general faint, and there is time for the work to
be deliberately and critically performed. Large instruments are to
be recommended for these districts as capable of revealing fainter
objects, though they are troublesome in several respects. They show
large numbers of nebulæ, especially if the observer is exploring the
region of Virgo, Coma Berenices, or Ursa Major; and he will have great
difficulty in identifying them and in feeling his way with certainty.
These complications are inseparable from the work, and, though
chiefly affecting large apertures, should not always be shunned; for
a telescope capable of displaying very faint nebulæ is also capable
of showing faint comets. Many comets have eluded discovery by the
inadequate reach of the instruments in the hands of comet-seekers;
and the statement recently made that there are only about one hundred
nebulæ liable to be mistaken for comets is not accurate, because comets
in certain positions are of the last degree of faintness, and there is
no identifying them from small nebulæ except by means of their motion.
Mr. Brooks says:—“Medium magnifying powers, with necessarily
moderate-sized fields, are better than very low powers and large
fields. While with the latter a large amount of the sky can be swept
over in a given time, the work is not so well done, and a faint comet
would be easily swept over and not seen. A small region, _thoroughly
worked_, is far more likely to be successful. This gives a feeling of
satisfaction with the work performed, even with negative results. In
support of this I may remark that, during all the years I have engaged
in comet-seeking, not a single comet has been discovered by another
astronomer in a region of the heavens that I had just previously
searched; so that I have never had occasion to feel that I had swept
over a comet and missed seeing it. Aside from the obvious requirement
of good eyesight, capable of detecting exceedingly faint objects,
a good telescope of at least moderate aperture, and a familiarity
attained by experience with the large number of nebulæ resembling
telescopic comets, the comet-seeker, to be successful, must possess
in a high degree the qualities of patience, perseverance, energy, and
enthusiasm. I have the highest admiration for the man or woman who
discovers a comet, because I know of the hard and thorough work which
the success implies.”
Mr. Brooks’s experience and success in this branch give weight to
his suggestions, and there can be little doubt that his commendation
of moderate powers is fully justified. I believe he usually sweeps
with a power of 40 (field of 1° 20′) on the 10-1/8-inch equatoreal
of his observatory. Speaking for myself, I find powers of 32 (field
1-1/4°) and 40 (field 1°) perform very satisfactorily on my 10-inch
With-Browning reflector, having frequently tried them on faint
nebulæ and comets. Sometimes I employ a power of 60, field 50′; but
for ordinary purposes this is too high. It is a good plan to sweep
with a moderate power, say of 40, and to keep a higher magnifier at
hand to examine any suspicious objects that may be picked up. With
power 32 I often encounter forms, the real character of which is
uncertain. In such cases I clamp the telescope and apply the power
60, which generally exhibits the objects as several minute stars
grouped together, or possibly nebulæ, in which case I proceed to
identify them. With lower magnifiers than 30 there must always be
considerable danger of sweeping over faint comets. Some of these are
only of the 10th, 11th, or 12th mag., and less than 1′ diameter, and
must certainly elude detection unless adequate power is brought to
bear upon them. Dr. Doberck mentioned in the L. A. S. Journal, vol.
vi. p. 236, an instrument for comet-seeking, 3½ inches in aperture,
power about 10, and field of 5°, which was bought in 1842 by the late
Mr. Cooper at Markree. But though with such a telescope a very large
portion of the firmament might be swept in one night, there would be
serious disadvantages; for small faint comets would pass through the
field unseen, and render the work abortive. The necessary conditions
of the case go far to support the view that moderate powers and
fields are best; for a search, to be thorough and satisfactory, must
be done critically, and with a power capable of revealing the smallest
specimens of comets.
_Annual Rate of Discovery._—Arranging cometary discoveries during the
century from 1782 to 1881 into periods of 20 years, and comparing
the annual average with that during the last eight years, we get the
following numbers:—
Comets Annual
Period. found. average.
1782-1801 25 1·25
1802-1821 26 1·30
1822-1841 36 1·80
1842-1861 83 4·15
1862-1881 79 3·95
1882-1889 40 5·00
These discoveries seem to have been greatly accelerated about the year
1845. The yearly average between 1842 and 1881 was about 4; but between
1882 and 1889 it increased to 5, owing mainly to the diligence of
Barnard and Brooks.
The months in which the largest number of cometary discoveries have
been effected are July and August, the figures since 1782 being—
Comets
Month. found. Percentage.
January 22 7·6
February 20 6·9
March 18 6·2
April 25 8·7
May 17 5·9
June 21 7·3
July 34 11·8
August 38 13·2
September 22 7·6
October 20 6·9
November 26 9·0
December 26 9·0
Of 289 comets discovered during the last 108 years, 123 belonged to
the first six months, while no less than 166 belonged to the last half
of the year.
Though comets are not confined to any special region of the heavens,
there is no doubt that the vicinity of the Sun is the spot to which
the comet-seeker should direct his chief attention. It is here where
the majority of the discoveries have been made; and theoretically this
should be so, seeing that the Sun is the controlling influence of the
cometary flights, and that his position must be regarded as a sort of
focus of their convergence and divergence. Hence the most likely spots
are over the western horizon after sunset and the eastern horizon
before sunrise. The twilight and zodiacal light, together with the
mist at low altitudes, are impediments which are inseparable from this
work; but they need not interfere to any serious extent if the observer
is careful to make the best of his opportunities. But though special
attention is recommended to the neighbourhood of the Sun, other regions
should not be altogether neglected, for comets are occasionally found
in nearly the opposite part of the heavens to the Sun’s place, as, for
example, Zona’s Comet of November 1890. In order to save time, and to
prevent troublesome references during the progress of sweeping, the
brighter nebulæ should be marked upon a star-chart, so that, as they
enter the field, they may be instantly identified.
_Telescopic Comets_ vary in size to a considerable degree. In diameter
they generally range from about 1′ to 7′, and are usually round, with
a bright centre like the globular clusters Messier 2, 3, 13, 15, 49,
and 92, as seen with a low power; but occasionally they are faint
diffused masses, like the planetary nebula near β Ursæ Majoris, M. 97,
or the large nebula S. of ξ Cassiopeiæ, in the New General Catalogue,
No. 185, R.A. 0^h 33^m, Dec. 47° 44′ N. In brightness they range
from being visible to the naked eye to objects of the last degree of
faintness. They average some 2′ or 3′ diameter, but are sometimes less
than 1′; so that the power of the sweeper should be capable of readily
showing an object of this size as it passes through the field. The
observer should turn his instrument upon the small planetary nebula
N.G.C. 1501, R.A. 3^h 57^m, Dec. 60° 37′ N. in Camelopardus. It is
about 1′ diameter. He should also pick up N.G.C. 6654, R.A. 18^h 27^m,
Dec. 73° 6′ N., which is a star of about 12½ mag. involved in a pretty
conspicuous nebulosity. Swift describes the latter as looking just
like a comet. N.G.C. 6217, R.A. 16^h 38^m, Dec. 78° 25′ N., is also a
small nebulosity which might easily be overlooked with a low power.
Let the observer examine the three objects named, and he will gather
a good idea of a small telescopic comet, especially from N.G.C. 6654,
which may be readily found, as it is in the same field as χ Draconis,
and visible at any time of the year and night. N.G.C. 6643, R.A. 18^h
23^m, Dec. 74° 32′ N., is near the latter, but it is a brighter object.
The observer will find two tolerably plain nebulæ in the same field at
about R.A. 6^h 52^m, Dec. 85° 56′; so that they are only 4° from the
pole. They are N.G.C. 2276 and 2300. These objects ought not to elude
detection in any instrument properly adapted for comet-seeking.
_Ascertaining Positions._—No observer should be without the means
of determining exact positions. A ring-micrometer and comprehensive
star-catalogues are most important accessories of the amateur. When a
suspicious object is found its precise position should be instantly
measured; but if no micrometer is at hand, the observer should
carefully note the place relatively to adjoining stars, and then,
after a short interval, re-observe it for traces of motion. In these
comparisons the low-power eyepiece should be exchanged for one of
greater amplification, because this will render a slight motion more
readily sensible. If the suspicious object proves to be a comet, the
extent and direction of its daily motion should be computed from the
change in the observed places, and the information telegraphed to the
Royal Observatory, Greenwich. A statement should also be given as to
the diameter and brightness of the object; we may then be satisfied
that it will be readily picked up at some of the many stations where
prompt attention is given to this class of observation. Amateurs who do
not attempt to obtain exact positions are sometimes condemned for their
negligence in this respect, and most unjustly so. By far the hardest
part of the work falls to them, and professional astronomers ought to
be indebted to amateurs for leaving to their care an important feature
of these observations. If the latter are to undertake the labour of
measuring as well as discovering comets, then there will be nothing
left in this line for the elaborate instruments of observatories to do.
Yet, while thus objecting to amateurs, with their generally incomplete
and inefficient appliances, being expected to perform the work both
of discovery and exact observation, it cannot be denied that there is
a great necessity for them to have the means of measurement, and to
utilize them during the first few observations, which are usually made
before the comet has been seen elsewhere, and will therefore possess
great value if precise.
_Dr. Doberck’s Hints._—Dr. Doberck has given some useful hints in
connection with this subject:—“In order to be as sure as possible of
ultimate success it is not enough to sweep with the instrument and
watch any suspicious object for proper motion. It is better to procure
a large map such as Argelander’s, and, comparing the image seen in
the comet-seeker with the map, to insert all the nebulous objects
according as they are discovered. At the end of the watch they are
then compared with the catalogues of nebulæ and clusters of stars. A
general catalogue facilitates this, but is never quite sufficient, as
there seems to be no limit to the number of objects in the sky, and
more are constantly being catalogued. In the course of time an observer
learns to remember the objects he has seen before in the seeker, and
at last he need not consult the map at all. The subsequent observation
of a newly-found comet is best made with the ring-micrometer if the
telescope is not equatoreally mounted. In the latter case it should be
made by aid of a steel-bar micrometer. As soon as three observations
are available the first approximation to a parabolic orbit can
generally be determined: the calculation of which is quite elementary,
and would be enjoyed by many amateur astronomers who are fond of
figures and would easily get used to Olbers’s method. Only the three
positions must not be so near each other as to lie on a great circle.”
_Prizes for Discoveries._—The Vienna Academy of Sciences formerly gave
a gold medal to the discoverer of every new comet. These presentations
were discontinued in about 1880; and Mr. H. H. Warner then offered a
prize of $200 for every unexpected comet found in the United States
or in Canada. This prize was continued in subsequent years, and the
conditions were amended so as to include observers in Europe. Many
of these prizes were gained by Barnard and Brooks; but they have not
been re-offered during the past year or two. Mr. Warner, however,
contemplates renewing them. The Astronomical Society of the Pacific now
awards a bronze medal to all such discoverers.
FOOTNOTES:
[41] Donati’s Comet of 1858 and Coggia’s Comet of 1874 may be mentioned
as good examples of the gradual approach and development of these
visitors witnessed by means of the telescope.
[42] It ought, perhaps, in the present state of our knowledge, to be
termed “the Neptune of comets;” for it has the longest period of any
comet whose path has been definitely ascertained by multiple returns to
perihelion.
[43] Encke’s Comet has the shortest period of all the known comets.
[44] Newton conjectured that comets formed “the aliment by which suns
are sustained,” his opinion being that the former bodies finally
coalesced with the suns round which they revolved. He remarked:—“I
cannot say when the Comet of 1680 will fall into the Sun,—possibly
after five or six revolutions; but whenever that time shall arrive, the
heat of the Sun will be raised by it to such a point that our globe
will be burnt and all the animals upon it will perish.”
CHAPTER XV.
_METEORS AND METEORIC OBSERVATIONS._
Ancient ideas concerning Meteors.—Meteoric
Apparitions.—Radiation of Meteors.—Identity of Meteors
and Comets.—Aerolites.—Fireballs.—Differences of
Motion.—Nomenclature of Meteor-Systems.—Meteor-Storms.—Telescopic
Meteors.—Meteor-Showers.—Varieties of Meteors.—Heights.—Meteoric
Observations.
“As oft along the still and pure serene
At nightfall, glides a sudden trail of fire,
Attracting with involuntary heed
The eye to follow it, erewhile it rest;
And seems some star that shifted place in heaven.”
DANTE.
No one can contemplate the firmament for long on a clear moonless
night without noticing one or more of those luminous objects called
shooting-stars. They are particularly numerous in the autumnal months,
and will sometimes attract special attention either by their frequency
of apparition or by their excessive brilliancy in individual cases.
For many ages little was known of these bodies, though some of the
ancient philosophers appear to have formed correct ideas as to their
astronomical nature. Humboldt says that Diogenes of Apollonia, who
probably belonged to the period intermediate between Anaxagoras and
Democritus, expressed the opinion that, “together with the visible
stars, there are invisible ones which are therefore without names.
These sometimes fall upon the Earth and are extinguished, as took place
with the star of stone which fell at Ægos Potamoi.” Plutarch, in the
‘Life of Lysander,’ remarks:—“Falling stars are not emanations or
rejected portions thrown off from the ethereal fire, which when they
come into our atmosphere are extinguished after being kindled: they
are, rather, celestial bodies which, having once had an impetus of
revolution, fall, or are cast down to the Earth, and are precipitated,
not only on inhabited countries, but also, and in greater numbers,
beyond these into the great sea, so that they remain concealed.”
In later times, however, opinions became less rational. Falling stars
were considered to be of a purely terrestrial nature, and originated by
exhalations in the upper regions of the air. Shakespeare expressed the
popular belief when he wrote:—
“I shall fall
Like a bright exhalation in the evening,
And no man see me more.”
Another theory, attributed to Laplace, Arago, and others, was that
meteors were ejections from lunar volcanoes. But these explanations
were not altogether satisfactory in their application. The truth is,
that men had commenced to theorize before they had begun to observe and
accumulate facts. They had learnt little or nothing as to the numbers,
directions, and appearances of meteors, and therefore possessed no
materials on which to found any plausible hypothesis to account for
them.
_Meteoric Apparitions._—The occasional apparition of brilliant
detonating fireballs, the occurrence of remarkable star-showers, the
precipitation upon the Earth’s surface of stony masses, were facts
which could be verified from many independent sources, and they set
men thinking how to account for the strange and startling freaks of
nature as exhibited in such phenomena. But though records existed of
exceptionally large meteors and of meteor-showers, the descriptions
were imperfect and failed in the most important details. The observers
were usually unprepared for witnessing such events, and gave
exaggerated and inaccurate accounts of what they had seen. The vivid
brightness of a fireball (overpowering the lustre of the stars, and
even vieing with the Moon in splendour), the flaming train left in its
wake (curling itself up into grotesque shapes, as it drifted and died
away), the form of the nucleus with its jets and sparks, and the final
explosion, with the reverberations it caused, were all alluded to by
the enthusiastic observer; but it was only in rare cases that the more
valuable features were placed on record. The _direction_ and _duration_
of the meteor’s flight amongst the stars were facts of greater
significance than the mere visible aspect of the object; but they were
seldom regarded. Hence the early observations proved of little weight
in inducing just conceptions as to the phenomena of meteors.
There is, perhaps, no celestial event which can compare, as regards
its striking aspect and interesting features, with that of a meteoric
display of the most brilliant kind. A large comet, a total solar
eclipse, a bright display of aurora, have each their attractive and
imposing forms; but the effect produced is hardly equal to that during
the Earth’s _rencontre_ with a dense meteor-swarm. The firmament
becomes alive with shooting-stars of every magnitude; their incessant
flights are directed to every point of the compass for several hours;
and the scene is so animated, and one of such peculiarly impressive and
novel character, that it can never be forgotten by those who have been
among its fortunate spectators.
_Radiation of Meteors._—Heis, in Germany, was the pioneer in this
branch of practical astronomy. About half a century ago he began
systematic observations, and gathered many useful data. Schmidt, at
Bonn and Athens, followed his example; and in England Prof. Alexander
Herschel and Mr. R. P. Greg devoted themselves to the subject with
highly successful results. Their collective labours revealed a large
number of well-defined systems of meteors, and enabled them to publish
tables of the radiant-points. The investigations were more precise than
formerly, and conducted on methods ensuring more accurate and plentiful
materials. The radiation of meteors from fixed points in the sky had
been observed before in regard to the great display which occurred
in November 1833; but the meteors that fell on ordinary nights were
regarded as sporadic, until Heis and his immediate successors showed
they were reducible to an orderly arrangement and that every one of
them had its radiant-point and its origin in a definite meteor-stream.
The apparently divergent flights from a common centre are simply due
to the effects of perspective on bodies really moving in parallel
directions and collected into groups more or less scattered.
[Illustration: Fig. 52.
Radiation of Meteors
(Shower of early Perseids from 32°+53°, July 28-Aug. 1, 1878.)]
_Identity of Meteors and Comets._—The mystery concerning these
fugitive objects and their vagaries of appearance was not always to
remain concealed. Denison Olmsted had, in his work on ‘The Mechanism
of the Heavens,’ published in 1850, stated that the constitution of
the body to which the meteors of 1833 belonged bore “a strong analogy
to comets.” Reichenbach, in 1858, wrote a paper in which it was sought
to prove that a comet is a swarm of meteorites. Prof. Kirkwood, in
1861, also concluded that “meteors and meteoric rings are the debris
of ancient but now disintegrated comets, whose matter has become
distributed around their orbits.” But it remained for Schiaparelli, of
Milan, in 1866, to demonstrate the identity of meteoric and cometary
systems. Others had reasoned up to it, and observers had amassed many
useful observations bearing on the subject; but absolute proof was
wanting until Schiaparelli supplied it. He computed elements for a
well-known shower of meteors occurring on August 10th, and found the
orbit presented a very close resemblance to that of Comet III. 1862;
and he detected a similar analogy between the November meteors and
Comet I. 1866. The orbit of the April meteors was afterwards shown
by Galle and Weiss to agree with the path of Comet I. 1861; and a
meteor-shower occurring at the end of November was found to coincide
with Biela’s Comet. Facts like these could not be disproved. Comets
were thenceforth known to be the parents—the derivative source—of
meteors. Thus two important classes of objects became as one, the
differences observed being merely those of aspect due to the variable
conditions under which they were presented. The great meteor-shower of
November was found to be the dispersed materials of Tempel’s Comet of
1866 seen in detail and from a near standpoint. Every meteoric display
was known to be the visible effects of the collision of the Earth with
a comet or with the great stream of planetary fragments describing a
cometary orbit.
[Illustration: Fig. 53.
1. Double meteor, Dec. 29, 1886. 2. Curved meteor, Dec. 25, 1886.
3. Fireball, Sept. 7, 1888.
]
[Illustration: Fig. 54.
Meteorite found in Chili in 1866.]
[Illustration: Fig. 55.
Meteorite which fell at Orgueil in 1864.]
_Aerolites._—Meteors enter our atmosphere with such great velocity
that the friction induced by their impact is sufficient to destroy
them by combustion. They rarely approach the Earth’s surface within 15
miles. Occasionally, however, a slow-moving meteor of large size, and
formed of a very compact substance, will penetrate entirely through the
air-strata and fall upon the Earth’s surface. Many instances of the
kind have been recorded, and a few of these are quoted below:—
1478 B.C. The Parian chronicle records that an aerolite or
thunder-stone fell in the island of Crete. This appears to be the
earliest stone-fall described in history.
654 B.C. A shower of stones descended near Rome.
465 B.C. A stone, surrounded with fire, fell in Thrace. This stone is
referred to by several ancient writers. It was termed the “Mother of
the Gods” and is said to have fallen at the feet of the poet Pindar.
52 B.C. A shower of iron descended at Lucania, in the time of Crassus.
1492 A.D. A stone weighing 262 lb. fell at Ensisheim, in Alsace.
1642. A stone of 4 lb. fell near Woodbridge, in Suffolk.
1795, Dec. 13. A stone of 56 lb. fell at Wold Cottage, Thwing,
Yorkshire.
1860, July 14. A shower of aerolites fell at Dhurmsala, in India. A
tremendous detonation attended their descent, and the natives became
greatly alarmed. They supposed the stones to have been thrown by some
of their deities from the summit of the Himalayas, and many of them
were preserved as objects of religious veneration.
1864, May 14. A very large meteor was observed in France. At Montauban
and the neighbourhood deafening explosions occurred, and showers of
stones fell near the villages of Orgueil and Nohic.
1876, April 20. A piece of iron weighing 7-3/4 lb. fell at Rowton,
Shropshire.
1881, March 14. A stone weighing 3 lb. 8-1/4 oz. fell at
Middlesborough, Yorkshire, on a part of the North-Eastern Railway
Company’s branch line. The descent of the aerolite was witnessed by an
inspector and three platelayers, who were working about fifty yards
distant. At first they became aware of a whizzing or rushing noise in
the air, immediately followed by the sudden blow of a body striking
the ground near. The hole, 11 inches deep, which the stone made was
found directly after, and the stone was extracted.
Many other examples might be given, but the above will be sufficient
for our purpose. Records of this nature were discredited in former
times; but more modern researches have long since placed their reality
beyond all question. The fall of stones from the sky is no longer
regarded as a mere legendary tale, but as one of the well-assured
operations of nature.
Meteoric stones and irons have been classified according to the
ingredients of their composition. Those in which iron is found in
considerable amount are termed siderites, those containing an admixture
of iron and stone, siderolites, and those consisting almost entirely of
stone are known as aerolites. The siderite which fell in Shropshire on
April 20, 1876, forms only the seventh recorded instance where a mass
of meteoric iron has been actually seen to fall.
_Fireballs._—The table on p. 268 gives the dates, heights, &c. of
fifteen fireballs observed during the last quarter of a century.
Fireballs are sometimes detonating, though more often silent. The
fireball of Nov. 23, 1877, gave a sound like salvoes of artillery,
and doors and windows were shaken violently. At Chester the noise of
its explosion was compared to loud but distant thunder. Lieut.-Col.
Tupman says that “thunder, to be loud, must be within five miles;
hence it appears that the violence of the explosion must have been at
least a hundred times greater than a peal of thunder, the intensity of
sound-waves diminishing as the square of the distance.” “The explosion
of a 13-inch bomb-shell, consisting of some 200 lb. of iron, would not
have produced a sound of one hundredth part of the intensity of the
meteor-explosion.” This fireball must therefore have been an object of
considerable mass before its dissolution; and it is fortunate that such
bodies are usually destroyed by the effects of combustion before they
reach the Earth’s surface.
These phenomena exhibit many varieties of appearance.
+——————————————+——————+————————————————————+————————+——————+
| | | Height. | Real | |
| Date of | +——————————+————————-+ Length |Velo- |
| Apparition. |G.M.T.| At Ap- |At Disap-|of Path.|city. |
| | |pearance. |pearance.| | |
+——————————————+——————+————————————————————+————————+——————+
| | h m| miles. | miles. | miles. |miles.|
|1865, April 29| 12 42| 52 | 37 | 75 |20 |
|1868, Sept. 5| 8 5| 250 | 85 | 1200 |28 |
|1869, Nov. 6| 6 50| 90 | 27 | 170 |35 |
|1872, July 22| 8 55| 77 | 37 | 88 | |
|1874, Aug. 10| 11 53| 77 | 33 | 105 |17 |
|1875, Sept. 3| 9 55| 75 | 40 | 35 |27 |
|1875, Sept. 14| 8 28| 52 | 13 | 104 |13 |
|1876, Sept. 24| 6 30| 58 | 16 | 45 |15 |
|1877, Nov. 23| 8 25| 95 | 14 | 135 |17½ |
|1878, June 7| 9 53| 65 | 37 | 160 |19 |
|1879, Feb. 23| 14 53| 60 | 7 | 102 |14½ |
|1886, Nov. 17| 7 18| 96 | 21 | 123 |17½ |
|1887, May 8| 8 22| 70 | 14 | 110 |18 |
|1888, Aug. 13| 11 33| 78 | 47 | 46 | |
|1889, May 29| 10 44| 58 | 23 | 76 | 8½ |
+——————————————+——————+————————————————————+————————+——————+
+——————————————+————————-+————————————————+
| |Radiant- | |
| Date of | Point. | |
| Apparition. | | Authority. |
| |R.A. Dec.| |
+——————————————+————————-+————————————————+
| | ° ° | |
|1865, April 29| 73 +47 |A. S. Herschel. |
|1868, Sept. 5| 14 -2 |G. von Niessl. |
|1869, Nov. 6| 62 +37 |A. S. Herschel. |
|1872, July 22|246 -11 |T. H. Waller. |
|1874, Aug. 10|325 -17 |W. H. Wood. |
|1875, Sept. 3|311 +52 |G. L. Tupman. |
|1875, Sept. 14|348 -0 |G. L. Tupman. |
|1876, Sept. 24|285 +35 |A. S. Herschel. |
|1877, Nov. 23| 62 +21 |G. L. Tupman. |
|1878, June 7|247 -25 |A. S. Herschel. |
|1879, Feb. 23|310 +55 |J. E. Clark. |
|1886, Nov. 17| 34 +19 |W. F. Denning. |
|1887, May 8|191 -5 |W. F. Denning. |
|1888, Aug. 13| 43 +56 |W. F. Denning. |
|1889, May 29|216 7 |D. Booth. |
+——————————————+————————-+————————————————+
[Illustration: Fig. 56.
Fireball of Nov. 23, 1877, 8^h 24^m, emerging from behind a cloud.
(Drawn by J. Plant, Salford.)]
Sometimes there is no visible explosion; the bright nucleus slowly
dies out until reduced to a faint spark before final disappearance.
Several outbursts of light are often noted; and a curious halting
motion has been observed in regard to large slow-moving meteors. I have
occasionally remarked a succession of four brilliant flashes given by
individual fireballs. These flashes, though sometimes of startling
intensity, are somewhat different to the transient vividness of
lightning; they come more softly, and remind one forcibly of moonlight
breaking suddenly from the clear intervals in passing clouds.
Fireballs differ vastly from shooting-stars in point of size; but their
origin is identical. The August meteor-shower yields the smallest
shooting-stars and the largest type of fireballs. The great display of
meteors on Nov. 27, 1885, not only presented us with large and small
members, but it also furnished us with a siderite or piece of iron,
presumably from Biela’s Comet. This fell at Mazapil, Mexico; and as
considerable interest is attached to the case, I quote a part of the
discoverer’s statement:—
“It was at about 9 o’clock on the night of November 27th, when I went
out to the corral to feed certain horses: suddenly I heard a loud
sizzing noise, exactly as though something red-hot was being plunged
into cold water; and almost instantly there followed a somewhat loud
thud. At once the corral was covered with a phosphorescent light;
while suspended in the air were small luminous sparks, as though from
a rocket.... A number of people came running towards me; and when we
had recovered from our fright we saw the light disappear, and bringing
lanterns to look for the cause found a hole in the ground, and in it a
ball of light. We retired to a distance, fearing it would explode and
harm us. Looking up to the sky, we saw from time to time exhalations of
stars, which soon went out without noise. We returned after a little,
and found in the hole a hot stone which we could barely handle; this,
on the next day, we saw looked like a piece of iron. All night it
rained stars; but we saw none fall to the ground, as they all seemed to
be extinguished while yet very high up.”
This is the first observed instance in which a meteorite has actually
reached the Earth’s surface during the progress of a star-shower.
If its identity with the meteors of Biela’s Comet is admitted, then
all classes of meteoric phenomena would appear to have a community of
origin.
_Differences of Motion._—Great differences are observed in the velocity
of meteors. An observer may notice all varieties on the same night
of observation. Some will move very slowly, others shoot quickly
across the sky. These differences are occasioned by the astronomical
conditions affecting the position of the meteor-orbit relatively to
the motion of the Earth. Thus the meteors of Nov. 13 move with great
velocity (44 miles per second), because they come directly from that
part of the heavens towards which the Earth is moving; hence the
orbital speed of the Earth (18½ miles per second) and meteors (26 miles
per second) is combined in the observed effects. But in the case of
the meteor-shower of Nov. 27 the motions are extremely slow (about 10
miles per second), as the Earth and the meteors are travelling nearly
parallel in the same direction, and the latter have to overtake the
Earth.
_Nomenclature of Meteor-Systems._—It is customary to name the showers
after the constellation from which the meteors appear to diverge. Thus
the meteors of April 20 are called _Lyrids_, the radiant being in Lyra;
the meteors of August 10 are termed _Perseids_, the point of emanation
being in Perseus. The two great streams of November are known as the
_Leonids_ (13th) and _Andromedes_ (27th). Several showers are often
visible in the same constellation; and when it is desired to name these
according to the above system, it is necessary to add the approximate
star to distinguish them. Thus, in August there are showers of μ
Perseids, ε Perseids, and α Perseids, in addition to the well-known
Perseids of August 10.
_Meteor-Storms._—On Nov. 12, 1799, Humboldt, at Cumana, in South
America, saw “thousands of bolides and falling stars succeed each other
during four hours.” On Nov. 12, 1833, this shower recurred, and was
witnessed with magnificent effect in America. One observer stated that
between 4 and 6 A.M. (Nov. 13) about 1000 meteors per minute might
have been counted! Another display occurred on Nov. 13, 1866, and on
this occasion 8485 meteors were enumerated by several observers at
Greenwich. A different system gave us a brilliant exhibition on Nov.
27, 1872, when 33,000 meteors were counted by Denza and his assistants
at Moncalieri, in Italy, between the hours of 5^h 50^m and 10^h 30^m
P.M. A repetition of this phenomenon occurred on Nov. 27, 1885, when
the same observers counted nearly 40,000 meteors between 6^h and 10^h
P.M.
[Illustration: Fig. 57.
VERTICAL.
Flight of Telescopic Meteors seen by W. R. Brooks, Nov. 28, 1883.]
_Telescopic Meteors._—Observers who are engaged in seeking for comets
or studying variable stars employ low powers and large fields, and
during the progress of their work notice a considerable number of
small meteors. At some periods these bodies are more plentiful than
at others, and appear in such rapid succession that the observer’s
attention is distracted from the special work he is pursuing to
watch them more narrowly and record their numbers. Schmidt saw 146
telescopic meteors during ten years. They ranged between the 7th and
11th mags. Winnecke in the year 1854 noticed 105 of these objects
on thirty-two evenings of observation with a 3-inch finder, power
15, and field of 3°. I have also remarked many of these objects when
using the comet-eyepieces of my 10-inch reflector[45], and find they
are apparently more numerous than the ordinary naked-eye meteors
in the proportion of 22 to 1. It would be supposed from the great
rapidity with which the latter shoot across the firmament that the
smaller telescopic meteors are scarcely distinguishable by their
motion, as they must dart through the field instantaneously and only
be perceptible as lines of light. But this impression is altogether
inconsistent with the appearances observed. They possess no such
velocity, but usually move with extreme slowness, and not unfrequently
the whole of the path is comprised within the same field of view.
The eye is enabled to follow them as they leisurely traverse their
courses, and to note peculiarities of aspect. Of course, there are
considerable differences of speed observed, but as a rule the rate is
decidedly slow and far less than that shown by naked-eye meteors. I
believe that telescopic meteors are situated at great heights in the
atmosphere, and that their diminutive size and slowness of movement
are due to their remoteness. This conclusion will hardly be avoided
by anyone who attentively studies the several classes of meteors in
their various aspects. Unfortunately no attempt appears to have been
hitherto made to determine the actual heights of telescopic meteors,
owing to the difficulty of obtaining two reliable observations of the
same object. The only way of securing such data would be for several
observers to watch certain selected regions by prearrangement either
with a low-power telescope or field-glass, and record the exact times
and paths of the meteors seen. On a comparison of the results a good
double observation of the same object might be found, in which case the
real path could be readily computed.
Future observers should note the different forms of telescopic meteors.
Safarik has divided them into four classes, viz.:—(1) Well-defined
star-like objects of very small size; (2) Large luminous bodies of
some minutes of arc in diameter; (3) Well-defined disks of a very
perceptible diameter brighter at the border than at the centre, which
gives them the aspect of hollow transparent shells; and (4) faint
diffused masses of irregular shape, considerable size, and different
colours. He has seen hundreds of meteors of every magnitude from the
2nd down to the 12th pass through the field of his 6½-inch reflector
(ordinary power 32, field 54′). On Aug. 30, 1880, 9^h to 15^h he
observed between 50 and 100 telescopic meteors, and many others were
seen on the following night. Whenever a shower of these bodies, such
as that witnessed by Brooks on Nov. 28, 1883, occurs, observers should
notice whether the objects participate in a common direction of motion;
because, if so, the radiant-point will admit of determination. The
horary rate of their apparition ought also to be ascertained. Those
who habitually search for comets should invariably make a note of
telescopic meteors, as such records would aid inquiries into the
relative frequency of these phenomena.
_Meteor Showers._—The following short list includes the principal
displays of the year:—
+———————————————-+——————————————-+————————+——————————————-+——————————+
| Name of | Duration. |Date of | Radiant- | Sun’s |
| Shower. | | Max. | Point. |Longitude.|
+———————————————-+——————————————-+————————+——————————————-+——————————+
| | | | α δ | |
| | | | ° ° | ° |
|Quadrantids |Dec. 28-Jan. 4 |Jan. 2 | 229·8 +52·5 | 281·6 |
|Lyrids |April 16-22 |April 20| 269·7 +32·5 | 31·3 |
|η Aquarids |April 30-May 6 |May 6 | 337·6 - 2·1 | 46·3 |
|δ Aquarids |July 23-Aug. 25|July 28 | 339·4 -11·6 | 125·6 |
|Perseids |July 8-Aug. 22 |Aug. 10 | 45·9 +56·9 | 138·5 |
|Orionids |Oct. 9-29 |Oct. 18 | 92·1 +15·5 | 205·9 |
|Leonids |Nov. 9-17 |Nov. 13 | 150·0 +22·9 | 231·5 |
|Andromedes |Nov. 25-30 |Nov. 27 | 25·3 +43·8 | 245·8 |
|Geminids |Dec. 1-14 |Dec. 10 | 108·1 +32·6 | 259·5 |
+———————————————-+——————————————-+————————+——————————————-+——————————+
_Notes._
_Quadrantids._ Heis was the first to determine this radiant accurately.
It was subsequently observed by Masters and Prof. Herschel (1863-4).
The radiant is circumpolar in this latitude, but low down during the
greater part of the night, hence the display is usually seen to the
best advantage on the morning of Jan. 2.
_Lyrids._ Attention was first drawn to the April meteors by Herrick in
the United States. Active displays occurred in 1863 and 1884.
η _Aquarids._ Further observations are urgently required of this
stream. The radiant is only visible for a short time before sunrise.
There is a considerable difference between my results and those secured
by Lieut.-Col. Tupman, the discoverer of this system in 1870, whose
observations place the radiant at 326½—2½ April 29-May 3. These May
Aquarids are interesting from the fact that they present an orbital
resemblance to Halley’s Comet, which makes a near approach to the Earth
on May 4, twelve days before reaching the descending node.
δ _Aquarids._ The meteoric epoch, July 26-30, was first pointed out
by Quetelet many years ago. Biot also found, from the oldest Chinese
observations, a general maximum between July 18 and 27 (Humboldt).
Showers of Aquarids were remarked by Schmidt, Tupman (1870), and
others; but it was not known until my observations in 1878 that the
Aquarids formed the special display of the epoch, and that there were
many early Perseids visible at the same time.
_Perseids._ Muschenbroeck, in his work on ‘Natural Philosophy,’ printed
in 1762, mentions that he observed shooting-stars to be more numerous
in August than in the other months of the year. Quetelet, in 1835, was,
however, the first to attribute a definite maximum to the 9th-10th.
This stream is remarkable for its extended duration, and for the
obvious displacement which occurs from night to night in the place of
its radiant. It furnishes an annual display of considerable strength,
and is, perhaps, the best known system of all.
_Orionids._ Profs. Schmidt and Herschel were the first to discover
the Orionids as the most brilliant display of the October period, and
accurately determined its radiant in 1863-4-5. Herrick recorded a
shower at 99° +26°, Oct. 20-26, 1839, and Zezioli in 1868 recorded many
meteors which were ascribed to a radiant at 111° +29°; but there is no
doubt that the Orionids were observed in both these cases, though the
radiant was badly assigned.
The radiant of the Orionids shows no displacement like that of the
Perseids.
_Leonids._ Observed from the earliest times. Humboldt and Bonpland saw
it well on the night of November 11-12, 1799, and the phenomenon at its
magnificent return on November 12, 1833, was ably discussed by Olmsted.
It furnished a splendid shower in 1866, November 13, and many meteors
were seen at the few subsequent returns. I observed fairly conspicuous
showers of Leonids in 1879 and 1888. There is no doubt the meteors form
a complete ellipse, for the earth encounters a few of them at every
passage through the node. Grand displays may be expected at the end of
this century.
_Andromedes._ Observed by Brandes, at Hamburg, Dec. 7, 1798. It also
recurred in 1838; the very brilliant showers of November 27, 1872 and
1885, are still fresh in the memory. It is uncertain whether this group
forms an unbroken stream; if so, the regions far removed from the
parent comet must be extremely attenuated. Some of the meteors were
seen in 1877 and 1879. The radiant is diffuse to the extent of 7° or
10°. Returns of the shower should be looked for in 1892 and 1898.
_Geminids._ Mr. Greg first called attention to the importance of this
shower. It was well observed by Prof. Herschel in 1861-3-4, and some
later years.
There are an enormous number of minor systems, but these are generally
feeble, and interesting only to the regular observer of meteors. Many
showers are so slightly manifested that they yield but one visible
meteor in 6 or 7 hours, and on the same night of observation there are
often as many as 50 or 60 different systems in operation. I gave a list
of 918 radiant-points of showers observed at Bristol in the ‘Monthly
Notices,’ May 1890, and other catalogues will be found in the ‘British
Association Reports’ for 1874 and 1878.
_Varieties of Meteors._—The amateur who systematically watches for
meteors will occasionally remark instances of anomalous character.
I have sometimes observed meteors which are apparently very near,
and move with enormous velocity. They are mere gleams of pale light,
which have little analogy to ordinary shooting-stars, and suggest an
electric origin, though I do not know whether the marvellous quickness
with which they flash upon the eye is not to be held responsible for
the impression of nearness. They are somewhat rare, and one may watch
through several entire nights without a single example, but as far as
my memory serves I must have witnessed some scores of these meteoric
flashes.
One of the most interesting class of meteors includes those which
move so slowly that the eye is enabled to note the details of their
appearance. Some of these objects are small when first seen, but
enlarge considerably under the increasing temperature, and after a
great slackening of speed (due to atmospheric resistance) their nuclei
are finally spent in thick streams of luminous dust. On Dec. 28, 1888,
I recorded a meteor which on its first apparition was tolerably bright,
small, and compact. It moved slowly, and I had an excellent view of
its passage. The nucleus quickly expanded, though with no increase of
brilliancy. Towards the end it assumed a sensible disk, and at the
last phase the mass spread or deployed itself into a wide stream of
fine ashes and disappeared. The whole phenomenon was so curious, and
observed with such distinctness, that I made the above sketch of it
directly afterwards.
[Illustration: Fig. 58.
Meteor of Dec. 28, 1888, 6^h 17^m.]
_Heights of Meteors._—Usually the height of meteors at their first
appearance is less than 90 miles, and at disappearance more than 40
miles. From a comparison of a large number of computations I derived
the following average values:—
Beginning height 76·4 miles (683 meteors)
End height 50·8 ” (756 ” )
But if fireballs and the smaller shooting-stars are separated I
find the usual heights at disappearance are:—fireballs, 30 miles;
shooting-stars, 54 miles. Fireballs therefore approach much nearer to
the Earth’s surface before disruption than the ordinary falling stars.
[Illustration: Fig. 59.
Large Meteor, and successive appearances of its streak, seen at Cape
Jask, in the Persian Gulf, on June 8, 1883, 7^h 51^m to 8^h 33^m.]
A very slight acquaintance with trigonometry will enable anyone to
compute the real path of a meteor if two or more observations, made at
distant stations, are available for the purpose. The observed courses
of the meteor should be marked upon a celestial globe, and extended
backwards to the point where they mutually intersect; this will be the
_radiant-point_. The globe having been set for the time and latitude,
the apparent tracks should also be prolonged in a forward direction
until they meet the horizon, this will indicate the _Earth-points_, or
azimuths of the place where the meteor would have been precipitated
on the Earth had it been enabled to continue its flight so far. The
azimuths and altitudes of the beginning and end of the path, and the
azimuths of the Earth-point should then be read off, and by means of
a reliable map and a protractor their points of intersection over the
Earth’s surface may be readily found by lines drawn from the two
places of observation. From the spot where the Earth-points intersect a
straight line should also be drawn in the direction of the radiant, and
it is along this line the meteor’s motion was directed. The coordinates
of the observed points of appearance and disappearance of the meteor,
at the two stations, would intersect this line at identical points were
the observations perfectly accurate, but this is rarely the case. The
distance between the observer’s station and the places over which the
meteor began and ended is easily derived from the map, and the height
of the object may be found by adding the logarithm of the distance to
the log. of the tangent of the altitude. Thus, if the end of a meteor
is witnessed from London in azimuth 130° W. of S. (alt. 25°), and from
Bristol in azimuth 216° W. of S. (alt. 30°) the place of intersection
on the map will be at Warwick, so that the meteor must have disappeared
when vertically over this city. London is distant from Warwick about
86 miles, and from Bristol 70 miles, and the resulting height of the
meteor is:—
London. Bristol.
86 log. 1·93450 70 log. 1·84510
25° tan 9·66867 30° tan 9·76144
———- ———-
1·60317 = 40·1 1·60654 = 40·4
so that the observations accord very closely in fixing the height at a
little exceeding 40 miles at disappearance, but a slight correction is
necessary to allow for the Earth’s curvature. There are other methods
of computing the heights, one of which is explained by Prof. A. S.
Herschel in a paper entitled “Height of a Meteor” (‘Monthly Notices,’
vol. xxv. p. 251).
_Meteoric Observations._—A large number of meteor-showers still await
discovery, and there are features even in connection with the best
known streams which remain to be elucidated. Such doubts as now exist
are only to be cleared away by assiduous observation made with the
utmost accuracy possible both of the _directions_ and _durations_ of
meteors.
This attractive field of investigation has certainly been neglected
in recent years, and the reason of this may perhaps be found in the
complications inseparable from it, in the need of great patience
and scrupulous care in observation, and the necessity of gaining
experience before the observer can feel a reliance on his work,
and draw safe conclusions. Meteors are so fugitive, so diverse and
erratic in their apparitions, as to be quite beyond the scope of
instrumental refinements. They must necessarily be observed under many
disadvantages. Positions have to be fixed from very hurried and often
imperfect impressions. But these drawbacks, formidable as they at
first appear, may be severally overcome by practice, by careful regard
for the conditions under which meteors are displayed, and the marked
differences of aspect induced by these conditions. When the observer
has acquired a practical knowledge he will proceed with confidence in
his work, and avoid many of the difficulties surrounding it.
In recording meteor-tracks for the purpose of discovering the
radiant-points, the chief feature in which precision is essential is
the _direction_ of flight. A perfectly straight wand, held in the hand
for the purpose, should be projected upon the path of every meteor
directly it is seen, and then when the eye has quickly noted the
position and slope relatively to the fixed stars near, it should be
reproduced on the chart or celestial globe. The time, mag., estimated
duration, and details of appearance should be registered in a tabular
form, with the R.A. and Dec. of the beginning-point and probable
radiant. The end-point and length of path may be left until next day,
in order to save valuable time. The wand is a great assistance to the
eye in retaining the approximate directions and noting the places.
If a meteor belongs to the slow, trained class, or if it belongs to
the swift, streak-leaving order, the path may be very accurately
noted, for the wand can be adjusted to its direction before the meteor
or its visible offcome has died away. In the case of short, quick
meteors, devoid of either streaks or trains, and generally shooting
from radiants at high altitudes, they are more difficult to secure, as
they vanish before one may turn, and the observer must rely upon the
mere impression he received. But even these succumb to experience, and
will be found to resolve themselves into a number of sharply defined
radiants scarcely less certain than the positions derived from the
streaked or trained meteors.
These positions are only to be fixed by the exercise of much cautious
discrimination on the part of the observer, for the direction of the
flight is not sufficient, alone, to indicate it. The visible aspect of
the meteor has to be equally considered, for the place of its radiant
imparts certain peculiarities to it which are rarely to be mistaken.
First, _the astronomical position_ of the radiant. If the radiant is
at, or within 50° of, the Earth’s apex (a point 90° preceding the Sun
along the ecliptic, and towards which the Earth’s motion is directed)
the meteors generally leave streaks, especially the brighter ones, and
move with great speed. They are usually white, exhibiting a high degree
of incandescence. If the radiant is near the anti-apex or anywhere
in the anti-apex half-sphere the meteors are streakless, they travel
slowly or very slowly, and often leave trails of yellowish sparks.
Bearing these facts in mind the region may be assigned in which any
radiant is situated, if not the exact position of the radiant itself.
If, say, on Aug. 10, at midnight a swift, streaked meteor is seen
shooting from the Pleiades towards Aldebaran, just risen, the radiant
is either in Musca, Triangulum, or Andromeda. But if the meteor is
slow, with a train, then we must go further back in the direction of
its flight, and seek the radiant in the S. or S.W. sky. If the motion
is very slow, the radiant may be as far away as Aquila. Second, _the
sensible position of the radiant_. A low radiant yields long-pathed
meteors, characterized by slowness of speed and a flaky appearance
either of the streaks or trains. A radiant near the zenith gives
short, darting meteors, with rather dense streaks or trains. These
nearly vertical meteors have a less extensive range of atmosphere to
penetrate than the horizontal meteors, which are sometimes abnormally
long. In the case of brilliant meteors, however, the paths occasionally
extend over considerable arcs though the radiant may be high. Third,
_the position of the radiant relatively to the path of a meteor_. If a
meteor is close to its radiant its track is usually slow, and appears
greatly foreshortened by the effects of perspective. It is travelling
(approaching) nearly in the line of sight, and the streak or offcome
of sparks is especially dense because it is seen through its entire
depth; and the nucleus in such a case has a brushy diffused appearance.
Such meteors often traverse sinuous, or curved paths of 2°, 3°, or 4°,
and they are readily distinguishable from other meteors far from the
radiants to which they belong.
A good method of tabulating meteor-tracks is that adopted by
Lieut.-Col. Tupman in his catalogue published by the British
Association in 1874. I have adopted the same form, and herewith append
a copy of my register of a few isolated bright meteors observed in the
autumn of 1890:—
+————————+——————+——————+————————————————————-+——————+————-+
| | | | Observed Path | | |
| Date |G.M.T.| Mag. +————————-+——————————-+Length|Dura-|
| 1890. | | | From | To | of |tion.|
| | | |R.A. Dec.| R.A. Dec | Path.| |
+————————+——————+——————+————————————————————-+——————+————-+
| | h m| | o o | o o | o | sec.|
|Oct+. 17| 10 37| >1 | 219+61 | 255+65 | 16 | 3·5 |
| 19| 10 35| 1 | 61½+26 | 44½+27½ | 15½ | 0·7 |
| 19| 12 0| ½ ☽ | 326-8 | 319-10 | 7 | 0·5 |
| 25| 17 18| >♃ | 168+34 | 180+24 | 14½ | 0·8 |
| 26| 7 33| ♃ | 329+69 | 243+51 | 42 | 4·0 |
|Nov. 1 | 7 1| >1 | 278+49 | 244+11½ | 46 | 6·0 |
| 1 | 9 17| >1 | 345+11 | 307+1½ | 39 | 4·0 |
| 5 | 10 40| >♀ | 28½-25 | 25½-29½ | 5½ | 0·7 |
| 16 | 11 15| ♃ | 274+77 | 265½+67 | 10 | 1·5 |
+————————+——————+——————+————————————————————-+——————+————-+
+————————+—————————————————————+————————+
| | | |
| Date | Appearance. |Probable|
| 1890. | |Radiant.|
| | | |
+————————+—————————————————————+————————+
| | | o o |
|Oct+. 17| V. slow, B. train. | 204+56.|
| 19| Swift, streak. | Orion. |
| 19| Swift, streak. | Orion. |
| 25 |Swift, streak. | Lynx. |
| 26| Slow. | 32+18. |
|Nov. 1 | Very slow. | 50+15. |
| 1 | Slow. | 50+15. |
| 5 | Swift, strk. 15 sec.| Taurus.|
| 16 | Not very swift. | Auriga.|
+————————+—————————————————————+————————+
The _duration of flight_ is a most important element to estimate
correctly, as it affords data wherewith the real velocity may be
computed, and enables the nature of the orbit in which the meteor is
moving to be definitely assigned. This feature is, however, one of the
most difficult of all to derive with satisfactory precision. In the
case of very slow meteors lasting several seconds, it is easy by means
of a stop-watch, or by other methods, to get the times of flight within
narrow limits of error, but the swifter class of meteors complete their
visible trajectories in the fraction of a second, and are gone before
any effort can be made to gauge their durations, so that a value has to
be attributed which is little better than a mere guess.
Every adopted radiant-point should be based on at least five paths,
unless the conditions are special, and these must show a very definite
centre, and present family resemblances. It is often possible to detect
a good centre from very few paths, when the radiant is low on the
horizon, or when it occupies an isolated position.
In recording meteors the details of their appearances should also be
appended to the paths. Foreshortened and crooked courses, fluctuations
of brightness, halting motion, spark-trains, phosphorescent streaks,
broken streaks, and other features must be invariably noted when
observed, as likely to assist in fully comprehending these bodies. A
streak will sometimes brighten up perceptibly after the head has died
out.
One of the principal aims of future observers should be to ascertain
the visible duration of meteor-showers, and the displacement or fixed
position of the radiants during the period of their continuance. The
Perseids seem to endure for forty-six nights (July 8-August 22) while
the radiant moves from 3° +49° to 76° +57°. The Lyrids also exhibit
a shifting radiant, and it is highly probable some other showers are
to be included in the same category. In investigating these, the
observations of single nights should be kept separate, and the radiant
determined from each set of paths. The positions when compared will
then exhibit the rate and direction of the displacement. As to radiants
which are apparently stationary[46] during long intervals, these should
be closely observed. Are the centres of radiation, as successively
determined, identical, allowing for the slight errors of observation?
Are they continuously in operation, or intermittent? Meteors with
motions in declination and near their radiants will be specially
valuable in settling these questions, and if observed at more than one
station will possess great significance. If it can be proved that a
radiant is fixed and continuous during a few weeks, there can be no
reason why it may not be stationary for a much more lengthy interval,
unless the circumstances are exceptional.
Though I have pointed out the urgency of noting the directions and
durations of meteors, there are other features in such observations
that must not be disregarded. If the paths are being recorded for the
particular purpose of getting duplicate observations and calculating
the heights, then it is desirable to note the beginning-and end-points
of the flights as exactly as possible, for unless this is done the
combined paths will show great discordances. Those who have acquired
a familiar knowledge of the constellations will, however, experience
little trouble in insuring accuracy in these records.
Observers, particularly those residing in towns, must be constantly on
their guard against mistakes in identifying meteors from terrestrial
objects such as fire-balloons and the various forms of pyrotechnic
display. That such caution is necessary will be admitted when we read
the two following letters, which were published in the ‘Times’ some
years ago:—
“SIR,—
“A large meteor was seen to-night at 8.27, moving very slowly along
the northern horizon, from west to east, at an altitude of about 8
deg. It was at least three times as brilliant as Venus, remaining
visible for nearly five minutes, moving slower than any hitherto
observed. I should be glad to receive observations made at more
favourable stations....
“I remain, Sir, your obedient Servant,
“THOMAS CRUMPLEN.
“Mr. Slater’s Observatory, Euston Road, August 10th.”
“SIR,—
“The ‘large meteor’ seen by Mr. Crumplen on Monday evening at 8.27,
three times as brilliant as Venus, and moving from west to east,
was a fire-balloon sent up shortly after 8 o’clock from the Eton
and Middlesex Cricket Ground, Primrose Hill, as a _finale_ to some
athletic sports which had taken place during the afternoon.
“I am, Sir, your obedient Servant,
“B. C. C.
“St. John’s Wood, August 12th.”
In concluding this chapter I may briefly mention that an old idea
concerning meteors was that they originated gales of wind, and that, in
fact, they were the usual precursors of stormy weather. This belief is
thus expressed in Dryden’s ‘Virgil’:—
“Oft shalt thou see, ere brooding storms arise,
Star after star glide headlong down the skies,
And, where they shot, long trails of lingering light,
Sweep far behind, and gild the shades of night.”
FOOTNOTES:
[45] During the seven months from May to November 1890 I noted
ninety-five telescopic meteors while engaged in comet-seeking.
[46] A list of these was published in the ‘Monthly Notices,’ vol. 1. p.
466. See also ‘Monthly Notices,’ vol. xlv. pp. 93 _et seq._
CHAPTER XVI.
_THE STARS._
Sidereal Work.—Greek alphabet.—Learning the Names of the
Stars.—The Constellation figures.—Means of Measurement.—Dividing
power.—Number of Stars.—Magnitudes.—The Milky Way.—Scintillation
of the Stars.—Star-Disks.—Distance of the Stars.—Proper Motion of
Stars.—Double Stars and Binary Systems.—Variable Stars.—New or
Temporary Stars.—Star Colours.—Groups of Stars.—Further Observations.
“Ten thousand suns appear
Of elder beam; which ask no leave to shine
Of our terrestrial star, nor borrow light
From the proud regent of our scanty day.”
BARBAULD.
The planetary observer has to accept such opportunities as are given
him; he must use his telescope at the particular seasons when his
objects are well presented. These are limited in number, and months may
pass without one of them coming under favourable review. In stellar
work no such irregularities can affect the progress of observations.
The student of sidereal astronomy has a vast field to explore, and a
diversity of objects of infinite extent. They are so various in their
lustre, in their grouping, and in their colours, that the observer’s
interest is actively retained in his work, and we often find him
pursuing it with unflagging diligence through many years. No doubt
there would be many others employing their energies in this rich field
of labour but for the uninteresting character of star-disks, which
are mere points of light, and therefore incapable of displaying any
detail. Those who study the Sun, Moon, or planets have a large amount
of surface-configuration to examine and delineate, and this is ever
undergoing real or apparent changes. But this is wholly wanting in
the telescopic images of stars, which exhibit a sameness and lack
of detail that is not satisfying to the tastes of every observer.
True there are some beautiful contrasts of colour and many striking
differences of magnitude in double stars; there are also the varying
position and distance of binary systems, the curious and mysterious
fluctuations in variable stars, and some other peculiarities of stellar
phenomena which must, and ever will, attract all the attention that
such important and pleasing features deserve. And these, it must be
conceded, form adequate compensation for any other shortcomings. The
observer who is led to study the stars by comparisons of colour and
magnitude or measures of position, will not only find ample materials
for a life-long research, but will meet with many objects affording him
special entertainment. And his work, if rightly directed and accurately
performed, will certainly add something to our knowledge of a branch in
which he will certainly find much delectation.
_Greek Alphabet._—The amateur must, at the outset of his career,
thoroughly master the Greek alphabet. This will prevent many
time-wasting references afterwards, and avoid the doubt and confusion
that must otherwise result. The naked-eye stars in each constellation
have Greek letters affixed to them on our celestial globes and
star-maps.
α Alpha
β Beta
γ Gamma
δ Delta
ε Epsīlon
ζ Zēta
η Eta
θ Theta
ι Iota
κ Kappa
λ Lambda
μ Mu
ν Nu
ξ Xi
ο Omīcron
π Pi
ρ Rho
σ Sigma
τ Tau
υ Upsīlon
φ Phi
χ Chi
ψ Psi
ω Omĕga.
The letters are applied progressively to the stars (generally according
to brightness) in each constellation. The 1st-mag. stars frequently
have a duplicate name. Thus α Leonis is also known as Regulus, and α
Canis Majoris as Sirius, the Dog-star.
_Learning the Names of the Stars._—A knowledge of the stars as they
are presented in the nocturnal sky may be regarded as the entrance to
the more advanced and difficult branches of the science, and forms
the young observer’s introductory lesson. When he has learnt a few of
the principal constellations, and can point them out to his friends,
he already begins to feel more at home with the subject, and regards
it with a different eye to what he did before when the names and
configurations of the stars were alike unknown to him. He no longer
views the heavens as a mysterious assemblage of confusing objects, for
here and there he espies certain well-known groups always preserving
the same relative positions to each other. The unconscious gaze he
formerly directed to the sky has given way to the intelligent look of
recognition with which he now surveys the firmament.
An acquaintance with the leading constellations, and with the names
or the letters of the brighter stars in each, becomes very important
in some departments of observation, and various methods have been
suggested as likely to impress the positions and names on the memory.
The beginner must first be content to get familiar with a few of the
brighter stars, and make these the base for extending his knowledge.
The objects are so numerous that it is impossible his primary attempts
can be anything like complete. He must advance step by step in
his survey, and feel his way cautiously, setting out from certain
conspicuous stars with which he has already become conversant. A
lantern and a series of star-maps are the only aids required, and
with these he ought to make satisfactory progress. The stars as they
are seen in the sky may be compared with those figured in the maps,
and their names and the constellations in which they lie may then be
identified. It is an excellent plan as conducing to fix the positions
indelibly in the memory to construct maps from personal observation,
and to compare these afterwards with the published maps for
identification of the constituent stars. This plan, if repeated several
times, has the effect of impressing the positions of the leading stars
forcibly upon the observer’s mind.
It is not intended to give, in this place, any details as to the
places or distribution of the stars. Without diagrams, such a
description could not be made readily intelligible. To those, however,
who are commencing their studies, I would recommend the northern sky as
the most suitable region to aid their initiatory efforts. For
“He who would scan the figured sky
Its brightest gems to tell,
Must first direct his mind’s eye north
And learn the Bear’s stars well.”
The seven bright stars of Ursa Major are familiar to nearly everyone.
Two of them, called the Pointers, serve to direct the eye to the Polar
star, which, though not a brilliant one, stands out prominently in a
region comparatively bare of large stars. It is important to know the
Polar star, as it is situated near the centre of the apparent motion
of the firmament. When the student has assured himself as to the
northern stars he will turn his attention southwards, and recognize the
beautiful Orion and the curious groups in Taurus. He will also observe,
much further east, the well-known sickle of Leo, and in time become
acquainted with the many other constellations that make the winter sky
so attractive.
[Illustration: Fig. 60.
The constellation Orion.]
_The Constellation Figures._—The observer will soon realize that the
creatures after which the constellations have been named bear no
resemblance to the configuration of the stars they represent. If we
look for a Bear amongst the stars of Ursa, for a Bull amid the stars of
Taurus, or for a flying Swan in the stars of Cygnus we shall utterly
fail to find it. The names appear to have been originally given, not
because of individual likenesses between them and the star-groups
to which they are applied, but simply on account of the necessity
of dividing the sky into parts, and giving each a distinguishing
appellation, so that it might be conveniently referred to. There were
pressing needs for a system of stellar nomenclature, and the plan of
grouping the stars into imaginary figures was the one adopted to avoid
the confusion of looking upon the sky as a whole. There are some who
object to the method of the Chaldean shepherds because the series
of grotesque figures on our star-maps and globes bear no natural
analogies. But it would be unwise to attempt an innovation in what has
been handed down to us from the myths of a remote antiquity, for
“Time doth consecrate,
And what is grey with age, becomes religion.”
[Illustration: Fig. 61.
Diagram illustrating the Measurement of Angles of Position.
(In measuring angles of position the larger star is always understood
as central in the field. The north point is zero, and the angles are
reckoned from this point towards the east. If a star has a faint
component lying exactly east or following it, then the angle is 90°;
if the smaller star is south, the angle is 180°; and so on.)]
_Means of Measurement._—A micrometer becomes an indispensable
instrument to those who make sidereal observations of an exact
character. Without such means it will be impossible to determine
either positions or distances except by mere estimation, and this
is not sufficiently precise for double-star work. With a reliable
micrometer[47] excellent results may be obtained, especially with
regard to the varying angles of binary systems. Frequent remeasurement
of these is desirable for comparison with the predicted places in
cases where the orbits have been computed. In this department of
astronomy the names of Herschel, South, Struve, Dawes, Dembowski,
Burnham, and others are honourably associated, and it is notable that
refracting-telescopes have accomplished nearly the whole of the work.
But reflectors are little less capable, though their powers seem to
have been but rarely employed in this field. Mr. Tarrant has lately
secured a large number of accurate measures with a 10-inch reflector
by Calver, and if care is taken to secure correct adjustment of the
mirrors, there is no reason why this form of instrument should not
be nearly as effective as its rival. Mr. Tarrant advises those who
use reflectors in observing double stars “to test the centering of
the flat at intervals during the observations, as the slightest shift
of the large mirror in its cell will frequently occasion a spurious
image which, if it by chance happens to fall where the companion is
expected to be seen, will often lead to the conclusion that it has
been observed. In addition to this, any wings or the slightest flare
around a bright star will generally completely obliterate every trace
of the companion, especially if close and of small magnitude, and such
defects will in nine cases out of ten be found to be due to defective
adjustment. Undoubtedly for very close unequal pairs the refractor
possesses great advantages over a reflector of equal aperture; in the
case of close double stars the components of which are nearly equal
there appears to be little, if any, difference between the two classes
of instruments; while for any detail connected with the colour of stars
the reflector certainly comes to the fore from its being perfectly
achromatic.” These remarks from a practical man will go far to negative
the disparaging statements sometimes made with regard to reflectors and
stellar work, and ought to encourage other amateurs possessing these
instruments to take up this branch in a systematic way.
_Dividing Power._—This mainly depends upon the aperture, and it was
made the subject of careful investigation and experiment by Dawes,
who found that the diameters of the star-disks varied inversely as
the aperture of the telescope. Adopting an inch as the standard, he
ascertained that this aperture divided stars of the sixth magnitude
4″·56 apart, and on this base he constructed the following table:—
Aperture Least
in inches. separable
distance.
″
1·0 4·56
1·6 2·85
2·0 2·28
2·25 2·03
2·5 1·82
2·75 1·66
3·0 1·52
3·5 1·30
3·8 1·20
4·0 1·14
4·5 1·01
5·0 0·91
5·5 0·83
6·0 0·76
6·5 0·70
7·0 0·65
7·5 0·61
8·0 0·57
8·5 0·536
9·0 0.507
9·5 0.480
10·0 0·456
12.0 0·380
15·0 0·304
20·0 0·228
25·0 0·182
30·0 0·152
Dallmeyer, the optician, confirmed these values by remarking:—“In all
the calculations I have made, I find that 4·33 divided by the aperture
gives the separating power. Thus, 4·33 inches separates 1
″.” But a
good deal depends upon the character of the seeing and upon other
conditions. A large aperture will sometimes fail to reveal a difficult
and close _comes_ to a bright star when a smaller aperture will
succeed. This is due to the position of the bright diffraction-ring,
which in a large instrument may overlap the faint companion and obscure
it, while in a small one the ring falls outside and the small star is
visible[48]. Dawes concluded that “tests of separation of double stars
are not tests of excellence of figure,” and he gave much valuable
information with regard to micrometers and double-star observations
generally in the ‘Monthly Notices,’ vol. xxvii. pp. 217-238. This paper
will well repay attentive reading.
_Number of Stars._—In the northern hemisphere there are about 5000[49]
stars perceptible to the naked eye. This is less than an observer
would suppose from a casual glance at the firmament, but hasty ideas
are often inaccurate. The scintillation of the stars and the fact that
many small stars are momentarily glimpsed but cannot be held steadily
have a tendency to occasion an exaggerated estimate of their numbers.
Authorities differ as to the total of naked-eye stars. Sir R. S. Ball
says “the number of stars which can be seen with the unaided eye in
England may be estimated at about 3000.” Gore gives 4000. Backhouse
mentions 5600 as visible in the northern hemisphere. Argelander, who
has charted 324,188 stars between 2° S. of the equator and the N.
pole, gives the following numbers of stars up to the 9th magnitude:—
1st. 2nd. 3rd. 4th. 5th.
20 65 190 425 1100
6th. 7th. 8th. 9th.
3200 13,000 40,000 142,000
With every decrease in magnitude there is a great increase in numbers,
and if this is extended to still smaller magnitudes down to the 15th
or 16th we can readily understand that there exist vast multitudes
of fainter stars. Paul Henry, of the Paris Observatory, says there
are about 1,500,000 stars of the 11th mag., and Dr. Schönfield, of
Bonn, gives 3,250,000 as of the 11½ mag. It is probable that by means
of photography and the largest telescopes considerably more than 50
millions of stars may be charted, but this is a mere approximation.
Roberts has photographed 16,206 stars within an area of four square
degrees in a very rich region of the Galaxy near η Cygni, and Gore
computes that were the distribution equal to this over the whole
firmament the number of stars would reach 167 millions. He also
remarks that in the Paris photographs of the Pleiades, 2326 stars are
shown in a space covering about three square degrees, and this gives
for the entire sky a total of 33 millions. It is, however, manifest
that unusually plentiful spots in the heavens cannot be accepted as
affording a criterion of the whole.
_Magnitudes._—According to Argelander’s figures, above quoted, each
magnitude exhibits a rise of about 300 per cent. But authorities rarely
agree as to scale, as the following comparison between Sir J. Herschel
and Struve will show:—
H. S.
4·0 3·6
4·5 4·1
5·0 4·6
5·5 5·05
6·0 5·5
6·5 5·95
7·0 6·4
7·5 6·85
8·0 7·3
8·5 7·7
9·0 8·1
9·5 8·5
10·0 8·8
10·5 9·1
11·0 9·3
11·5 9·6
12·0 9·8
12·5 10·0
13·0 10·18
13·5 10·36
14·0 10·54
14·5 10·71
15·0 10·87
16·0 11·13
17·0 11·38
18·0 11·61
19·0 11·82
20·0 12·00
Argelander’s magnitudes come between those of Herschel and Struve.
Such disagreements are perplexing to observers, and it is fortunate
that in regard to the naked-eye stars we are now furnished with
a more consistent and accurate series of magnitudes. Photometric
determinations of the light of 4260 stars not fainter than the 6th
mag., and between the N. pole and 30° S. declination, were made at
Harvard College Observatory, and similar measures of 2784 stars
between the N. pole and 10° S. declination were effected at the Oxford
University Observatory, and the results published in 1885. The two
catalogues are in very satisfactory agreement, the accordances within
one tenth of a mag. being 31 per cent., within one quarter of a mag.
71 per cent., and within one third of a magnitude 95 per cent. The
photometers used in the two independent researches were constructed on
very different principles, and the substantial agreement in the results
indicates that “a great step has been accomplished towards an accurate
knowledge of the relative lustre of the stars” (‘Monthly Notices,’ vol.
xlvi. p. 277).
_The Milky Way._—On dark nights when the Moon is absent and the air
clear, a broad zone of glimmering, filmy material is seen to stretch
irregularly across the heavens. It may be likened to a milky river
running very unevenly amongst the constellations, and showing many
curves and branches along its course. On very favourable occasions the
unaided eye glimpses many hundreds of glittering points on this light
background. A field-glass reveals some thousands, and shows that it is
entirely composed of stars the blended and confused lustre of which
occasions that track of whiteness which is so evident to the eye. In a
good telescope stars and star-dust exist in countless profusion, and
great diversity is apparent in their numbers and manner of grouping. In
certain regions the stars are concentrated into swarms, and the sky is
aglow with them; while in others there are very few, and dark cavernous
openings offer a striking contrast to the silvery sheen of surrounding
stars. There are many of these void spaces in Scorpio, and a circular
one in Sagittarius R.A. 17^h 56^m, Dec.-27° 51′ has been particularly
remarked. These inequalities of grouping may be easily recognized with
the naked eye, especially in Cygnus, where bright star-lit regions
frequently alternate with dark void spaces. In the southern sky
there is a noteworthy instance. Near the brilliant stars of Crux and
Centaurus and closely surrounded by the Milky Way there is a large
black vacancy very obvious at a glance, and so striking to ordinary
observers that it is known as the “Coal-sack,” a name applied to it by
the early navigators of the southern seas.
The course of the Milky Way may be described generally as flowing
through Auriga, the club of Orion, feet of Gemini, western part of
Monoceros, Argo Navis, Crux, feet of Centaurus, Circinus, Ara, where
it separates into two branches, the western of which traverses the
northern part of the tail of Scorpio, eastern side of Serpens, Taurus
Poniatowski, Anser, and Cygnus. The eastern branch crosses the tail
of Scorpio, the bow of Sagittarius, Antinous, Aquila, Vulpecula, and
then enters Cygnus, where it reunites with the other branch. It thence
passes through Cepheus, Cassiopeia, Perseus, and enters Auriga. In
breadth it varies greatly, being in some places only 4° or 5°, whereas
in others it reaches 20°. It is, of course, best visible when twilight
is absent, but it is sometimes very plain, even at midsummer, for at
this season some of its more conspicuous sections are favourably placed
for observation. It is supposed that fully nine tenths of the total
number of stars in the firmament are included within the borders, of
the Milky Way.
Some of the ancient philosophers, including Democritus, formed just
conceptions as to the real nature of this appearance. Though they
lacked instruments wherewith to observe the stars forming it, they yet
saw them with the eye of reason. But very vague and incorrect notions
prevailed in early times, when superstition was rife, as to many
celestial phenomena. Some of the ancient poets and learned men refer
to the Galaxy as the path by which heroes ascended to heaven. Thus we
read in Ovid:—
“A way there is in heaven’s extended plain,
Which when the skies are clear is seen below,
And mortals, by the name of Milky, know;
The ground-work is of stars, through which the road
Lies open to great Jupiter’s abode.”
_Scintillation of the Stars._—The rapid variations of light known
as the “twinkling” of the stars received notice from many ancient
observers, including Aristotle, Ptolemy, and others, and they severally
endeavoured to account for it, but not in a manner altogether
satisfactory. At low altitudes bright stars exhibit this twinkling or
scintillation in a striking degree, but it is much less perceptible in
stars placed at considerable elevations. Sirius, the brightest star in
the sky, is a noted twinkler. His excessive lustre and invariably low
position are conditions eminently favourable to induce this effect.
But the planets seldom exhibit scintillation in a very marked degree.
The light of Jupiter and Saturn is steady, even when these planets
are close to the horizon. Mercury, however, twinkles most obviously,
and Venus and Mars, when low down, are often similarly affected,
especially in stormy weather when the air is much disturbed. Hooke,
in 1667, concluded that the scintillation was due “to irregular
refractions of the light of the stars by differently heated layers
of atmosphere.” M. Arago said it arose “from the peculiar properties
possessed by the constituent rays of light, of moving with different
velocities through the strata of the atmosphere, and of producing what
are called interferences.” More recently, M. Montigney has conducted
some interesting researches into this subject, and he believes “that
not only is twinkling caused, to a great extent, by the deviations
of portions of a star’s light altogether away from us by variable
layers of atmosphere, but it is also affected, both in frequency and
in the colours displayed, by the nature of the light emitted by the
individual star.” The planets are little subject to scintillation, as
they present disks of sensible size, and thus are enabled to neutralize
the effect of atmospheric interferences. It is curious, however, that
the steadiness of telescopic images does not appear to be much improved
at high altitudes, and that the phenomenon of scintillation still
operates powerfully as observed from mountainous stations. In February
1888, Dr. Pernter, of the Vienna Academy of Sciences, found “that the
scintillation of Sirius was actually greater at the top of Sonnblick,
10,000 feet high, than it was at the base of the mountain, and he
formed the opinion that scintillation has its origin in the _upper_
strata of the atmosphere and not in the lower as usually assumed.”
It would appear from this that lofty situations do not possess all
the advantages claimed for them in regard to the employment of large
telescopes.
_Star-Disks._—The stars as observed in telescopes are shorn of the
false rays apparent to the naked eye, and they are seen with small
spurious disks. That the disks are spurious is evident from the fact
that the larger the telescope employed, the smaller the star-disks
become. And moreover, when a star is occulted by the Moon, it
disappears instantaneously. There is no gradual diminution of lustre;
the star vanishes with great suddenness. Bright stars, like Aldebaran
or Regulus, have been watched up to the Moon’s limb, and observers
have been sometimes startled at the abruptness with which they were
blotted out. An appreciable disk could not be withdrawn in this
instantaneous manner; it would exhibit a perceptible decadence as the
Moon increasingly overlapped it, but no such appearance is observed.
On the occasion of the occultation of Jupiter on Aug. 7, 1889, the
planet’s diameter was 41″·4, and the disappearance occupied 85
seconds. Now had Aldebaran or Regulus a real disk of only 1″ it would
prevent their sudden extinctions, and their disappearances would be
spread over perceptible though short intervals of time[50]. But there
is every reason to conclude that the actual disks are to be represented
by a small fraction of 1″, so that the largest instrument and the
highest powers fail to reveal it. In this connection, Mr. Gore, in
his ‘Scenery of the Heavens,’ p. 152, says:—“Let us take the case of
α Centauri, which is, as far as is known at present, the nearest fixed
star to the Earth. The distance of this star is about 25 billions of
miles. From comparisons made between its light and the Moon, it has
been found that its intrinsic brilliancy must be about four times that
of the Sun. Supposing its greater lustre is due to its greater size—a
not improbable supposition—it would subtend, if placed at the Sun’s
distance, an angle twice as great, or about 1°, and hence we find that
the angle subtended at its distance of 25 billions of miles would be
about 1/76th of a second of arc, which the most powerful telescope yet
constructed would be incapable of showing as a visible disk.”
_Distance of the Stars._—The distances of the outer planets Uranus and
Neptune, mentioned in an earlier chapter of this work, are sufficiently
large to amaze us; but the distances of the stars may be said to be
relatively infinite. For many years the problem of stellar distances
defied all attempts to resolve it. At length, in 1838-39, Bessell,
Henderson, and Struve obtained results for three stars—viz. 61 Cygni,
α Centauri, and α Lyræ,—which practically settled the question. More
recent measures of stellar parallax, while correcting the earlier
values, have virtually corroborated them; though the figures adopted
can only be regarded as approximations, owing to the difficult and
delicate nature of the work. The binary star α Centauri appears to be
the nearest of all; it has a parallax of 0″·75, and its distance from
us is equal to 275,000 times the distance of the Sun. Light traversing
space at the rate of 187,000 miles per second would occupy 4-1/3 years
in crossing this interval. In the Northern hemisphere 61 Cygni is the
nearest star, with a parallax of 0″·44 and a distance of about 470,000
times the Sun’s distance. Light would take more than seven years in
reaching us from this star, α Lyræ has a parallax of 0″·15, equal to
nearly 22 light-years. α Crucis shows a very small parallax (0″·03),
and its distance is excessively remote—equal to about 108 light-years!
_Proper Motion of Stars._—A very slight motion affects the places
of many of the so-called fixed stars. This must, after the lapse of
long intervals of time, materially alter the configuration of the
constellations. But the change is a very gradual one, and must operate
through many centuries before its effects will become appreciable in
a general way. The greatest proper motion yet observed is that in
regard to two small stars (one in Ursa Major and the other in Piscis
Australis), which amounts to about 7″ annually. Another motion has
been recognized, viz. in the line of sight. Dr. Huggins made the
initiatory efforts in this research by measuring the displacement
of the F line in the spectrum of Sirius. The work has been actively
pursued at the observatories of Greenwich and Rugby, and with
interesting results. While certain stars exhibit a motion of approach,
others display a motion of recession. Thus Vega, Arcturus, and Pollux
are approaching us at the rate of about 40 miles per second; while
Rigel is receding at the rate of 17 miles per second, Castor at the
rate of 19, Regulus 14, Betelgeuse 25, and Aldebaran 31. Sirius, in
the years from 1875 to 1878, was receding from us at the rate of 22
miles per second; but this decreased in subsequent years, and in
1884-85 the star was approaching with a motion of about 22 miles per
second. In 1886 and 1887 this rate was increased to about 30 miles per
second, as observed both at Greenwich and Rugby. This confirms the idea
that Sirius is moving in an elliptical orbit. Similar observations,
in regard to the variable star Algol, have revealed that changes of
velocity are connected with its changes of lustre. Before minimum the
star recedes at the rate of 24½ miles per second, while after minimum
the star approaches with a speed of 28½ miles per second (‘Monthly
Notices,’ vol. 1. p. 241).
_Double Stars and Binary Systems._—Telescopic power will often reveal
two stars where but one is seen by the naked eye. Sometimes the
juxtaposition of such stars is merely accidental; though they are
placed nearly in the same line of sight the conjunction is an optical
one only, and no connection apparently subsists between them. In other
cases, however, pairs are found which have a physical relation, for one
is revolving round the other; and these are termed _binary_ stars. Sir
W. Herschel was the first to announce them, from definite observations,
in 1802. Of double stars more than 10,000 are now known; many of these
are telescopic, but the list includes some fine examples of naked-eye
stars.
[Illustration: Fig. 62.
Double Stars.
β Orionis. γ Leonis. α Ursæ Minoris. γ Virginis.
δ Cygni. γ Arietis. γ Andromedæ. δ Serpentis.
]
Double stars are excellent telescopic tests. A very close pair affords
a good criterion as to the defining capacity of an instrument; while
a pair more widely separated and of greatly unequal magnitude, like
that of α Lyræ, offers a test of the light-grasping power. But in these
delicate observations, as, indeed, in all others, the character of the
seeing exercises an important and variable influence. A double star
that is well shown on one night becomes utterly obliterated on another,
owing to the unsteadiness and flaring of the image. On such occasions
as the latter one is reminded of the “twitching, twirling, wrinkling,
and horrible moulding” of which Sir John Herschel complained, and
which unfortunately forms a too common experience of the astronomical
observer. A close double, of nearly equal magnitudes, requires a steady
night, such as is suitable for planetary details; but a wide double
consisting of a bright and a minute star rather needs a very clear sky
than the perfection of definition. Certain doubles, such as θ Aurigæ, δ
Cygni, and ζ Herculis, are often more easily seen in twilight than on a
dark sky; and some experienced observers, conscious of this advantage,
have secured excellent measures in daylight. Mr. Gledhill says:—“Such
stars as γ Leonis and γ Virginis are best measured before or very soon
after sunset” (‘Observatory,’ vol. iii. p. 54).
_List of Double Stars._
[Abbreviations in col. 9:—β., Burnham; T., Tarrant; S., Schiaparelli;
L., Leavenworth; E., Engelmann; P., Perrotin; Hσ., H. Struve; M., Maw.]
+—--+-—————————————-+————-—————————-+——————-+———————+——————+——————+—————+
| | |Position, 1890.| |Posit- | | | |
|No.| Name of Star. +——————-+——————-+ Mags. | ion- |Dis- |Epoch.|Obser-|
| | | R.A. | Dec. | | Angle |tance.| | ver.|
+—--+-—————————————-+————-—————————-+———————+——————-+——————+——————+—————+
| | | h m | ° ′ | | o | ″ | | |
| 1.|δ Equulei |21 9·1|+ 9 34 | 4½ 5 | 189·9 | 0·25 |1887·7| β. |
| | | Most rapid binary known. Period 11½ years |
| | | (Wrublewsky). Disc. 1852 by O. Struve. |
| 2.|Piazzi 109 | 1 51·0|+ 1 20 | 7 7 | 206·3 | 0·28 |1888·1| S. |
| | | An excessively close and difficult object. Binary.
| 3.|β Delphini |20 32·4|+14 13 | 3½ 5½| 310·1 | 0·29 |1888·6| β. |
| | | A rapid binary. Period 26 years (Doubjago). |
| | | Disc. 1873 by Burnham. |
| 4.|γ^2 Andromedæ | 1 57·1|+41 48 | 5 6 | 277·6 | 0·35 |1884·8| L. |
| | | Distance in Oct. 1889 less than 0″·1, and very |
| | | difficult with 36-inch (Burnham). |
| 5.|γ Coronæ Bor. |15 38·1|+26 39 | 4 7 | 126·6 | 0·38 |1887·5| S. |
| | | A close binary. Period 95½ years (Doberck). |
| | | Colours greenish-white and purple. |
| 6.|55 Tauri | 4 13·6|+16 16 | 6½ 8 | 76·4 | 0·43 |1887·6| S. |
| | | A binary. Difficult object with a 10-inch. |
| 7.|λ Cassiopeiæ | 0 25·7|+53 55 | 6½ 6½ | 146·9 | 0·45 |1887·3| T. |
| | | Another close binary. Distance of components |
| | | shows little change. |
| 8.|ζ Boötis |14 35·9|+14 12 | 4 4 | 293·4 | 0·51 |1887·5| S. |
| | | A binary pair, of equal mags. Period 127 |
| | | years (Doberck). |
| 9.|42 Comæ Bor. |13 4·7|+18 7 | 5½ 6 | 189·6 | 0·55 |1889·1| L. |
| | | A close binary, of short period; about 25¾ years.|
| | | Disc, in 1827 by O. Struve. |
|10.|λ Cygni |20 43·1|+36 8 | 5 7½ | 70·6 | 0·63 |1888·8| Hσ.|
| | | A binary. The distance between the components |
| | | is increasing. |
|11.|ζ Coronæ Bor. |15 18·7|+30 41 | 5½ 6 | 178·5 | 0·63 |1886·5| T. |
| | | A well-known binary, of short period; 41½ years |
| | | (Doberck). |
|12.|ω Leonis | 9 22·6|+ 9 32 | 5½ 7 | 96·8 | 0·70 |1889·1| L. |
| | | A close pair, but not difficult. Binary. Period |
| | | 114½ years (Doberck). |
|13.|15 Lyncis | 6 47·8|+58 34 | 5 6 | 5·9 | 0·77 |1890·3| M. |
| | | A probable binary, the position and distance |
| | | exhibiting a gradual increase. |
|14.|ι Orionis | 5 1·9|+ 8 21 | 5½ 7 | 193·9 | 0·99 |1889·0| L. |
| | | Triple. A low power shows many stars here. |
|15.|ζ Cancri, A.B. | 8 5·9|+18 0 | 5 6 | 40·3 | 1·05 |1889·2| L. |
| | | A triple star. A.C. Pos. 134°·4; Dist. 5″·36; |
| | | Mag. 7; 1878·3 (Hall). |
|16.|ν Scorpii, A.B.|16 5·6|-19 10 | 4 7 | 9·3 | 1·08 |1886·5| T. |
| | | A quadruple star, forming one of the finest |
| | | systems in the sky. |
|17.|π Cephei |23 4·4|+74 47 | 5 7½ | 32·5 | 1·16 |1888·7| Hσ.|
| | | Binary. Becoming more difficult with decrease of |
| | | distance. Yellow and purple. |
|18.|ε Arietis | 2 52·9|+20 54 | 5½ 6 | 202·2 | 1·28 |1889·7| L. |
| | | Distance increasing. Good dividing-test for a |
| | | 4-inch aperture (T.). |
|19.|λ Ophiuchi |16 25·4|+ 2 13 | 4½ 5½ | 42·6 | 1·55 |1888·4| L. |
| | | Binary, but period not yet ascertained with |
| | | accuracy. Yellow and bluish. |
|20.|ζ Herculis |16 37·1|+31 48 | 3 6½ | 65·8 | 1·68 |1890·7| M. |
| | | A fine, rather close binary. Period 34½ years |
| | | (Doberck). Single in 1865. Yellow and red. |
|21.|ξ Ursæ Maj. |11 12·3|+32 9 | 4 5 | 222·7 | 1·63 |1889·3| S. |
| | | One of the first-computed binaries. Period |
| | | 63 years (Breen). Excellent object. |
|22.|δ Cygni |19 41·5|+44 52 | 3 8 | 317·7 | 1·66 |1885·5| T. |
| | | A well-known binary. Period 376·7 years (Gore). |
| | | Test for 4½-inch. Pale yellow and sea-green. |
|23.|33 Orionis | 5 25·5|+ 3 12 | 5 6 | 32·8 | 1·81 |1887·1| T. |
| | | Just visible in a 3-inch. White and pale blue. |
|24.|θ Aurigæ, A.B. | 5 52·2|+37 12 | 3 8 | 2·5 | 1·98 |1885·1| T. |
| | | A similar pair to δ Cygni, though the distance |
| | | is wider. |
|25.|70 Ophiuchi |18 0·0|+ 2 32 | 4 6 | 348·7 | 2·16 |1889·3| β. |
| | | Binary. Period nearly 88 years (Gore). Good |
| | | object for a 3-inch. Yellow and purple. |
|26.|ι Leonis |11 18·2|+11 8 | 4½ 7½ | 62·0 | 2·56 |1889·2| L. |
| | | Binary; but distance shows little variation since|
| | | 1839. Yellowish and blue. |
|27.|ε Boötis |14 40·2|+27 32 | 3 5½ | 328·1 | 2·88 |1885·4| T. |
| | | A very interesting object, and visible in a small|
| | | instrument. |
|28.|α Scorpii |16 22·7|-26 11 | 1 8 | 271·7 | 2·92 |1880·0| β. |
| | | This pair forms an atmospheric rather than an |
| | | optical test. |
|29.|γ Ceti | 2 37·6|+ 2 46 | 3 7 | 289·7 | 2·94 |1883·9| P. |
| | | A binary system. Test for a 2½-inch. |
| | | Yellow and blue. |
|30.|α Piscium | 1 56·3|+ 2 14 | 5 6 | 321·9 | 3·03 |1886·9| T. |
| | | A probable binary, but since 1831 not much change|
| | | in position or distance. |
|31.|ζ Aquarii |22 23·1|- 0 35 | 4 4 | 325·8 | 3·08 |1889·9| L. |
| | | A fine binary, with very long period. 1625 years |
| | | (Doberck). |
|32.|ε^1 Lyræ |18 40·7|+39 34 | 4½ 6½ | 15·3 | 3·24 |1877·4|Dob- |
| | | | | | | | |erck.|
|33.|ε^2 Lyræ |18 40·7|+39 30 | 5 5 | 137·6 | 2·45 |1877·4| Hall|
| | | These stars form a wide double (distance 3′ 27″),|
| | | just separable by the naked eye. A 2½-inch |
| | | shows a fine double-double. A 4-inch reveals |
| | | three faint stare between. |
|34.|ε Hydræ | 8 41·0|+ 6 49 | 4 7 | 226·5 | 3·16 |1889·1| β. |
| | | A new _comes_, Pos. 154°·4; Dist. 0″·26; Mag. 6, |
| | | 1889; 36-inch, power 3300! β. |
|35.|γ Leonis, A.B. |10 13·9|+20 24 | 2 4 | 114·6 | 3·51 |1889·3| β. |
| | | A fine binary. Period 407 years (Doberck). |
| | | Readily seen in a 3-inch. |
|36.|δ Serpentis |15 29·6|+10 55 | 3 5 | 189·9 | 3·52 |1886·6|Ball.|
| | | Probably binary. Fine object in small instruments.|
|37.|α Canis Maj. | 6 40·3|-16 34 | 1 10 | 359·7 | 4·19 |1890·3| β. |
| | | Brilliant binary. Period 58·5 years (Gore). |
| | | Colours white and yellow. |
|38.|α Herculis |17 9·6|+14 31 | 3 4½ | 114·5 | 4·58 |1885·5| T. |
| | | A splendid object. Orange and bluish green. |
|39.|ζ Cassiopeiæ | 0 42·4|+57 14 | 4 8 | 184·7 | 4·76 |1888·3| M. |
| | | Binary. Period 195 years (Gruber). Difficult |
| | | object for 2-1/4-inch (Johnson). |
|40.|γ Virginis |12 36·1|- 0 51 | 3 3 | 153·9 | 5·45 |1889·3| L. |
| | | Well-known binary. Period 182 years |
| | | (J. Herschel). Single in 1836. |
|41.|α Geminorum | 7 27·6|+32 8 | 2 3 | 229·4 | 5·68 |1889·2| L. |
| | | Very fine object. Binary; Period doubtful |
| | | (Mädler 232 years, Doberck 1001 years). |
|42.|π Boötis |14 35·6|+16 54 | 4 6 | 104·3 | 6·04 |1885·4| T. |
| | | This pair has exhibited little change in pos. or |
| | | dist. since 1781. |
|43.|α^2 Capricorni,|20 11·9|-12 53 | 3 15 | 149·7 | 6·30 |1879·7| β. |
| | A.B. | Good light-test for 6-inches. Companion double; |
| | | pos. 240°, dist. 1″·5. |
|44.|δ Geminorum | 7 13·5|+22 11 | 3½ 9 | 207·2 | 6·98 |1886·1| T. |
| | | Rather wide pair of unequal mags. Difficult with |
| | | small apertures. |
|45.|γ Arietis | 1 47·5|+18 45 | 4½ 5 | 178·3 | 8·78 |1886·9| T. |
| | | A fine, easy object. Discovered in 1664 by Hooke.|
|46.|ι Ursæ Maj. | 8 51·7|+48 28 | 3 12 | 356·7 | 9·56 |1883·4| E. |
| | | Well seen in a 4-inch, powers 80 and 130. |
| | | Good light-test. |
|47.|β Orionis | 5 9·3|- 8 20 | 1 9 | 202·0 | 9·61 |1887·2| T. |
| | | A fine object for small instruments. Visible in a|
| | | 2-inch refractor. |
|48.|γ^1 Andromedæ | 1 57·1|+41 48 | 3 6 | 62·6 |10·50 |1876·0|Hall.|
| | | A splendid pair, stationary in relative positions|
| | | (see no. 4). |
|49.|γ Delphini |20 41·6|+15 44 | 4 6 | 271·2 | 11·35|1879·7|Hall.|
| | | Estimates of the colour of this pair differ, and |
| | | change is inferred. |
|50.|σ Orionis, A.D.| 5 33·2|- 2 40 | 4 10½| 236·8 | 11·62|1875·2| |
| | | Multiple. Fine group here. Schröter saw 12 stars,|
| | | Struve 18. |
|51.|β Scorpii |15 59·0|-19 30 | 2 5½| 26·7 | 12·72|1879·7| β. |
| | | The brighter star is a close double; Pos. 87°, |
| | | Dist. 0″·73 (Burnham). |
|52.|ζ Ursæ Maj. |13 19·5| +55 30| 2 4 | 150·5 | 14·38|1886·2| T. |
| | | Fine object for small instruments. Other stars in|
| | | the field. |
|53.|α Centauri |14 32·1| -60 23| 1 2 | 202·9 | 17·12|1888·6| S. |
| | | A fine southern binary with Period of 80·3 years |
| | | (Elkin). |
|54.|α Ursæ Min. | 1 18·5| +88 43| 2 9 | 210·1 | 18·60| | |
| | | Good test for a 2-inch. Dawes saw it with |
| | | 1-3/10-inch, Ward with 1-1/4 inch. |
|55.|61 Cygni |21 2·0| +38 12| 5 6 | 121·0 | 20·58|1887·7| S. |
| | | Probably a binary of long period (782½ years, |
| | | Peters; 1159 years, Mann). |
|56.|33 Arietis | 2 34·3| +26 35| 5 8 | 0·3 | 29·76|1879·7| β. |
| | | A distant and easy pair in small instruments. |
|57.|β Cygni |19 26·3| +27 44| 3 7 | 55·1 | 34·32|1879·7| β. |
| | | A beautiful pair, colours golden yellow and |
| | | smalt blue. |
|58.|β Geminorum | 7 38·6| +28 18| 2 14 | 274·9 | 43·00|1877·9| β. |
| | | Disc. by Burnham, who also finds the companion |
| | | double; dist. 1″·4 (1879·2). |
|59.|α′ Capricorni |20 11·9| -12 53| | 219·7 | 44·55|1879·7| β. |
| | | α^1 and α^2 Capricorni (No. 43) form a naked-eye |
| | | double; Pos. 291°, Dist. 373″·4. |
|60.|α Canis Min. | 7 33·6| + 5 30| 1 14 | 317·3 | 44·62|1877·9| β. |
| | | Difficult object; just seen steadily by Dawes |
| | | with 8-1/4-inch refractor. |
|61.|β Lyræ, A.B. |18 46·0| +33 14| 3 7 | 148·9 | 45·20|1886·9| T. |
| | | There are three other faint and distant |
| | | components. | |
|62.|α Lyræ |18 33·2| +38 41| 1 11 | 156·1 | 48·00|1879·7| β. |
| | | Good light-test for a 3-inch. There are other |
| | | more distant companions. |
|63.|α Cassiopeiæ | 0 34·3| +55 56| 2 13½| 280·2 | 61·33|1879·7| β. |
| | | The 36-inch refractor shows a very faint _comes_;|
| | | Dist. 17″·5 (Burnham). |
|64.|α Canis Maj., | 6 40·3| -16 34| 1 13 | 114·9 | 71·39|1877·5|Hall.|
| | A.C. | This faint and distant companion to Sirius was |
| | | disc. by Marth. |
|65.|α Andromedæ | 0 2·7| +28 29| 2 11 | 271·6 | 71·60|1878·6| G. |
| | | A wide double, visible in a 3-inch, but _comes_ |
| | | very faint. |
|66.|α Tauri | 4 29·6| +16 17| 1 12 | 34·1 |114·96|1879·7| β. |
| | | Good light-test for a 3-inch. Very faint _comes_ |
| | | Pos. 109°; Dis. 30″·4 (Burnham). |
+——-+——————————————-+——————-+——————-+——————-+——————-+——————+——————+—————+
The determination of the angles of position and distance of double
stars forms a very important and extensive branch of work in connection
with sidereal astronomy. In cases where double stars preserve
stationary places relatively to each other, there is clearly no need
for frequent re-observation. But in those numerous instances where the
two components form a binary system it is desirable to obtain as many
measures as possible, so as either to verify the calculated orbit or
to furnish the materials for an orbit if one has not been computed
before. Dr. Doberck, whose name is well known in these researches,
mentioned, in 1882, that ample data for purposes of computation had
not been secured for more than thirty or forty binaries out of between
five and six hundred such systems that were probably known to exist.
Sir W. Herschel, in 1803, estimated the period of revolution of α
Geminorum as 342 yrs. 2 mths. and of γ Virginis as 1200 yrs. Orbits[51]
do not appear, however, to have been computed until 1827, when Savery
of Paris showed that the companion of ξ Ursæ Majoris was revolving in
an ellipse with a period of 58-1/4 years. The accomplished Encke also
turned his attention to this work, and adopted a more elaborate method;
and many others have pursued the subject with very interesting and
valuable results. On pp. 302-305 is a selected list of some of the most
noteworthy double and binary stars, arranged according to the distance
between the components.
In compiling the above list, I have used some of the latest measures
available, as most of these doubles are binary systems, and therefore
in motion, so that a few years effect a perceptible difference in the
angles of position and distance of the components. Some of the pairs
are closing up, others are opening, and thus it happens that a binary
star, divided with great difficulty to-day, may become an easy object
some years hence, and _vice versâ_. In fact, as telescopic tests they
are constantly varying.
Before leaving this part of the subject it may be interesting to refer
individually to a few brilliant examples of double stars.
α _Canis Majoris_ (_Sirius_). A red star according to ancient records,
but it is now intensely white. In 1844 Bessel inferred from certain
little irregularities in the proper motion of this star that it
consisted of a binary system with a period of about half a century[52].
Peters confirmed this idea in 1851, and it was observationally verified
eleven years afterwards. On Jan. 31, 1862, Alvan Clark, jun., while
testing a new 18½-inch refractor, discovered a very faint companion
10″ distant. Measures in the few subsequent years proved that the
position-angle was decreasing, while the distance showed a slight
extension. In 1872 it was about 11″·50, but since then the two stars
have been approaching each other, and Mr. Burnham’s measures in April
1890 gave the distance as only 4″·19. It is now, therefore, a very
difficult object, and only visible in large instruments. In England it
is never easy, owing to its southern position, and it has been little
observed, but it is satisfactory to note that the large refractors at
Washington, Princeton, and Chicago, U.S.A., have been often employed
on this object in recent years. Mann gives a period of 51·22 years for
this interesting binary, and places the time of periastron-passage as
1890·55. This differs from Gore’s orbit, quoted in the table.
β _Orionis_ (_Rigel_). A favourite test-object for small instruments.
The companion has been seen with only 1½-inch aperture by experienced
observers familiar with the object, and accustomed to its appearance
in larger telescopes. The beginner may, however, esteem himself
fortunate if he distinguishes the smaller star with 3 inches of
aperture. When he has done this he may afterwards succeed with 2½
inches only, and quite possibly with 2 inches. He can ascertain his
ability in this direction by inserting cardboard diaphragms of the
diameters referred to in the dew-cap of his telescope. This object
is not a binary; the component stars are fixed relatively to each
other, and merely form an optical double. The colours are pale yellow
and sapphire blue. Burnham thought the smaller star was elongated,
as though a very close double, but the 36-inch at Mount Hamilton has
disproved the idea.
α _Lyræ_ (_Vega_). Another well-known object, and one upon which
amateurs are constantly testing their means. The companion star is
extremely faint, and small instruments would have no chance with it
but for its comparatively wide distance from Vega. Were it much nearer
it would be obliterated in the glare. This is a more difficult pair
than that of Rigel, though certain lynx-eyed observers have glimpsed
the minute star with ridiculously small apertures. It is no mean feat,
however, to discern the star with a 3-inch telescope. Webb saw it
more easily with a power of 80 than with 144 on a 3-7/10-inch. There
are many other stars in the same field, though more distant than the
companion alluded to. With power 60 on my 10-inch reflector, I counted
eighteen stars in the field with Vega on Oct. 9, 1889, though the full
Moon was shining at the time. Several faint stars have been alleged to
exist much closer to Vega than the well-known _comes_; but these have
resisted the great American refractors, and it may be safely assumed
that they were ghosts produced by a faulty image.
α _Ursæ Minoris_ (_Polaris_). This double, from its constant visibility
in northern latitudes, from its unvarying brightness, and from the
relatively stationary positions of the stars composing it, forms
an excellent test for small instruments. But it is a comparatively
easy object, and ought to be seen in a 2-inch telescope. With this
aperture the primitive efforts of a young observer will probably be
disappointing. If possible he should first look at the pair through a
3-or 4-inch, and then he will know exactly what he may expect to see
with inferior means. A difficult object is often readily glimpsed in a
small telescope after the eye has become acquainted with it in a larger
one. Experience of this kind is very requisite, and it is by thus
educating the eye that observers are gradually enabled to reach objects
which appeared hopelessly beyond them at their first attempts. The
companion to Polaris, like that of Rigel and Vega, though situated in
nearly the same line of sight is not physically related to the larger
star, the contiguity of the objects being accidental. Some dubious
observations have been made of _comites_ nearer to Polaris than the one
to which we have been adverting; but Burnham does not see these, and we
are forced to conclude that they have no objective existence.
α _Scorpii_ (_Antares_). A fiery-red star, with a rather close, faint
companion. This object being in 26° of S. declination is rarely
seen with advantage in places with latitudes far north. Atmospheric
disturbance usually affects the image in such degree that the smaller
star is merged in the contortions of the larger. This pair is, however,
interesting from the circumstance that it is frequently liable to
occultation by the Moon. A night on which this double star can be
distinctly seen may be regarded as an exceptional one in point of
definition. It appears to have been discovered nearly half a century
ago by Grant and Mitchel.
_Variable Stars._—A proportion of the stars exhibit fluctuations in
their visible brightness. In most cases, however, the variation is
but slight, though there are instances in which the differences are
considerable. The fluctuations are periodical in nature and capable
of being exactly determined. But the character of the variation and
the period are very dissimilar in different stars. Some are of short
period, and their variations occupy a few days only; others, however,
are more gradual, and twelve months or more may represent the complete
cycle of their changes. These alterations of brightness generally
escape the notice of casual observers of the heavens. To them the
stars appear as constant in their relative magnitudes as they are in
their relative positions. But a close observer of the firmament, who
habitually watches and records the comparative lustre of the stars,
must soon discover numerous evidences of change. He will remark certain
stars which alternately grow bright and faint, and, in fact, display
a regular oscillation of brilliancy. In the case of a pair of stars
he may possibly notice that the superior lustre is emitted first by
one and then by the other. The observation of these variables dates
from a period anterior to the invention of the telescope. Nearly three
centuries ago Fabricius remarked that ο Ceti (Mira) suffered
a great diminution of light; for while it was of the 3rd mag. in Aug.
1596, it became invisible in the following autumn. It was re-observed
by Holwarda in 1639, and as he appears to have been the first to
estimate its period, some authors, including Argelander, have credited
him with the discovery. The star has a period of about 331·3 days. Its
variations are somewhat erratic, for at max. it is sometimes only 4th
mag., while at others it is as bright as 2nd mag., and its min. are
equally inconsistent.
β Persei (Algol) is another and perhaps the best known of all the
variable stars. Its changes are very rapid, for it passes through its
various gradations of brilliancy in less than three days. It was first
noticed by Montanari in 1669, though it was left for Goodricke in
1782 to ascertain its period. The normal mag. of the star is 2·2, and
it only shows distinct variation during the five hours which precede
and follow a minimum, when it declines to 3·7 mag. Its period is
shortening, for in 1782 it was 2^d 20^h 48^m 59^s·4, in 1842, 2^d 40^h
48^m 55^s·2, and at present Chandler finds it 2^d 20^h 48^m 51^s. As
to the causes which contribute to these variations, they are invested
in mystery. It has been conjectured that dark spots on the surfaces
of the stars may, by the effects of rotation, introduce the observed
alternations. Another surmise is that the temporary diminutions of
lustre are to be ascribed to the interposition of dark satellites, and
this theory seems tenable in regard to stars of the Algol type. It is
satisfactory to note that a large amount of systematic work is being
done in this important and delicate branch of research. Such stars as
are subject to variation have been classed as follows:—1. Temporary or
new stars; 2. Stars having long and pretty regular variation; 3. Stars
irregularly variable; 4. Stars varying in short periods; 5. Stars of
the type of Algol, which are liable to temporary diminutions of lustre.
On the preceding page is a list of the most noteworthy variable stars.
_List of Variable Stars._
+——————————————+——————————————————-+——————————————+——————————————————-+
| Name of Star.| Position, 1890. | Mags. | Period. |
| +————————-+————————-+ | |
| | R.A. | Dec. | | |
+——————————————+——————————————————-+——————————————+——————————————————-+
| | h m | ° ′ | | |
| μ Cephei | 0 52.5 | +81 17 | 7.2 9.4 | 2^d 11^h 50^m |
| ο Ceti | 2 13.8 | - 3 29 | 2 0 | 331-1/3 days |
| β Persei | 3 1.0 | +40 32 | 2.2 3.7 | 2^d 20^h 49^m |
| λ Tauri | 3 54·6 | +12 11 | 3.4 4.2 | 3^d 22^h 52^m |
| U Orionis | 5 49·3 | +20 9 | 6 12½ | |
| ζ Geminorum | 6 8.2 | +22 32 | 3.2 4·2 | 135-151 days |
| ζ Geminorum | 6 57.6 | +20 44 | 3.7 4·5 | 10^d 3^h 43^m |
| L_{2} Puppis | 7 10·2 | -44 28 | 3.5 6.3 | 136 days |
| R Canis Maj. | 7 14.5 | -16 11 | 6.2 6.8 | 1^d 3^h 16^m |
| U Geminorum | 7 48.6 | +22 18 | 9 14 | 71-126 days |
| S Cancri | 8 37.7 | +19 26 | 8.2 11·7 | 9^d 11^h 38^m |
| ζ Argûs | 10 40.8 | -59 6 | 1 6 | 46 or 67 yrs.? |
| R Hydræ | 13 23.7 | -22 43 | 4 10 | 436 days |
| δ Libræ | 14 55.1 | -8 5 | 4.9 6·1 | 2^d 7^h 51^m |
| U Coronæ | 15 13.7 | +32 3 | 7.6 8·8 | 3^d 10^h 51^m |
| α Herculis | 17 9.6 | +14 31 | 3.1 3.9 | 88^d 12^h (irreg.)|
| U Ophiuchi | 17 11.0 | + 1 20 | 6 6.7 | 0^d 20^h 8^m |
| β Lyræ | 18 46.0 | +33 14 | 3.5 4.5 | 12^d 21^h 47^m |
| χ Cygni | 19 46·3 | +32 38 | 4-6.5 13 | 406 days |
| ζ Aquilæ | 19 46.9 | + 0 44 | 3.6 4.7 | 7^d 4^h 14^m |
| Y Cygni | 20 47.7 | +34 15 | 7.1 7.9 | 1^d 11^h 57^m |
| μ Cephei | 21 40.1 | +58 16 | 3.6 4·8 | 432 days? |
| δ Cephei | 22 25.1 | +57 51 | 3.7 4.8 | 5^d 8^h 48^m |
+——————————————+——————————————————-+——————————————+——————————————————-+
+——————————————+——————————————————-+
| Name of Star.| Observer. |
| | |
+——————————————+——————————————————-+
| | |
| μ Cephei | Ceraski, 1880. |
| ο Ceti | Fabricius, 1596. |
| β Persei | Montanari, 1669. |
| λ Tauri | Baxendell, 1848. |
| U Orionis | Gore, 1885. |
| ζ Geminorum | Schmidt, 1865. |
| ζ Geminorum | Schmidt, 1847. |
| L_{2} Puppis | Gould, 1872. |
| R Canis Maj. | Sawyer, 1887. |
| U Geminorum | Hind, 1855. |
| S Cancri | Hind, 1848. |
| ζ Argûs | Burchell, 1827. |
| R Hydræ | Maraldi, 1704. |
| δ Libræ | Schmidt, 1859. |
| U Coronæ | Winnecke, 1869. |
| α Herculis | W. Herschel, 1795.|
| U Ophiuchi | Sawyer, 1881. |
| β Lyræ | Goodricke, 1784. |
| χ Cygni | Kirch, 1686. |
| ζ Aquilæ | Pigott, 1784. |
| Y Cygni | Chandler, 1886. |
| μ Cephei | Hind, 1848. |
| δ Cephei | Goodricke, 1784. |
+——————————————+——————————————————-+
_New or Temporary Stars._—These stars (sometimes classed with variable
stars) furnish us with rare instances of vast physical changes
occurring among sidereal objects, usually so steadfast and endurable.
The alternating lustre of certain variable stars represents phenomena
of regular recurrence, and is probably to be explained by simple
causes; but the sudden outbursts and rapid decline of temporary stars
are facts of a more startling character, and need a more exceptional
explanation. The first of these objects recorded in history appears
to have been seen in Scorpio 134 years before the Christian era, and
it suggested to Hipparchus of Rhodes the idea of forming a catalogue
of stars, so that in future ages observers might have the means of
recognizing new stars or any other changes in the configuration of the
heavens. Hipparchus completed his catalogue in 128 B.C.; it contained
1025 stars, and forms one of the most valuable memorials we possess of
the labours of the ancient astronomers. Another temporary star is said
to have appeared in 130 A.D., but this and several other objects of
presumably similar character noticed in later years may just possibly
have been comets, and considerable doubt hangs over the descriptions.
It will therefore be safest to confine our remarks to more modern and
better attested instances of these phenomena[53]:—
1572, November 11.—The famous star of Tycho Brahe. He thus described
his first view of it:—“One evening when I was considering, as
usual, the celestial vault, the aspect of which is so familiar to
me, I perceived with indescribable astonishment a bright star of
extraordinary magnitude near the zenith in the constellation of
Cassiopeia.” He adds:—“The new star was destitute of a tail, or of
any appearance of nebulosity; it resembled the other stars in all
respects, only that it twinkled even more than stars of the first
magnitude. In brightness it surpassed Sirius, α Lyræ, or Jupiter. It
could be compared in this respect only to Venus when she is nearest
the earth (when a fourth part of her illuminated surface is turned
towards us). Persons who were gifted with good sight could distinguish
the star in the daytime, even at noon, when the sky was clear.” This
brilliant NOVA began to fade early in Dec. 1572, and in April and May
1573 it resembled a star of the 2nd mag., in July and Aug. one of the
3rd mag., and in Oct. and Nov. one of the 4th mag. In March 1574 the
star completely disappeared (to the naked eye), after a visibility of
about 17 months. It exhibited some curious variations of colour. It
was white when most brilliant; it then became yellow, and afterwards
red, so that its hue in the early part of 1573 was similar to that
of Mars. But in May it again became white, and continued so until it
ceased to be visible. The position of this star (for 1890) is R.A.
0^h 18^m 41^s, Dec. +63° 32′·2. It was supposed to be a reapparition
of the brilliant stars which shone between Cepheus and Cassiopeia in
945 and 1264, and to have possibly been associated with the “Star of
Bethlehem;” but there is no reliable evidence on which this view can
be supported, as the alleged “stars” of 945 and 1264 were undoubtedly
comets, misdescribed in old records. Cornelius Gemma is reputed to have
seen the celebrated star of 1572 a few days before Tycho Brahe, viz.,
on November 8, 1572.
1604, October 10.—Discovered by Brunowski, who announced it to
Kepler. It was brighter than a star of the 1st mag., also than Mars,
Jupiter, or Saturn, which were not far distant at the time. It did
not begin to fade immediately; for a month after its discovery it was
still brighter than Jupiter, and of a white lustre. At the middle of
November it surpassed Antares, but was inferior to Arcturus. In April
1605 it had fallen to the 3rd mag., and went on decreasing until in
October it could scarcely be seen with the naked eye owing to the
twilight resulting from its proximity to the Sun. In March 1606 it was
invisible. The position of this object was about midway between ξ and
58 Ophiuchi, or at R.A. 17^h 24^m, Dec.-21° 207′ (1890).
1670, June 20.—Discovered by the Carthusian Monk Anthelme in R.A. 19^h
43^m 3^s, Dec. +27° 3′ (1890), a few degrees east of β Cygni. It was
of the 3rd mag., and continued in view, with constantly fluctuating
brightness, for nearly two years. At the end of March 1672 it was
6th mag., and has never reappeared. Hind found a small, hazy, and
ill-defined star in the same place, but this is probably not the same
as Anthelme’s star of 1670.
1848, April 28.—During the long interval of 178 years separating 1670
from 1848 not a single new star appears to have revealed itself.
Observers had multiplied, astronomical instruments had been much
improved, star-catalogues were plentiful, and yet the sidereal heavens
gave no intimation of a stellar outburst. No better proof than this
could be afforded as to the great rarity of temporary stars. At length,
in the spring of 1848, the spell was broken, and Mr. Hind announced
that a new star of a reddish-yellow colour had appeared in Ophiuchus,
R.A. 16^h 53^m 20^s, Dec.-12° 43′ (1890). He expressed himself as
certain that no star brighter than the 9th mag. had been there previous
to April 5. After rising to the 4th mag. it soon faded, and in 1851
could only be observed in large instruments. In 1875 it was still
visible as a very minute star.
1860, May 21.—M. Auwers, of Konigsberg, noticed a star of the 7th mag.
near the centre of the bright resolvable nebula (M. 80), lying between
α and β Scorpii, R.A. 16^h 10^m 29^s, Dec.-22° 42′ (1890). On May 18
the star was not there, and it disappeared altogether in three weeks.
It was independently seen by Pogson on May 28, and to him it seemed
that “the nebula had been _replaced_ by a star, so entirely were its
dim rays overpowered by the concentrated blaze in their midst.”
1866, May 12.—Discovered by Birmingham at Tuam. It was of the 2nd mag.,
and situated in Corona, R.A. 15^h 54^m 54^s, Dec. +26° 14′ (1890).
The outburst must have been very sudden, as Schmidt, at Athens, was
observing this region three hours before the new star was detected, and
is certain it was then fainter than the 4th mag. The star was found
to be identical with one on Argelander’s charts estimated as 9½ mag.
It faded from the 2nd to the 6th mag. by May 20, and was thereafter
invisible to the naked eye.
1876, Nov. 24.—A yellow star of the 3rd mag. was seen by the ever
vigilant Schmidt at Athens near ρ Cygni, and where no such star existed
on Nov. 20. The position of the object was R.A. 21^h 37^m 23^s, Dec.
+42° 20′ (1890). It soon grew fainter, so that on Dec. 13 it was of the
6th mag. and devoid of colour. In the spectroscope it presented much
the same lines as Birmingham’s star of May 1866. In addition to the
continuous spectrum it showed bright lines of hydrogen.
1885, August 31.—Dr. Hartwig announced the appearance of a star-like
nucleus in the great nebula (M. 31) of Andromeda, R.A. 0^h 36^m 43^s,
Dec. +40° 40′ (1890). Other observers soon corroborated the discovery.
The star appears to have been first seen on Aug. 19; it was not visible
on the preceding night. On Sept. 1 its mag. was 6·5, on Sept. 2, 7·3,
on Sept. 3, 7·2, Sept. 4, 8·0, Sept. 18, 9·2, &c. On Feb. 7, 1886,
it had dwindled down to the 16th mag., according to an estimate made
by Prof. Hall with the great Washington refractor. The spectrum was
continuous, and Proctor and Gore considered “that the evidence of the
spectroscope showed that the new star was situated _in_ the nebula.”
The phenomena presented by the temporary stars alluded to are so
different to those of ordinary variables that it is very questionable
whether they ought to be classed together. Our knowledge of the former
would no doubt progress more rapidly were they specially looked for
and more instances discovered. Those who have acquired a familiar
acquaintance with the naked-eye stars should examine them as often as
possible with this end in view. Some of these objects lose light so
quickly that unless they are caught near the maximum they are likely to
escape altogether, and this shows the necessity of being constantly on
the alert for their appearance. I have frequently, while watching for
meteors, reviewed the different constellations in the hope of picking
up a new object, but have never succeeded in doing so.
_Star Colours_ form another interesting department of sidereal
astronomy. It is obviously desirable to record the hues presented,
not only by double stars and binary systems, but by isolated stars
also, as changes of tint have been strongly suspected. Cicero, Seneca,
Ptolemy, and others speak of Sirius as a red star, whereas it is
now an intense white; and if we rely on ancient descriptions similar
changes appear to have affected some other prominent stars. But the
old records cannot be implicitly trusted, owing to the errors of
transcribers and translators; and Mr. Lynn (‘Observatory’, vol. ix. p.
104) quotes facts tending to disprove the idea that Sirius was formerly
a red star. In the majority of cases double stars are of the same
colour, but there are many pairs in which the complementary colours
are very decided. Chambers remarks that the brighter star is usually
of a ruddy or orange hue, and the smaller one blue or green. “Single
stars of a fiery red or deep orange are not uncommon, but isolated
blue or green stars are very rare. Amongst conspicuous stars β Libræ
(green) appears to be the only instance.” As an example of fiery-red
stars Antares may be mentioned; Aldebaran is deep reddish orange, and
Betelgeuse reddish orange. Amongst the more prominent stars Capella,
Rigel, and Procyon may be mentioned as showing a bluish tinge, Altair
and Vega are greenish, Arcturus is yellow, while Sirius, Deneb,
Polaris, Fomalhaut, and Regulus are white. Mr. Birmingham published a
catalogue of “The Red Stars” in the ‘Transactions of the Royal Irish
Academy’, for August 1877, and Mr. Chambers has a _working-catalogue_
of 719 such objects in the ‘Monthly Notices,’ vol. xlvii. pp. 348-387.
The region of Cygnus appears to be especially prolific in red stars,
and many of these objects are variable. In a paper read at a recent
meeting of the Astronomical Society of the Pacific Mr. Pierson stated
that in binary systems where the stars are of equal magnitude the
colours are invariably the same, while those differing in magnitude
differ also in colour and the larger star is always nearer the red end
of the spectrum than its secondary. In the estimation of star-colours
reflecting-telescopes are very reliable owing to their perfect
achromatism.
_Groups of Stars._—Great dissimilarity is apparent in the clustering of
stars. The heavens furnish us with all gradations—from the loose, open
groups like that in Coma Berenices, in the Pleiades, or in Præsepe, to
the compact globular clusters, in which some thousands of stars are so
densely congregated that considerable optical power is required to
disintegrate them. Some, it is true, yield more easily than others.
The great cluster (Messier 13) in Hercules readily displays the swarms
of stars of which it is composed; but others are so difficult that it
is only in the largest instruments they are resolved into star-dust.
Further references to these wonderful objects will be made in the next
chapter, and some of the principal examples described; our purpose
here is to allude to a few of the more scattered groups, and to some
noteworthy instances of multiple stars.
_Coma Berenices._ A naked-eye cluster, consisting of many stars,
chiefly from the 5th to 6th mags. A telescope adds a number of smaller
stars. Nebulæ may be often swept up hereabouts, as it is not far north
of the rich nebulous region of Virgo.
_The Pleiades._ Six stars are usually distinguished by the naked eye,
and a seventh is occasionally remarked. Möstlin (the instructor of
Kepler) counted fourteen, Miss Airy has drawn twelve, and Carrington,
like Möstlin, saw fourteen. In 1877 I distinctly made out fourteen
stars in this group. The telescope reveals a considerable number
of small stars and Tempel’s large nebula near Merope. Kepler saw
thirty-two stars with a telescope, and Hooke seventy-eight; but Wolf,
at Paris, after three years of unremitting labour with a 4-foot
reflector, catalogued 671 stars in the group. A photograph, however,
with a 12-inch refractor showed 1421 stars; and a more recent negative
includes no less than 2326. There is an interesting little triangle
close to the brightest star, Alcyone; and several of the leading stars
are involved in nebulosity, discovered by means of photography.
_Præsepe._ A fine group of small stars, divisible by the unaided eye
on a clear night. Chambers says the components are not visible without
a telescope; while Webb notes that the group is just resolvable by the
naked eye. Thirty-six stars were glimpsed with Galilei’s telescope;
but modern instruments show many more. Marth, using Lassell’s 4-foot
reflector at Malta, discovered several faint nebulæ and nebulous stars
in this cluster.
χ _Persei._ Perceptible to the eye as a patch of hazy material lying
between the constellations Cassiopeia and Persei. In a telescope it
forms a double cluster, and is one of the richest and most beautiful
objects that the sky affords. The tyro who first beholds it is
astonished at the marvellous profusion of stars. It can be fairly well
seen in a good field-glass, but its chief beauties only come out in
a telescope, and the larger the aperture the more striking will they
appear. It is on groups of this character that the advantage of large
instruments is fully realized. The power should be very low, so that
the whole of the two clusters may be seen in the field. An eyepiece of
40, field 65′, on my 10-inch reflector, presents this object in its
most imposing form.
κ _Crucis_. Sir J. Herschel’s observations at the Cape have made this
object familiar to northern observers. It is composed of more than 100
stars, from the 7th mag. downwards; and some of the brighter ones are
highly coloured, so that the general effect is greatly enhanced and
fully justifies Herschel’s statement that the group may be likened to
“a superb piece of fancy jewelry.”
ζ _Ursæ Majoris_ (_Mizar_). This group is interesting both as a
naked-eye and as a telescopic object. There is a 5th mag. star, named
Alcor, about 11½′ distant from Mizar, and the former was considered
a good test-object for unaided vision by the Arabian astronomers.
But the star has probably brightened; for it can now be easily seen,
and certainly offers no criterion of good vision. Mizar is a fine
telescopic double, the companion being 4th mag. and distant 14½″. Any
small telescope will show it, and there is another 8th mag. star very
near.
σ _Orionis_. This appears as a double-quadruple star, with several
others in the same field. A 3-inch will reveal most of them, though
some of the fainter stars in the group will be beyond its reach.
θ _Orionis_. In the midst of the great nebula of Orion there is a
tolerably conspicuous quadruple star, the components of which form
a trapezium. This is visible in a 2-inch refractor. In 1826 Struve
discovered a fifth star, and in 1830 Sir J. Herschel found a sixth;
these were both situated a little outside the trapezium. All these
stars have been seen in a 3-inch telescope. The great 36-inch
equatoreal at Mount Hamilton has added several others; one was
detected by Alvan G. Clark (the maker of the object-glass) and another
by Barnard. These were excessively minute, and placed within the
trapezium. Barnard[54] has also glimpsed an extremely minute double
star exterior to the trapezium, and forming a triangle with the stars A
and C on the following diagram:—
[Illustration: Fig. 63.
The Trapezium in Orion, as seen with the 36-inch refractor.]
Several observers, including Huggins, Salter, and others, had
previously drawn faint stars in the interior of the trapezium; but
these could not be seen by Hall and Burnham in the large refractors at
Washington and Chicago, and were thus proved to have no real existence.
The new stars observed in the 36-inch telescope are only just within
the limits of its capacity, and therefore cannot be identical with
stars alleged to have been previously seen in small instruments.
The fifth and sixth stars in the trapezium have been supposed to be
variable, and not without reason; possibly the others are equally
liable to change, but this is only conjecture. Sir J. Herschel says
that to perceive the fifth and sixth stars “is one of the severest
tests that can be applied to a telescope” (‘Outlines,’ 11th edit. p.
610); yet Burnham saw them both readily in a 6-inch a few minutes
before sunrise on Mount Hamilton in September 1879.
β and ε Lyræ also form multiple groups, which will well repay
observation either with large or small telescopes.
_Further Observations._—Anyone who attempts to indicate with tolerable
fulness the methods and requirements of observation in the stellar
department of astronomy will find a heavy task lies before him; and
it is one to which he will be unable to do justice in a small space,
owing to the variety of matters to be referred to and the necessity
of being particular in regard to each one. In what follows I shall
merely make very brief allusions, as it is hoped the description
already given of past work will be a sufficient guide for the future.
Moreover, those who take up a special branch of inquiry will hardly
rest content with the meagre information usually conveyed in a general
work on astronomy, but will consult those authorities who deal more
exclusively with that branch. Double and binary stars may be said to
form one department, variable and temporary stars another, the colours
of stars a third, while many others may be signified—such as the
determination of star-magnitudes, positions, grouping, and distances;
also the proper motions and number of stars, besides photographic and
spectroscopic work,—each and all of which comprise a field of useful
and extensive inquiry. The amateur will of course choose his own sphere
of labour, consistently with his inclination and the character of his
appliances. In connection with double stars, valuable work yet remains
to be done, though the Herschels and the Struves gathered in the
bulk of the harvest and Burnham has gleaned much that was left. With
regard to bright stars, it may be assumed that very few, if any, close
companions, visible in moderately small glasses, now await discovery,
unless, indeed, in cases where the star forms part of a binary system
of long period, and the epoch of periastron has fallen in recent years.
But with telescopic stars there must be many interesting doubles, some
of them binaries, still unknown. These should be swept up and submitted
to measurement. A great desideratum in this branch is a new general
catalogue of double stars; for such a work would greatly facilitate
reference, and save the trouble of searching through different lists
in order to identify an object. Burnham has given some practical hints
on double-star work in the ‘Sidereal Messenger,’ and his remarks are
reproduced in that excellent work ‘Astronomy for Amateurs.’
As to variable stars, some of these permit of naked-eye estimation,
others need a field-glass, and there are some which require to be
followed in a good telescope. The observer who enters this department
may either desire to find new objects or to obtain further data with
regard to old ones. If the former, he cannot do better than watch
some of the suspected variables in Gore’s Catalogue of 736 objects,
published by the Royal Irish Academy. Whether suspected or known
variables are put under surveillance, the plan of comparison will be
the same. Several stars near the variable in position, and nearly equal
in light, should be compared with it, and the differences in lustre,
in tenths of a magnitude, recorded as frequently as possible. The
extent and period of the variation will become manifest by a discussion
of the results. The comparison-stars should of course be constant
in light, and, if naked-eye stars, they may be selected from the
_Uranometria Nova Oxoniensis_ or ‘Harvard Photometry.’ If telescopic
stars are required, then recourse must be had to comprehensive charts
such as Argelander’s _Durchmusterung_, which includes stars up to 9½
mag. Variable stars of the Algol type are especially likely to escape
recognition, as they retain a normal brilliancy except during the few
hours near the time of a minimum.
As to star-colours, it must be admitted that our knowledge is in
an unsatisfactory condition. The results of past observation show
discordances which are difficult to account for. When, however, all the
circumstances are considered, we need feel no surprise at this want of
unanimity. In certain cases it is probable that actual and periodical
changes occur in the colours of stars, though absolute proof is still
required. Atmospheric variations unquestionably affect the tints of
stars, and some alterations depend upon altitude, for a celestial
object seen through the dense lower air-strata near the horizon will
hardly preserve the same apparent hues when on the meridian. Telescopes
are also liable to induce false impressions of colour, and especially
by the employment of different eyepieces not equally achromatic. And
the observer’s judgment is sometimes at fault through physiological
influences, or he may have a systematic preference for certain hues
which little impress another observer. Those engaged in this branch
feel the want of a reliable and ready means of comparison, and several
have been tried; but there are objections to their use, and it seems
that the best objects are furnished by the stars themselves. Let the
observer study the colours of well-known stars, and familiarize his
eye with the distinctions in various cases (also with the differences
due to meteorological effects &c.); he will then gradually acquire
confidence, and may use these objects as standards. The difficulty will
be that they cannot be directly compared, in the same field, with other
stars; but relative differences may be noted by turning the telescope
from one object to the other. This will be better than forming
estimates on the basis of an artificial method, which will sure to be
troublesome to arrange, and probably erroneous in practice. In some
stars the colour is so curious as to be attributed with difficulty, and
with regard to faint stars colour-estimates are often unreliable; so
that it is not desirable to go below the 9th mag. unless a very large
instrument is employed.
The necessity of being constantly on the look-out for temporary
stars has been already mentioned. There is also the need for further
observations of such of these objects as still exist. They are,
however, very minute, and the observer will have to be careful as
to their identity. Though no great revival in brilliancy is to be
expected, these objects exhibited some singular fluctuations during
their decline, and it is important to keep them under view as long as
possible.
Many other departments of sidereal work are best left to the
professional astronomer. The derivation of accurate star-places, proper
motions, distances, &c. requires instruments of great refinement and
trained hands to use them. Researches such as these do not come within
the scope of ordinary amateurs. But a vast field is open to them in
respect to double and variable stars; and the physical relations of
many of the former greatly intensify the interest in this branch, and
make it necessary to secure frequent observations.
FOOTNOTES:
[47] There are several forms of this instrument: for particulars of
construction and use the reader is referred to Thornthwaite’s ‘Hints on
Telescopes,’ and Chambers’s ‘Astronomy,’ 4th ed. vol. ii.
[48] Mr. George Knott, of Cuckfield, mentions that the radius of the
first bright diffraction-ring of a stellar image, for a 7-1/3-inch
aperture, is 1″·01, and for one of 2 inches 3″·70 (‘Observatory,’
vol. vi. p. 19; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted
as giving 1″·25 for a 7-inch, 1″·61 for a 5½-inch, and 3″·57 for a
2·4-inch. These figures exceed the theoretical values, if the latter
are adopted from Sir G. B. Airy’s ‘Undulatory Theory of Optics,’ where,
for mean rays, we have:—
Radius of object-glass in inches × radius of bright ring in seconds =
3·70.
[49] The number visible to different persons varies according to
eyesight. Some observers see thirteen or fourteen stars in the
Pleiades, while others cannot discern more than six or seven.
[50] About 2 seconds. Sir W. Herschel found the diameter of α Lyræ with
a power of 6450 to be 0″·3553. Tycho Brahe, before the invention of
telescopes, estimated the diameter of Sirius as 120″. J. D. Cassini,
with a telescope 35 feet long, found the diameter of the same star 5″.
[51] Dr. Doberck gives some valuable information with reference to the
computation of binary star-orbits in ‘The Observatory,’ vol. ii. pp.
110 and 140.
[52] The star α Canis Minoris (Procyon) was also inferred to be a
binary and to have a similar period. Several close companions appear
to have been discovered (Ast. Nach. no. 2080). But Prof. Hall, using
the 25·8-inch refractor at Washington, says:—“I have never been able to
see any of these companions that would stand the test of sliding and
changing the eyepiece, turning the micrometer, &c., and am therefore
doubtful of their existence. This is an interesting star for the
powerful telescopes of the future.” It has been surmised that the
companion is a non-luminous one, and therefore invisible.
[53] It is remarkable that nearly all the temporary stars have appeared
in the region of the Milky Way.
[54] This expert comet-finder would appear to have more acute,
sensitive vision on faint stars than Burnham (see ‘Monthly Notices’,
vol. xlix. p. 354).
CHAPTER XVII.
_NEBULÆ AND CLUSTERS OF STARS._
Distinction.—Large number of Nebulæ and Clusters visible.—Varieties
of form and grouping.—Distribution.—Early Observations.—Variable
Nebulæ.—Nebulous Stars.—The Magellanic Clouds.—Double
Nebulæ.—Real dimensions of Nebulæ and Clusters.—Round Nebulæ and
Clusters.—Description of Objects.—Further Observations required.—Lists
of selected Objects.
_Distinction._—These objects, though classed together in catalogues,
offer some great distinctions which the observer will not be long
in recognizing. It was thought at one period that all nebulæ were
resolvable into stars[55], and that their nebulous aspect was merely
due to the confused light of remote star-clusters. But modern
telescopes, backed up by the unequivocal testimony of the spectroscope,
have shown that purely nebulous matter really exists in space. The
largest instruments cannot resolve it into stars, and it yields a
gaseous spectrum. The conjecture has been thrown out that it may be
considered as the unformed material of which suns and planets are made.
_Large Number visible._—D’Arrest once said that nebulæ are so numerous
as to be infinite, and his opinion is supported by the rapid increase
in the number known. Let us make a comparison. Messier inserted in
the _Connaissances des Temps_ for 1783 and 1784 (published in 1781) a
catalogue containing 103 nebulæ and star-clusters. Of these 68 were
new. In 1888 a new edition of Sir J. Herschel’s catalogue of 1864
(revised and extended by Dreyer) was printed by the Royal Astronomical
Society, and this includes 7840 objects![56] The labours of the
Herschels, of Lord Rosse, D’Arrest, Marth, Tempel, Stephan, and Swift
have vastly augmented our knowledge in this branch since the time of
Messier.
_Varieties of Form and Grouping._—A telescope reveals all grades
of condensation in stellar groups. Some consist of rather bright,
scattered stars, and are easily resolved. Others contain more stars,
but they are smaller, and greater power is required to show them.
Others again are condensed into globular clusters needing high powers
and good instruments to disconnect the mass of stars composing them.
Some are faint, and the stars so minute that they are only to be
distinguished from nebulæ in the finest telescopes. As to the nebulæ
properly so called, they exist in all forms. They may be either round,
elliptical, or in the form of a streak. Some are highly condensed in
their centres, others present well-defined circular disks like planets,
and a small proportion are in the form of rings[57]. Many peculiarities
of detail have been remarked, and a curious and complicated spiral
structure has been discovered in certain prominent nebulæ. One of these
has been termed the “Whirlpool” Nebula from its singular convolution of
form. Other objects have received distinctive appellations agreeably
to their appearance. Thus, there is the “Dumb-bell” Nebula, the “Crab”
Nebula, the “Horseshoe” Nebula, &c. Lord Rosse’s 6-foot reflector is in
a large degree responsible for the particular knowledge we possess of
many of these objects. The large mirror commands a grasp of light which
renders it very effective on forms of this character. An instrument of
small diameter is quite inadequate to deal with them. They can be seen,
it is true, and the general shape recognized in the most conspicuous
examples, but their details of structure are reserved for the greater
capacity of large apertures.
_Distribution._—With regard to distribution these objects exhibit the
utmost irregularity, for in certain regions of the heavens they are
found to be very plentiful, while in others they are singularly rare.
Thus, in Virgo, Coma Berenices, Leo, and Ursa Major large numbers of
nebulæ abound, while in Hercules, Draco, Cepheus, Perseus, Taurus,
Auriga, &c., very few are encountered. Taking the 7840 objects in the
New General Catalogue of 1888 it will be found that their distribution
in hours of Right Ascension is as follows:—
R.A. Nebulæ.
0 H. 387
I 428
II 398
III 300
IV 276
V 375
VI 171
VII 196
VIII 230
IX 362
X 404
XI 585
XII 858
XIII 504
XIV 375
XV 212
XVI 230
XVII 259
XVIII 203
XIX 117
XX 153
XXI 188
XXII 275
XXIII 354
The maximum is therefore reached at XII hours, while the minimum is
shown at XIX h. There is a secondary max. at I h., and a secondary min.
at VI h.
_Early Observations._—The nebula in Andromeda appears to have been the
one first discovered, for the distinguished Persian astronomer Al-Sûfi
(who died in 986 A.D.) was undoubtedly acquainted with it. The nebula
is figured upon a Dutch map of the stars nearly 400 years old. In 1612
Simon Marius redetected this object, and appropriately likened its
appearance to that of a “candle shining through a piece of horn.” In
1618 the nebula in Orion was certainly known, for Cysatus of Lucerne
compared it with the head of the fine comet visible in December of that
year. Huygens alighted upon the same object in 1656, and appears to
have been unconscious of its prior discovery. Only six “nebulæ or lucid
spots” were known in 1716, and enumerated by Halley in the ‘Phil.
Trans.’ vol. xxix. These included those of Andromeda and Orion. A third
was situated in the space between the bow and head of Sagittarius.
This is M. 22, and consists of a bright globular cluster of Stars.
The fourth was the fine star-group involving ω Centauri, which Halley
himself found in 1677. The fifth was another fine group in the right
foot of Antinous. This is M. 11, and was discovered by Kirch in 1681.
The sixth was the magnificent globular cluster (M. 13) in Hercules,
discovered by Halley in 1714.
In 1735 the Rev. W. Derham published a list of 16 of these objects, and
in 1761 Lacaille summarized 42 nebulæ and star-clusters which he had
observed in the southern sky. This was followed by Messier’s tables of
45 nebulæ &c. in 1771, and of 103 in 1781[58]. But these contributions,
important though they severally were, sunk into insignificance beside
the splendid results obtained by Sir W. Herschel, who during his
prolonged and systematic sweeps of the heavens picked up no less than
2500 new nebulæ and clusters which he formed into three catalogues
printed in the ‘Phil. Trans.’ as follows:—1786, 1000 objects, 1789,
1000 ditto, 1802, 500 ditto.
_Variable Nebulæ._—It is in the highest degree probable that changes
occur in the physical appearances of certain nebulæ, though the opinion
is not perhaps supported by a sufficient number of instances. Until Sir
W. Herschel began his review of the heavens very few nebulæ were known,
and the information possessed about them was very incomplete. The early
records, obtained with small and inferior telescopes, scarcely admit of
comparison with recent observations, for in matters of detail little
agreement will be found; and this proceeds certainly not so much from
real changes in the objects as from differences due to the variety
of instruments employed, to atmospheric vagaries, and to “personal
equation.” Bullialdus and Kirch in 1667 and 1676 and Le Gentil in 1759
supposed that remarkable changes were operating in the great elliptical
nebula of Andromeda. But G. P. Bond fully investigated the evidence,
and concluded that the variability of the object was by no means
proved. Some observers have represented the nucleus as stellar, while
others have drawn it as a gradual condensation, and Dr. Copeland has
shown that different magnifying powers alter the aspect of the nucleus,
“the lower powers making it more star-like, the higher ones more
soft-looking and extensive.”
Mairan and others entertained the view that the large irregular nebula
in Orion was subject to change. This object received much attention
from Sir W. Herschel, and he concluded that it underwent great
alteration between 1774 and 1811. D’Arrest, from his own researches
and a discussion of other results, expressed himself in 1872 that “the
observed changes in this vast mass of gas seem exclusively to turn out
to be temporary fluctuations of brightness.” Prof. Holden has arrived
at a similar conclusion, and says:—“The figure of the nebula has
remained the same from 1758 till now (if we except a change in its apex
about 1770, which seems quite possible); but in the brightness of its
parts undoubted variations have taken place, and such changes are still
going on”[59] (‘Monograph of the Nebula in Orion,’ p. 225).
Hind discovered a faint nebula, with a diameter of about 1′, on Oct.
11, 1852. It was situated in Taurus, the position being R.A. 4^h 15^m
33^s, Dec. +19° 15′·6 (for 1890), or about 2° W. of the star ε Tauri
(mag. 3·7). D’Arrest, on Oct. 3, 1861, searched for this object, but
found it had quite disappeared! A small round nebula was seen in 1868,
about 4′ preceding Hind’s, but this resisted some later attempts at
observation. In Oct. 1890, Burnham and Barnard, with the 36-inch
refractor of the Lick Observatory, saw two nebulæ here, one a very
small, condensed nebula, with a stellar nucleus, and the other an
exceedingly faint nebulosity about 45″ in diameter (see ‘Monthly
Notices,’ vol. li. pp. 94, 95).
The nebula surrounding the star ζ Argûs has been suspected of
variation, particularly by Abbott, of Hobart Town, Tasmania. Vols.
xxv., xxx., and xxxi. of the ‘Monthly Notices’ contain many references
to, and figures of, this interesting object. But the alleged changes
have not been substantiated, and there seems no reason to doubt that
they were purely imaginary.
The trifid nebula in Sagittarius (M. 20) is supposed by Prof. Holden to
have altered its position with reference to a triple star now situated
in the S. following part of the nebula. Sir J. Herschel, more than
half a century ago, had described this star as placed in the middle of
the vacuity by which the nebula is divided. Dreyer, however, points
out that the drawings of this object differ in many details, and that,
though changes of brightness may have taken place, it is difficult to
understand that the nebula should move so as to envelop the star in
about 1835, “after which no sensible change occurred again, so far as
published observations go.”
The nebula (M. 17) just N. of the bow of Sagittarius was also inferred
by Holden to have shifted its place relatively to the small stars
figured by Lassell in this object; but Dreyer adduces facts which
controvert this assumption. (See ‘Monthly Notices,’ vol. xlvii. pp.
412-420, where much valuable information will be found as to supposed
variable nebulæ.)
On Oct. 19, 1859, Tempel discovered a faint, large nebulosity attached
to the star Merope, one of the Pleiades, and at first mistook it for
a diffused comet. Its position is R.A. 3^h 39^m·6, Dec. +23° 26′
(1890). An impression soon gained ground that this object was variable;
for while Schmidt, Chacornac, Peters, and others saw it with small
instruments, it could not be discerned by D’Arrest and Schjellerup
with the large refractor at Copenhagen. Swift saw the nebula easily in
1874 with a 4½-inch refractor, and has observed it with the aperture
contracted to 2 inches. Backhouse re-observed it in 1882 with a
4-1/4-inch refractor. Yet in March 1881 Hough and Burnham sent a paper
to the Royal Astronomical Society with an endeavour to prove that the
nebula did not exist! They had frequently searched for it during the
preceding winter, but not a vestige of the object could be seen in the
18½-inch refractor at Chicago, and they regarded the supposed nebula
as due to the glow proceeding from Merope and neighbouring stars. But
photography has entirely refuted this negative evidence, and has
shown, not only Tempel’s nebula, but others involving the stars Maia,
Alcyone, and Electra belonging to this cluster. As to the alleged
variations in the Merope nebula, there is every reason to suppose these
were not real.
Proper motion has been suggested in regard to a very small, faint
nebula (N.G.C. 3236) a few degrees following α Leonis. But Dreyer has
disproved this by showing that there was no proper motion between 1865
and 1887, whence “it may be safely inferred that there has been none
since 1830, unless we are to believe, in this and similar cases, that
nebulæ in the good old days moved about as they liked, but have been on
their good behaviour since 1861.”
_Nebulous Stars._—This name was applied by Hipparchus and other ancient
observers to the clusters of stars which, to the naked eye, appear
as patches of nebulous light. Sir W. Herschel, in 1791, showed this
designation to be incorrect, and used it in connection with stars
actually involved in nebulosity. In sweeping the heavens he met with
several instances of this kind. Thus, 3° E.S.E. of ζ Persei he found
a star of the 9th mag. surrounded by a nebula 3′ in diameter. He
picked up another close to the star 63 Geminorum. This is a remarkable
object—a star of the 9th mag. surrounded by two dark and two bright
rings. On Feb. 3, 1864, Lord Rosse’s telescope showed an opening in
the outer bright ring, and the latter seemed connected with the inner
bright ring; so that the object presented the aspect of a spiral nebula
with a star in the centre. The diameter of the whole nebulosity is
45″. Key observed this object with an 18-inch reflector in 1868, and
described it as symmetrical—a central star, with intervening dark and
bright rings. He found a power of 510 the best, for, “like the annular
nebula in Lyra, it bears magnifying wonderfully well.” Since Herschel’s
time many nebulous stars have been discovered. There is one of about
6th mag. in R.A. 8^h 6^m·1, Dec.-12° 36′. The nebulosity round the
star fades away gradually, and its extreme diameter is 157″. There
is a 7th mag. star at R.A. 21^h 0^m 14^s, Dec. +67° 44′ involved in a
very large, faint nebulosity. This is a striking object, and I have
frequently picked it up while comet-seeking. The star has such a
foggy, veiled appearance that on first remarking it the observer thinks
his lenses are dewed, but on viewing neighbouring stars he sees them
sharp and clear on the dark sky, and the contrast is very pronounced.
The nebulous star is much isolated, though in a part of the sky where
small stars abound. This is one of Herschel’s discoveries and No. 7023
of the N. G. C.; Dreyer says he has seen the nebulosity particularly
distinct north and south of the star. In some cases a double star is
involved in nebulosity, and there are instances in which two double
stars are placed within an elliptical nebula.
_The Magellanic Clouds_[60].—These are marked as Nubecula Major and
Nubecula Minor on celestial globes and charts. They form two extensive
aggregations of nebulæ and star-clusters, and are readily visible to
the naked eye in or near Hydrus, and not far from the south pole of the
heavens. They may be likened to detached patches of the Milky Way. Sir
J. Herschel says the Nubecula Major is situated between the meridians
of 4^h 40^m and 6^h and the parallels of 66° and 72° of S. declination,
and extends over a space of some 42 square degrees. The Nubecula Minor
lies between 0^h 28^m and 1^h 15^m and 72° and 75° of S. declination,
and spreads over about 10 square degrees. The composition of these
objects is very complex and diversified, and affords very rich ground
for exploration with a large telescope. Nebulæ exist in profusion and
in every variety, and are intermingled with star-clusters varying in
condensation from the compact globular form to groups more loosely
scattered, and such as we often find in the Milky Way. Nearly three
hundred nebulæ and clusters are included in the major “cloud,” while
more than fifty others closely outlie its borders. In the minor about
forty such objects have been discovered. It is very strange to find
them collected together in this manner; for in other regions of the
firmament they are usually found separated, and certain classes appear
to have their own special zones or localities of distribution. Sir J.
Herschel pointed out that “globular clusters (except in one region of
small extent) and nebulæ of regular elliptic forms are comparatively
rare in the Milky Way, and are found congregated in the greatest
abundance in a part of the heavens most remote possible from that
circle, whereas in the Nubeculæ they are indiscriminately mixed with
the general starry ground and with irregular though small nebulæ.”
_Double Nebulæ._—Instances are not wanting of conspicuous double
nebulæ. M. 51 and 76, near ζ Ursæ and θ Andromedæ, may be classed in
this category. There is a very interesting, though a smaller object
just W. of α and β Geminorum, or in R.A. 7^h 18^m·6, Dec. +29° 43′.
Two bright, round nebulæ are separated by an interval of 28″. These
double nebulæ are usually round, and are sometimes resolvable into
stars. Whether they are physical or mere optical pairs has yet to be
ascertained. So many examples exist that it seems highly probable
they have a real connection, though no motion has yet been certainly
detected to prove they are binary systems. Such motion may, however,
be very slow, and require observations extending over a much longer
interval before it is revealed.
_Real Dimensions of Nebulæ and Clusters._—It may be readily imagined
that these objects are of immense size; for though placed at
distances of the utmost remoteness, they spread over perceptible and
comparatively large areas. Gore remarks that, on the assumption that
the globular cluster in Hercules (M. 13) is 5′ in diameter, and its
parallax one tenth of a second, its real diameter must be 3000 times
the Sun’s mean distance from the Earth, or nearly 280 billions of
miles! He further points out that, though this group contains as many
as 14,000 stars, according to Sir W. Herschel, yet each component may
be separated many millions of miles from the others, owing to the
vast dimensions of the group. Details like these are of course only
approximate, as the distance of a nebula or star-cluster has not yet
been definitely ascertained. The great nebulæ of Orion and Andromeda
must extend over prodigious regions in distance-space; but to quote
figures seems useless, in consequence of our inability to form just
conceptions of such immensity.
_Round Nebulæ and Clusters._—Resolvable nebulæ and clusters are
frequently circular in outline. The central condensation is an
indication of their globular form, though not always so, for many of
these objects become suddenly much brighter in the middle, and show an
apparently stellar nucleus. The material or stars forming the object
cannot therefore be equally distributed. Where it suddenly brightens
there is a great condensation, and in some cases several of these are
evident in the form of bright rings, intensifying as the nucleus is
approached. This irregular aggregation denotes the operation of “a
force of condensation directed from all parts towards the centre of
such systems.” In regard to planetary nebulæ, they cannot be globular
or they would exhibit a brightness increasing from the margin to the
centre. Their even luminosity throughout affords the evidence of a
special structure. Sir J. Herschel thought the planetary nebula (M. 97)
near β Ursæ Majoris must either be in the form of a hollow globe or a
flat circular disk lying perpendicular to the line of vision.
_Description of Nebulæ and Clusters of Stars._—The latter objects are
included in this chapter for several reasons. In a small telescope
nearly all such clusters exhibit the aspect of nebulæ, and they have
been catalogued with them, though, as already explained, some great
distinctions are to be drawn. To the naked eye the cluster Præsepe,
in Cancer, is usually visible as a patch of nebulosity, though on a
very clear, dark night stars may be glimpsed sparkling about the spot,
and a very small glass will suffice to show it as a nest of stars.
This object, and some others of a more difficult character (their
component stars being smaller and more compressed), are tabulated (I.)
at the end of this chapter. A summary (II.) of globular clusters is
also given, together with a list (III.) of nebulæ, a few of which are
resolvable into stars[61]. It must be understood that these selections,
though comprising many notable objects, are by no means exhaustive,
the intention being merely to indicate some typical examples of fine
nebulæ and clusters and of peculiarities of form or appearance, such
as planetary, annular, elliptical, and centrally condensed nebulæ
and loose, compressed, and globular clusters. Some of these objects
deserve individual references, as they present interesting details
to the telescopic observer and come within the reach of moderate
appliances.
_Great Nebula in Andromeda_ (M. 31). This object has often been
mistaken for a comet, for it is readily perceptible to the eye on a
moonless night. It is very large—4° by 2½°, according to Bond, with
a 15-inch refractor. He discovered a pair of dark streaks in the
brightest region of the nebula, and these may be well seen in a 10-inch
reflector. It is really triple; for about 25′ S. of the nucleus there
is a very bright, round, resolvable nebula, discovered by Le Gentil,
and a third, observed by Caroline Herschel, lies rather further to
the N.W. Photographs by Roberts show dark rings dividing the bright
interior parts of the nebula from the outer, and imparting to it a
decided spiral tendency. This nebula has hitherto resisted attempts
to resolve it into stars, though many hundreds have been seen in
the foreground. But its spectrum is continuous, so that its stellar
character is to be inferred.
_Great Nebula in Orion_ (M. 42). Visible to the naked eye just below
a line connecting β and ζ Orionis, and involving θ Orionis. It
exhibits an extremely complicated structure, and many of its irregular
branches and condensations may be discerned in small instruments. Sir
W. Herschel failed to resolve this object into stars with his 4-foot
reflector; but Lord Rosse, in 1844, thought he had effected it with his
6-foot mirror, though the conclusion was premature. The spectroscopic
researches of Huggins have shown this nebula to be composed of
incandescent gases, so that the stars telescopically observed in it are
probably in the foreground and entirely disconnected from the nebulous
mass. Effective photographs have been taken of it by Draper, Common,
and Roberts. It certainly forms one of the grandest objects in the
heavens.
The _Planetary_[62] _Nebula_ (M. 97). Discovered by Mechain in 1781.
In small telescopes it looks like a rather faint, round mass of
nebulosity, somewhat brighter in the middle than at the edges. In
Lord Rosse’s telescope it shows many details, including a spiral
arrangement and two dark spots in the middle inclosing bright, eye-like
condensations. The margin is fringed with protuberances, and from its
peculiar aspect this object has been called the “Owl” Nebula. Diameter
between 155″ and 160″. It may readily be picked up 2-1/4° S.E. of β
Ursæ Majoris. It yields a gaseous spectrum.
In Draco at R.A. 17^h 58^m 36^s, Dec. +66° 38′ there is a pretty small,
but exceedingly bright planetary nebula. With a low power it looks like
a star out of focus, but a high power expands it into a well-defined
planetary disk. As observed in Lord Rosse’s 3-ft. reflector on Sept.
17, 1873, this nebula exhibited “a round, well-defined disk of a full
blue colour, light very equable, diameter 22″·4, surrounded by an
extremely faint nebulosity.” This is an excellent object of its class.
_Spiral Nebula_ (M. 51). Discovered by Messier on Oct. 13, 1773.
It is situated in Canes Venatici, and 4° S.W. from ζ Ursæ Majoris.
An ordinary instrument will reveal it as a double nebula, and the
two parts will be seen to differ greatly in size. Messier gave the
distance separating them as 4′ 35″. Sir J. Herschel drew this object
as a bright, centrally condensed nebula, surrounded by a dark space
and then by a luminous ring divided through nearly one half of its
circumference. Closely outlying this he placed a bright round nebula.
Lord Rosse’s 6-foot showed something very different. In April 1845 its
spiral character was discovered; coils of nebulosity were observed
tending in a spiral form towards the centre, and the outlying nebula
was seen to be connected with it. Some striking drawings have been
published of this object. Those by Sir J. Herschel and Lord Rosse
differ essentially, and would scarcely be supposed to represent the
same nebula; but when we reflect that the instruments used were
respectively of 18 inches and 72 inches aperture, the cause of the
disparity becomes evident.
Another fine example of a spiral nebula is M. 99, in the northern
wing of Virgo, and 8° E. of β Leonis. This object was discovered by
Mechain; its spiral form of structure was detected by Lord Rosse in
1848. Diameter 2½′ Like M. 51 it gives a continuous spectrum and is
resolvable into stars.
[Illustration: Fig. 64.
1. Nebula with bright centre.
2. Planetary Nebula.
3. Ring-nebula in Lyra.
4. Star-cluster in Hercules.]
_The Crab Nebula in Taurus_ (M. 1). Discovered by Bevis in 1731, and
situated 1° N.E. of ζ Tauri. Its diameter is 5½′ by 3½′. An early
drawing with Lord Rosse’s telescope shows it with numerous radiations;
whence it was termed the Crab Nebula, from the supposed resemblance:
but later observations have given it quite another form. In 1877 there
was no trace of the nebulous arms: it appeared as a well-defined, oval
nebula with some irregularities of structure. This object is very plain
in small telescopes, and may be readily picked up from its proximity
to ζ Tauri; but in such instruments it is void of detail, and merely
presents a pale, oval nebulosity. It has not been clearly resolved,
though it has a mottled appearance, indicating a stellar composition,
in large apertures.
_The Dumb-bell Nebula_ (M. 27). Discovered by Messier in 1779, and
situated in Vulpecula—a region very rich in small stars. Diameter about
7′ or 8′. Its general form resembles a dumb-bell or hour-glass; hence
its name. Struve, Lord Rosse, and others have seen many stars in the
nebulous mass, but the latter is not resolvable. I have seen seven
stars in the nebula with a 10-inch reflector. Its peculiar shape is
perceptible in a small instrument. This object frequently serves to
illustrate books on Astronomy; but the drawings by Sir J. Herschel,
Lord Rosse, and others are curiously discordant, and show how greatly
differences in telescopic power may affect the observed appearance of
an object.
_The Ring-Nebula in Lyra_ (M. 57). Discovered by Messier between the
stars β and γ Lyræ. Diameter 80″ by 60″. This object is bright,
though rather small, and it will stand high powers. The dark centre may
possibly be glimpsed in a 3-inch refractor; I have seen it readily in
a 4-1/4-inch. It was at one period thought to be resolvable, but the
spectroscope has negatived the idea, and shown it probably consists of
nitrogen gas. A small star near the centre was frequently seen in Lord
Rosse’s telescope; but the 36-inch refractor at Mount Hamilton reveals
twelve stars projected on or within the ring, and several others
have been suspected. There is a faint star exterior to the ring, and
following it; this is visible in small telescopes. The space within the
ring is not quite dark, and the structure of the nebula is somewhat
complicated as seen in large instruments. Another fine instance of an
annular nebula may be found 3° preceding the 4th mag. star 41 Cygni,
but it is not so large or conspicuous as that in Lyra. Its diameter is
47″ by 41″. Several stars were seen sparkling in it by Lord Rosse,
who found the centre was filled with faint light and the N. side of
the ring broadest and brightest.
_Elliptical nebulæ_ are well represented by the pair (M. 81 and 82)
about 2° E. of δ (22) Ursæ Majoris. They are separated by about 38′
of declination, so that they may be observed in the same field of a
low-power eyepiece. The preceding one is very bright and large (8′
by 2′). The following one is a ray or streak of nebulosity (7′ by
3/4′). On May 21, 1871, the great Rosse telescope showed the latter
as a most extraordinary object, at least 10′ in length and crossed by
several dark bands. Roberts photographed these nebulæ on March 31,
1889. “The negative shows that the nucleus [of M. 81], which has not a
well-defined boundary, is surrounded by rings of nebulous or meteoric
matter, and that the outermost rings are discontinuous in the N.p.
and S.f. directions.” M. 82 is “probably a nebula seen edgeways, with
several nuclei of a nebulous character involved, and the rifts and
attenuated places in it are the divisions of the rings that would be
visible as such if we could photograph the nebula from the direction
perpendicular to its plane” (‘Monthly Notices,’ vol. xlix. p. 363).
This fine pair may be easily picked up in a small instrument. Another
grand object of this class (discovered by Caroline Herschel in 1783)
lies in R.A. 0^h 42^m·2, Dec.-25° 54′, between the stars β Ceti and α
Sculptoris.
_Globular clusters_ furnish us with many examples of easily resolved
and richly condensed balls of stars. M. 3 (discovered by Messier),
M. 5 (discovered by G. Kirch), and M. 13 (discovered by Halley) may
be selected as amongst the finest of these objects in the northern
hemisphere. They are severally visible to the naked eye, and may be
found in a telescope directed as follows:—M. 3, between Arcturus and
Cor. Caroli, and nearer the former; M. 5, 7° S.W. of α Serpentis and
close to the double star 5 Serpentis; M. 13, one third the distance
from ζ to ζ Herculis. They are brilliant objects from 5′ to 7′
diameter. With power 60 on my 10-inch reflector they are spangled with
stars, though not fully resolved. Smyth described M. 3 as consisting of
about 1000 small stars, blazing splendidly towards the centre. Webb
hardly resolved it with a 3-7/10-inch refractor. Another fine object of
this class (M. 80) will be encountered midway between α and β Scorpii.
Sir W. Herschel described it as the richest and most compact group of
stars in the sky, and it is noteworthy from the fact that a new star
burst forth near its centre in 1860. There is a magnificent cluster,
involving ω Centauri, which Sir J. Herschel considered as “beyond all
comparison the richest and largest object of the kind in the heavens.”
It is visible to the naked eye as a 4th mag. star, but residents in
northern latitudes are precluded from a view of it. Pegasus also
supplies us with some fine clusters; Maraldi picked up two in 1746
(M. 2 and 15), and these will respectively be found 5° N. of β and 4°
W.N.W. of ε Pegasi. They are to be classed amongst the grandest objects
of their kind.
In Cygnus, at R.A. 20^h 41^m 7^s, Dec. +30° 19′, near κ and especially
in the region immediately north-east, there exist irregular and
extensive streams of faint nebulosity which may be said to form a
telescopic milky way, Nebulæ and stars are curiously grouped together,
forming a remarkable arrangement which will well repay study. To see
these objects satisfactorily, a moonless night, free from haze or fog,
should be chosen, and the power should be moderately low, or some of
the more feeble nebulous films will be lost. The observer may spend
some agreeable hours in sweeping over this region, which is one of the
best in a wonderfully rich constellation.
_Further Observations._—The fact that Swift has discovered many
hundreds of nebulæ during the last few years affords indubitable proof
that considerable numbers of these objects still await detection. No
doubt they are generally small and faint, but it is necessary they
should be observed and catalogued, so that our knowledge in this
department may be rendered as complete as possible. New nebulæ are
sometimes mistaken for expected comets, and occasionally give rise to
misconceptions which would be altogether avoided were our data more
exhaustive.
Those who sweep for nebulæ must have the means of determining
positions, and a small telescope will be inadequate to the work
involved. A reflector of at least 10 inches, or refractor of 8 inches,
will be required; and a still larger instrument is desirable, for
to cope successfully with objects of this faint character needs
considerable grasp of light. The power employed should be moderate; it
must be high enough to reveal a very small nebula, but not so high as
to obliterate a large, diffused, and faint nebula. In forming his first
catalogue of 1000 nebulæ, Sir W. Herschel used a Newtonian reflector
of 18·7 inches aperture, power 157, field 15′ 14″; Swift’s recent
discoveries were effected with a 16-inch refractor and a periscopic
positive eyepiece, power 132, field 33′. With a low power a very
extensive field will be obtained, and a large part of the sky may soon
be examined, but it will be done ineffectively. It is better to use a
moderately high power, and thoroughly sweep a small region. The work is
somewhat different to comet-seeking; it must proceed more slowly and
requires greater caution, for every field has to be attentively and
steadily scanned. If the telescope is kept in motion, a faint nebula
will pass unseen. Some of these objects are so feeble that they are
only to be glimpsed by averted vision. When the eye is directed, say,
to the E. side, a faint momentary glow comes from the W. side of the
field; but the observer discerns nothing on looking directly for the
object. On again diverting his gaze he receives another impression of
faint nebulosity from the same point as before, and becomes conscious
of its reality. Frequently, while comet-seeking, I meet with a small
indefinite object, the character of which cannot be determined by
direct scrutiny. On withdrawing the eye to another part of the field,
however, the mystery is solved. If the object is a nebula, it flashes
very distinctly on the retina; but if a small cluster, the individual
stars are seen sparkling in it. These indirect views are usually so
effective that the trouble of applying higher powers is dispensed with.
The glow from a faint nebula or comet often apparently fluctuates in a
remarkable manner. Light-pulsations affect it, causing the nebulosity
to be intermittently visible. It flashes out and enlarges, then
becomes excessively feeble and indeterminate. The changes are not real,
but due to the faint and delicate nature of the object, which is only
fugitively glimpsed and presents itself differently with the slightest
change in the manner of viewing it. Burnham has said there is no such
thing as glimpsing an object; but he is wrong. It is the intermediate
step between steady visibility and absolute invisibility.
The work of sweeping for nebulæ is much delayed by the comparisons
necessary for the identification of objects. The path may be smoothed
by marking the known nebulæ on a good chart, like Argelander’s. The
observer may then see, by reference, whether the objects he encounters
have been picked up before. The labour of projecting all the nebulæ
contained in the New General Catalogue would of course be considerable,
and the observer will probably find it expedient to select certain
regions for examination, and map such nebulæ as are included within
their borders.
The discovery of new nebulæ offers an inviting field to amateurs.
Vast numbers of these objects have escaped previous observation, for
though the sky has been swept again and again, its stores have not
been nearly exhausted. Mr. Barnard recently stated that with the
powers of the great 36-inch refractor the number of known nebulæ (more
than 8000) might readily be doubled! As an example of their plentiful
distribution in certain regions it may be mentioned that Mr. Burnham
very recently discovered eighteen new nebulæ in a small area of 16′ by
5′·5 near the position in R.A. 13^h 38^m, Dec. 56° 20′ N. Near the pole
of the northern heavens there exist many unrecorded nebulæ, as this
region does not appear to have been thoroughly examined with a large
instrument. It is often the case that several nebulæ are clustered
near together. Whenever a new one is discovered the surrounding space
should therefore be carefully surveyed in search of others. The region
immediately outlying known objects may also be regarded as prolific
ground for new discoveries. After several hours’ employment in the work
of searching for nebulæ or comets the eye is enabled to discern faint
objects which were invisible at first, as it is in a better condition
to receive feeble impressions. While comet-seeking in 1889 and 1890 I
discovered ten new nebulæ, all near the N. pole, and their approximate
positions are given below:—
+————+————————————-+——————————————-+————————————————————————————--+
|Ref.|Date of |Position 1890. | |
|No. |Discovery. +——————-+——————-+ Description |
| | |R.A. | Dec. +| |
+————+————————————-+——————————————-+————————————————————————————--+
| | |h m s | ° ′ | |
| 1. |1889, Aug. 26|4 29 59|75 25·2|F., S., b. M., *12, n.p. |
| 2. |1890, Nov. 7 |4 40 19|78 7·9 |F., S., R. |
| 3. |1890, Oct. 19|4 46 38|68 9·8 |F., S., R., b. M.N., F. double|
| | | | | * s.f. |
| 4. |1890, Nov. 16|5 50 7 |80 31·0| v.F., S. |
| 5. |1890, Nov. 9 |6 11 45|83 1·9 |F., S., R., m. b. M. |
| 6. |1890, Oct. 17|6 59 26|85 45·0|v. F., v.v.S., 12′ s.s.f. |
| | | | | N.G.C. 2300 |
| 7. |1890, Nov. 7 |7 8 52 |80 7·4 |v. F., p. S., 22′ s. s. f. |
| | | | | N.G.C. 2336. |
| 8. |1890, Sept.14|7 23 24|85 30·0|F., S., E., 46′ s. f. N.G.C. |
| | | | | 2300. |
| 9. |1890, Sept. 8|8 21 37|86 7·4 |p. F., S., m. b. M., * n. f. |
| 10.|1890, Aug. 23|8 34 30|85 54·4|F., S., R., g. b. M., near |
| | | | | preceding. |
+————+————————————-+——————————————-+———————————————————————————-—-+
Abbreviations:—F., faint; S., small; R., round; M., middle; N.,
nucleus, E., extended; v., very; b., brighter; n., north; s., south;
f., following; p., pretty, preceding; m., much; g., gradually; *,
star; N.G.O., New General Catalogue.
No. 8 is placed centrally within a curious semicircle of stars, thus:—
[Illustration: Fig. 65.]
I.—CLUSTERS OF STARS.
+————————-+——————-+——————————————————+———————————————————————————————+
| | | Position, 1890. | |
| No. | No. +————————-+————————+ |
| N.G.C., | M., | | | Description. |
| 1888. | 1781. | R.A. | Dec. | |
+————————-+——————-+—————————+————————+———————————————————————————————+
| | | | | |
| | | h m | ° ′ | |
| 225. | | 0 37·1 | +61 3 | Stars 9th-10th mags. Between |
| | | | | γ and κ Cassiopeiæ. |
| 869. | | 2 11·3 | +56 38 | In Perseus. Stars 7th-14th |
| | | | | mags. |
| 1039. | 34. | 2 35·0 | +42 18 | A fine group, chiefly of 9th |
| | | | | mag. stars. |
| 1912. | 38. | 5 21·3 | +35 44 | Stars of various mags. In |
| | | | | Auriga. |
| 1960. | 36. | 5 29·0 | +34 4 | Stars of 9th-11th mags. Near |
| | | | | 1912. |
| 2099. | 37. | 5 45·1 | +32 31 | Stars and star-dust. 5° S. of |
| | | | | θ Aurigæ. |
| 2168. | 35. | 6 2·0 | +24 21 | Stars of 9th-16th mags, near |
| | | | | ζ Geminorum. |
| 2287. | 14. | 6 42·3 | -20 38 | Visible to naked eye. 4° S. of|
| | | | | Sirius. |
| 2437. | 46. | 7 36·8 | -14 34 | Nebula involved with cluster |
| | | | | of 8th-13th mag. stars. |
| 2477. | | 7 48·4 | -38 16 | Fine group of 12th mag. stars |
| | | | | near ζ Argûs. |
| 2516. | | 7 56·5 | -60 34 | Visible to naked eye. 200 |
| | | | | stars of 7th-13th mags. |
| 2547. | | 8 7·4 | -48 56 | Vis. n.e. Stars 7th-16th mags.|
| | | | | Diameter 20′. |
| 2548. | | 8 8·3 | -5 28 | Stars of 9th-13th mags. In |
| | | | | Monoceros. |
| 2632. | 44. | 8 34·0 | +20 22 | Præsepe. Group of bright stars|
| | | | | vis. n. e. |
| 2682. | 67. | 8 45·2 | +12 13 | Large group of stars of |
| | | | | 10th-15th mags. |
| 4755. | | 12 47·1 | -59 45 | Very large group about κ |
| | | | | Crucis. |
| 6121. | 4. | 16 16·9 | -26 16 | Close to Antares. Group and |
| | | | | line of stars through it. |
| 6603. | 24. | 18 12·0 | -18 28 | Stars of 15th mag. 3° N. of |
| | | | | μ Sagittarii. |
| 6611. | 16. | 18 12·7 | -13 50 | Group of at least 100 stars |
| | | | | of various mags. |
| 6705. | 11. | 18 45·1 | -6 24 | Stars of 11th mag. and |
| | | | | fainter. Fine object. |
| 6838. | 71. | 19 48·8 | +18 29 | Stars of 11th-16th mags. In |
| | | | | Sagitta. |
| 7243. | | 22 10·9 | +49 20 | A clustering of many bright |
| | | | | stars. |
| 7654. | 52. | 23 19·4 | +61 0 | Irregular group of 9th-13th |
| | | | | mag. stars. |
| 7789. | | 23 51·5 | +56 6 | Grand cluster of 11th-18th |
| | | | | mag. stars. |
+————————-+——————-+—————————+————————+———————————————————————————————+
II.—Globular Clusters of Stars.
+————————-+——————-+——————————————————+———————————————————————————————+
| | | Position, 1890. | |
| No. | No. +————————-+————————+ |
| N.G.C., | M., | | | Description. |
| 1888. | 1781. | R.A. | Dec. | |
+————————-+——————-+—————————+————————+———————————————————————————————+
| | | | | |
| | | h m | ° ′ | |
| 104. | | 0 19·1 | -72 42 | Very large; more than 15′ |
| | | | | diameter. |
| 288. | | 0 47·8 | -27 11 | Slightly elliptical. Stars |
| | | | | 12th-16th mags. |
| 362. | | 0 58·5 | -71 26 | Stars 13th-14th mags. |
| | | | | Diameter 4′. |
| 1261. | | 3 9·3 | -55 38 | Large. Stars and star-dust. |
| | | | | |
| 1851. | | 5 10·5 | -40 10 | Very bright and large. Fine |
| | | | | object. |
| 4147. | | 12 4·5 | +19 9 | Pretty large, round. Minute |
| | | | | stars. |
| 4590. | 68. | 12 33·7 | -26 9 | Much compressed group of 12th |
| | | | | mag. stars. |
| 5024. | 53. | 13 7·5 | +18 45 | Fine object. Chiefly 12th |
| | | | | mag. stars. |
| 5139. | | 13 20·2 | -46 44 | Very large; diameter 20′. |
| | | | | At ω Centauri. |
| 5272. | 3. | 13 37·1 | +28 56 | Visible to naked eye. |
| | | | | Diameter 7′. |
| 5634. | | 14 23·8 | - 5 29 | Very bright, considerably |
| | | | | large. Round. |
| 5904. | 5. | 15 13·0 | + 2 29 | Visible naked eye. Stars |
| | | | | 11th-15th mags. Diam. 5′. |
| 5986. | | 15 38·8 | -37 25 | Stars of 13th-15th mags. In |
| | | | | Lupus. |
| 6093. | 80. | 16 10·5 | -22 42 | Stars of 14th mag. Between |
| | | | | α and β Scorpii. |
| 6205. | 13. | 16 37·7 | +36 40 | Visible naked eye. A grand |
| | | | | object, in Hercules. |
| 6218. | 12. | 16 41·5 | - 1 45 | Stars of 10th mag. and |
| | | | | fainter. Diam. 4′. |
| 6254. | 10. | 16 51·4 | - 3 56 | Stars of 10th-15th mags. |
| | | | | Diameter 4′. |
| 6266. | 62. | 16 54·2 | -29 57 | Stars of 14th-16th mags. In |
| | | | | Scorpio. |
| 6333. | 9. | 17 12·8 | -18 24 | Much compressed group of |
| | | | | 14th mag. stars. Diam. 4′. |
| 6341. | 92. | 17 13·8 | +43 15 | A mass of stars and star-dust.|
| | | | | 7° N, π Herculis. |
| 6402. | 14. | 17 31·8 | - 3 11 | Chiefly stars 15th mag. |
| | | | | Diameter 4′. |
| 6656. | 22. | 18 29·7 | -24 0 | Stars of 11th-15th mags. |
| | | | | In Sagittarius. |
| 6779. | 56. | 19 12·3 | +30 0 | Stars 11th-14th mags. Between |
| | | | | β Cygni and γ Lyræ. |
| 6809. | 55. | 19 33·0 | -31 14 | Fine, large, round cluster of |
| | | | | stars 11th-13th mags. |
| 7078. | 15. | 21 24·7 | +11 41 | Group of stars and star-dus. |
| | | | | Diameter 5′. |
| 7089. | 2. | 21 27·8 | - 1 19 | Exceedingly small stars. |
| | | | | Diameter 5′. |
| 7099. | 30. | 21 34·1 | -23 41 | Stars 12th-16th mags. |
| | | | | Diameter 3′. |
+————————-+——————-+—————————+————————+———————————————————————————————+
III.—NEBULÆ.
+————————-+——————-+——————————————————+———————————————————————————————+
| | | Position, 1890. | |
| No. | No. +————————-+————————+ |
| N.G.C., | M., | | | Description. |
| 1888. | 1781. | R.A. | Dec. | |
+————————-+——————-+—————————+————————+———————————————————————————————+
| | | | | |
| | | h m | ° ′ | |
| 185. | | 0 32·9 | +47 44 | Very large; pretty bright. |
| | | | | Resolvable into stars. |
| 224. | 31. | 0 36·7 | +40 40 | Great nebula in Andromeda. |
| | | | | |
| 253. | | 0 42·2 | -25 54 | Very, very large and bright. |
| | | | | 24′ by 3′. |
| 598. | 33. | 1 27·6 | +29 57·1| Exceedingly bright and large.|
| | | | | Nucleus. |
| 650. | 76. | 1 35·4 | +51 1 | Very bright double nebula. |
| | | | | |
| 1365. | | 3 29·4 | -36 30 | Very bright and large. |
| | | | | Elliptical. |
| 1501. | | 3 57·5 | +60 37 | Pretty bright planetary |
| | | | | nebula. Diam. 1′. |
| 1514. | | 4 2·4 | +30 29 | Star of 9th mag. in nebula 3′ |
| | | | | diameter. |
| 1952. | 1. | 5 27·9 | +21 56 | Great Crab Nebula, near |
| | | | | ζ Tauri. Stars. |
| 1976. | 42. | 5 29·9 | - 5 28 | Great nebula involving θ |
| | | | | Orionis. |
| 1990. | | 5 30·6 | - 1 16 | Star (ε Orionis) involved in |
| | | | | nebulosity. |
| 2070. | | 5 39·5 | -69 9 | Visible to naked eye. Great |
| | | | | “looped” nebula. |
| 2392. | | 7 22·7 | +21 8 | Nebulous star of 9th mag. |
| | | | | |
| 2403. | | 7 26·2 | +65 50 | Very large and bright. |
| | | | | Elliptical. |
| 2655. | | 8 41·2 | +78 38 | Very bright. Condensed in the |
| | | | | middle. |
| 2681. | | 8 45·6 | +51 44 | Very large and bright. |
| | | | | Centre = star 10th mag. |
| 2683. | | 8 45·9 | +33 51 | Very large and bright. |
| | | | | Elliptical. |
| 2841. | | 9 14·4 | +51 26 | Very large and bright. |
| | | | | Centre = star 10th mag. |
| 2903. | | 9 25·9 | +22 0 | Large, elliptical, double |
| | | | | nebula. |
| 3031. | 81. | 9 46·5 | +69 35 | Exceedingly bright and large. |
| | | | | Elliptical. |
| 3034. | 82. | 9 46·7 | +70 13 | A bright ray. In field with |
| | | | | preceding. |
| 3242. | | 10 19·5 | -18 5 | Bright planetary nebula. |
| | | | | Diameter 45″. Blue. |
| 3372. | | 10 40·8 | -59 6 | Great nebula surrounding |
| | | | | ζ Argûs. |
| 3556. | | 11 5·4 | +56 16 | Large, rather bright. |
| | | | | Elliptical. |
| 3587. | 97. | 11 8·4 | +55 37 | Fine planetary nebula. Diam. |
| | | | | 3′.Near β Ursæ Majoris. |
| 3623. | 65. | 11 13·2 | +13 42 | Large, bright, elliptical. |
| | | | | Near following one. |
| 3627. | 66. | 11 14·5 | +13 36 | Large elliptical nebula. Near |
| | | | | β Leonis. |
| 4254. | 99. | 12 13·3 | +15 2 | Very fine 3-branched spiral |
| | | | | nebula. |
| 4321. | 100. | 12 17·4 | +16 26 | Very large 2-branched spiral |
| | | | | nebula. |
| 4382. | 85. | 12 19·9 | +18 48 | Very bright; pretty large. |
| | | | | Round. |
| 4472. | 49. | 12 24·2 | + 8 37 | Bright; round. Resolvable into|
| | | | | stars. |
| 4486. | 87. | 12 25·3 | +13 0 | Large; round. Bright centre. |
| | | | | Third of three. |
| 4565. | | 12 30·9 | +26 36 | A ray of bright nebulosity E. |
| | | | | of Coma. |
| 4736. | 94. | 12 45·7 | +41 43 | Large and bright. Nucleus. |
| | | | | Resolvable. |
| 5128. | | 13 19·0 } -42 27 | Very large and bright. |
| | | | | Elliptical. Bifid. |
| 5194. | 51. | 13 25·2 | +47 46 | Great spiral nebula near |
| | | | | ζ Ursæ Maj. |
| 5236. | 83. | 13 30·8 | +29 18 | Fine object. 3-branched |
| | | | | spiral. |
| 5367. | | 13 51·1 | -39 27 | Very large and bright. |
| | | | | Condensed in the middle. |
| 5907. | | 15 13·0 | +56 44 | Large, elliptical. Another |
| | | | | very close to it. |
| 6369. | | 17 22·6 | -23 40 | Pretty bright, small |
| | | | | ring-nebula. |
| 6514. | 20. | 17 55·7 | -23 1 | Bright; large. Trifid. Double |
| | | | | star involved. |
| 6523. | 8. | 17 56·9 | -24 23 | Bright, with loose cluster of |
| | | | | stars. |
| 6618. | 17. | 18 14·4 | -16 13 | Bright and extremely large. |
| | | | | 2-hooked. |
| 6720. | 57. | 18 49·5 | +32 54 | Ring-nebula between β and |
| | | | | γ Lyræ. |
| 6826. | | 19 41·8 | +50 16 | Pretty large and bright |
| | | | | planetary nebula. |
| 6853. | 27. | 19 54·9 | +22 25 | The “Dumb-bell” Nebula. Fine |
| | | | | object. |
| 6960. | | 20 41·1 | +30 19 | Large and bright, κ Cygni |
| | | | | involved. |
| 7009. | | 20 58·2 | -11 48 | Very bright, small, planetary |
| | | | | nebula. Elliptical. |
| 7662. | | 23 20·6 | +41 56 | Very bright, pretty small, |
| | | | | planetary or ring-nebula. |
+————————-+——————-+——————————————————+———————————————————————————————+
FOOTNOTES:
[55] Sir W. Herschel at first entertained this view, finding that with
every increase of telescopic power more nebulæ were resolved. But in
1791 he said, “perhaps it has been too hastily surmised that all milky
nebulosity is owing to starlight only.” Lacaille had remarked in 1755
that “it is not certain the whiteness of parts of the Milky Way is
caused by clusters of stars more closely packed together than in other
parts of the heavens.”
[56] This is exclusive of 47 new nebulæ discovered by Prof. Safford,
which form the appendix to the catalogue.
[57] Chambers says only four examples are known, but this is erroneous,
as Lord Rosse’s telescope has added five ring-nebulæ to the four
previously catalogued.
[58] Some of the nebulæ in Messier’s list were discovered by Mechain at
Paris, who, like Messier, earned celebrity by his cometary discoveries.
He was born at Laon in 1744, and died at Valencia in 1805.
[59] O. Struve had expressed views identical with these in 1857 (see
‘Monthly Notices,’ vol. xvii. p. 230).
[60] Humboldt says this “name is evidently derived from the voyage of
Magellan, although he was not the first who observed them.”
[61] I have selected the various objects in these lists from the New
General Catalogue.
[62] These forms are more numerous than the annular nebulæ. They often
exhibit a blue colour, and the spectroscope shows them to consist of
gas.
NOTES AND ADDITIONS.
LARGE AND SMALL TELESCOPES.
P. 19.—With reference to mountainous sites for large instruments,
a remark in Sir Isaac Newton’s ‘Opticks’ (1730) may be
quoted:—“Telescopes ... cannot be formed so as to take away that
confusion of rays which arises from the tremors of the atmosphere. The
only remedy is a most serene and quiet air, such as may perhaps be
found on the tops of the highest mountains above the grosser clouds.”
P. 27.—Lieut. Winterhalter, of the United States Navy, recently
visited a large number of European observatories, and in describing
that of Nice says:—“M. Perrotin declares that two hours’ work with
a large instrument is as fatiguing as eight with a small one, the
labour involved increasing in proportion to the cube of the aperture,
the chances of seeing decreasing in the same ratio, while it can
hardly be said that the advantages increase in like proportion.” The
Nice Observatory, and its splendid instruments (including a 30-inch
refractor), are due to the munificence of M. Bischoffsheim, who has
expended about five million francs upon them.
P. 36.—The large refractor to be erected on Wilson’s Peak of the Sierra
Madre range of mountains, in Southern California, is to be 40 inches in
diameter. The rough unground disks of glass are already in the hands of
the Clarkes, of Cambridgeport, Mass. It is estimated that the complete
object-glass and cell will cost something like $65,000, and the focal
length of the instrument will be about 58 feet.
THE SUN.
P. 100.—The last minimum of sun-spot frequency appears to have occurred
at the middle of 1889. Conspicuous spots were very rare in the first
half of 1890, but some fine groups were presented in the last half of
the year. On Aug. 31 I saw a group extending over 113,000 miles in
length, and on Nov. 27 there was another, which measured 123,700 miles.
P. 111.—Thompson’s cardboard disks have been favourably spoken of as
enabling observers to determine the positions of spots at any season of
the year.
MERCURY.
P. 137.—At the meeting of the British Astronomical Association on
Nov. 26, 1890, Mr. G. F. Chambers expressed his firm belief in the
existence of an intra-Mercurial planet. The President (Capt. W. Noble)
in his inaugural address pointed out the desirability of effecting
further observations, both of Mercury and Venus, with a view to
redetermine their rotation-periods. He justly remarked that moderately
small instruments might be fittingly employed in the work, and that
Schiaparelli’s deductions (mentioned on pp. 142 and 149) ought to be
accepted with extreme reserve pending their verification.
MARS.
P. 160.—Prof. W. H. Pickering observed some of the canals on Mars in
1890 with a 12-inch refractor, but was not able to double any of them.
He says that, in examining these objects, the power employed should
not “exceed one or two hundred.” This is quite contrary to the advice
of others, who recommend high magnifiers; and perhaps it accounts for
Prof. Pickering’s failure in recognizing the duple canals.
With the great 36-inch refractor Mr. Keeler saw, on July 5 and 6, 1890,
some curious white spots on the edges of the gibbous limb of Mars,
something similar to those visible on the unilluminated part of the
lunar disk. The canals were observed as feeble diffused bands. The two
satellites were seen by a lady visitor, though previously unaware of
their existence.
P. 161.—The method of deriving the rotation-period of Mars is
exemplified by Mr. Proctor in the ‘Monthly Notices,’ vol. xxviii. p.
38. An interesting paper, “On the Determination of the Rotation-Period
of Jupiter in 1835,” will be found in the ‘Memoirs,’ vol. ix.
PLANETOIDS.
P. 167.—The 308th planetoid was discovered by Charlois on March 5, 1891.
JUPITER.
P. 170.—Dupret, in Algiers, saw Jupiter with the naked eye on Sept. 26,
1890, and following days, twenty minutes before sunset.
P. 191.—M. Guillaume, during a recent transit of the shadow of
Jupiter’s second satellite, observed a duplicate shadow, fainter than
the ordinary one, which partly covered its southern side.
COMETS.
P. 250.—On Nov. 16, 1890, Dr. Spitaler, while looking for Zona’s Comet
with the 27-inch refractor of the Vienna Observatory, discovered a new
and very faint comet only 23′ distant from the object of his search.
That two of these bodies should be found almost simultaneously and so
near together must be regarded as a very singular coincidence.
METEORS.
P. 261.—Mr. Proctor held the view that certain meteorites may have
originally been ejected from the Sun. A recent writer thus summarizes
our knowledge of them:—“That they are independent bodies, moving in
orbits of their own in space; that these dark bodies are abundant in
the interplanetary spaces; that those within the near range of solar
or planetary attraction move with great velocity; that many swarms of
them follow well-known orbits; and that, in general, their origin is
undoubtedly the same as that of other celestial bodies” (‘Sidereal
Messenger,’ June 1890, p. 284).
P. 267.—On May 2, 1890, a brilliant fireball, leaving a long train of
fire and smoke, and exploding with a noise like thunder, was seen at
many places in Northern Iowa, Minnesota, U.S.A. Some fragments of the
meteor fell on a farm a few miles from the south line of Minnesota.
The largest piece was sold by auction for $100, but it soon transpired
that the person who sold it was only the lessee and not the owner
of the ground on which the meteor fell. The aerial visitor and its
purchase-money were therefore peremptorily seized by legal authorities,
pending the decision of a Court of Justice as to the rightful ownership.
P. 267.—On December 14, 1890, at 9^h 42^m a large fireball of dazzling
lustre, and giving a report like thunder, was widely observed in the
southern parts of England. At the end-point the fireball appears to
have been only 8 miles in height, and over a point near Brentwood, in
Essex.
THE STARS.
P. 309.—Prof. Chandler, of Cambridge, Mass., estimates that the total
number of variable stars visible with a common field-glass is about
2000, but with a large telescope there are probably hundreds of
thousands within reach. He further states that quite five sixths of the
variable stars are reddish in colour, and that the redness is usually a
function of the length of the period of variation. The redder the star
the longer its period.
P. 312.—In a recent communication to the Academy of Sciences, M.
Lescarbault (the alleged discoverer of Vulcan in 1859) announced that
on the night of January 11, 1891, he discovered a bright body in Leo
which he could not identify in any star-map, and hence concluded it
to be a new star, or one suddenly increased in brilliancy. The “new
star,” however, subsequently turned out to be the planet Saturn! This
ridiculous mistake (so easily avoidable with a little care) will
naturally divest the supposed discovery of Vulcan of the importance
attached to it by some writers, for M. Lescarbault obviously lacks the
experience and caution necessary to command credit.
NEBULÆ AND CLUSTERS OF STARS.
P. 327.—Mr. Roberts, from a comparison of his photographs, has found
distinct evidence of variability in the nucleus of the great nebula
in Andromeda. In some of the photographs the nucleus is shown to
be stellar, while in others there is no trace of this. Mr. Roberts
remarks:—“We may reasonably infer that the nucleus of the nebula is
variable, and that it will be practicable to study the character of
the variability without the necessity of giving long exposures of the
plates.” The period of the variation has now to be determined, and it
is advisable that telescopic observations of the nucleus should be
made with the view of confirming the photographic results. It would be
premature to regard the changes as demonstrated before they have been
submitted to thorough investigation.
P. 327.—In the _Comptes Rendus_ for March 2, 1891, M. Bigourdan has
a paper on the variability of the nebula N.G.C. 1186, situated near
Algol. This nebula was discovered by Sir W. Herschel in 1785, and
though Sir J. Herschel re-observed it in 1831, Lord Rosse looked for
it without success in 1854 and 1864. On Nov. 8, 1863, D’Arrest failed
to detect the nebula, though he searched for it with assiduity at a
time when the sky was very favourable. He was led to conclude that
the object did not exist. M. Bigourdan finds that the nebula is again
visible in the position indicated by the two Herschels, viz. R.A. 2^h
54^m 20^s, Dec. +42° 10′, he having observed it on Jan. 31 and Feb. 26,
1891. It is difficult to believe that this object could have escaped
the scrutiny of Lord Rosse and D’Arrest in 1854, 1863, and 1864; hence
the variation is probably real. The nebula may be easily found, as it
is very near the binary B.D. +42° (1123 G.C.), the position of which
for 1891 is R.A. 2^h 58^m 6^s, Dec. +42° 29’ (‘Nature,’ March 12, 1891).
P. 329.—While examining the Pleiades on the night of November 14,
1890, Mr. Barnard discovered a new and considerably bright, round,
cometary nebula 36″ S. and 9″ following Merope. The reason why this
nebula has not been detected by photography is because it is so near
Merope that the over-exposed light from the star obliterates it. But
it is certainly very strange that the object alluded to has never
been telescopically discovered before; for the Pleiades have been
scrutinized repeatedly with all sorts of telescopes, and particularly
since Tempel announced his discovery of a large faint nebula involving
Merope in 1859. Mr. Barnard says the new nebula is 30″ in diameter,
and that it is visible in a 12-inch refractor when Merope is hidden
with a wire.
INDEX.
Action in Sun-spots, Cyclonic, 108.
Active volcanoes on the Moon, 120.
Adams theoretically discovers Neptune, 222.
Advantage of Equatoreals, 54.
Aerolites, 264.
Air and water on the Moon, Absence of, 115.
Algol, 310.
Alleged satellite of Venus, 152.
Almanacks, 83.
Alphabet, Greek, 287.
Alpine Valley, 127.
Altitudes of markings on Jupiter, 185.
Amateur’s first view of Mercury, 139.
Ancient ideas concerning meteors, 260.
Andromeda, Great Nebula in, 334.
Andromedes, 276.
Angles of Position, Measurement of, 291, 306.
Announcement of a new comet, 244.
Annual rate of cometary discoveries, 255.
Antares, 309.
Anthelme, Discoverer of a new star in 1670, 313.
Apennines, 132.
Aperture and Power required for Comet-seeking, 252.
Apparitions, Meteoric, 261.
Appearance of Comets, 228.
—— of Mars, 155.
Aquarids, 275.
Archimedes, 127.
Argelander’s magnitudes of stars, 294.
Aristarchus, 120.
Ascertaining positions of Comets, 257.
Aspect of the rings of Saturn, 204.
Atmosphere of Jupiter, 177.
—— of Mars, 161.
—— of Mercury, 139.
—— of Venus, 151.
Atmospheric undulations, 29.
Attractions of Telescopic work, 85.
Auwers, Discoverer of a new star in 1860, 314.
Bacon, Roger, Early hints on refracted rays, 3.
Barnard, His cometary discoveries, 255.
—— observes Brooks’s multiple Comet, 239.
—— observes new stars in the Trapezium, 319.
—— observes a new nebula in the Pleiades, 351.
Beauty and brilliancy of Venus, 145,
Belts of Sun-spots, 104,
—— on Jupiter, 172.
—— on Saturn, 198.
—— on Uranus, 218.
Berthon’s dynamometer, 50.
Biela’s Comet, 238.
Bigourdan observes a variable Nebula, 351.
Binary Stars, 300.
Birmingham discovers a new star in 1866, 314.
Bond, G. P., discovers Crape-ring of Saturn, 202.
Brahe’s, Tycho, new star of 1572, 312.
Bright objects near the Sun, 107.
Brightness and position of Jupiter, 170.
Brooks on Comet-seeking, 253.
—— on Occultation of Jupiter, 187.
—— on Shower of telescopic Meteors, 272, 274.
Brooks’s double Comet of 1889, 239.
Brorsen’s Comet, 239.
Browning and reflecting-telescopes, 60.
Brunowski discovers a new star, 313.
Burnham, Discoverer of double Stars, 31, 320.
—— discovers a group of 18 new nebulæ, 341.
——, Measures of the companion to Sirius, 307.
—— on the inutility of “stops,” 58.
Calver compares light of reflectors and refractors, 37.
——, Maker of glass specula, 16, 17.
Canal-shaped markings on Mars, 159.
Canis Majoris α, 307.
Cassegrain’s reflecting-telescope, 10.
Cassini, Diameter of his object-glasses, 9.
—— discovers four satellites and the divided ring of Saturn, 9.
——, Observations of Jupiter, 172.
——, —— of Saturn, 198.
——, —— of Venus, 147.
Celestial Globe, 63.
Centauri α, Diameter and distance, 299.
Ceres, 168.
Chambers on Coloured Stars, 316.
—— on the intra-Mercurial Planet, 348.
Chandler on Variable Stars, 350.
Changes, Lunar, 120.
—— on Jupiter, 182.
—— on Mars, 163.
—— on Saturn, 206.
Charts of Mars, 158.
Cheapness of Telescopes, 57.
Choice of Telescopes, 38.
Clark, Alvan, & Sons, make large object-glasses, 18.
——, discovers the companion to Sirius, 307.
Cleaning lenses, 59.
Clusters of Stars, 317.
Coggia’s Comet of 1874, 233.
Colour of Jupiter, 171.
—— of Mars, 155.
—— of Saturn, 195.
—— of Uranus, 217.
Colouring of the eclipsed Moon, 119.
Colours of Stars, 315.
Coma Berenices, 317.
COMETS AND COMET-SEEKING, 227.
Ideas concerning Comets, 227.
Appearance of Comets, 228.
Large number visible, 228.
Nature of apparition, 229.
Tenuity, 229.
Differences of orbit, 230.
Discoveries of Comets, 230.
Large Comets, 231.
Periodical Comets, 234.
Halley’s Comet, 236.
Encke’s Comet, 236.
Biela’s Comet, 238.
Brooks’s double Comet, 239.
Brorsen’s Comet, 239.
Faye’s Comet, 240.
D’Arrest’s Comet, 240.
Pons-Winnecke’s Comet, 240.
Tuttle’s Comet, 241.
Grouping of Periodical Comets, 241.
Further Observations required, 243.
Nomenclature of Comets, 246.
Curiosities of Comets, 248.
Naked-eye Comets, 248.
Comet-seeking, 249.
English weather and Comet-seeking, 251.
Aperture and Power required, 252.
Annual rate of discovery, 255.
Telescopic Comets, 256.
Ascertaining positions, 257.
Dr. Doberck’s hints, 258.
Prizes for Discoveries, 258.
Common, His large Reflectors, 15;
Their performance, 28.
Computation of a Meteor’s real path, 278.
Conjunctions, Planetary, 225.
Constellation figures, The, 290.
Cooke & Sons mount a 24·8-inch refractor, 18;
Its barren record, 25.
Copernicus, 127.
Course of the Milky Way, 296.
“Crab” Nebula in Taurus (M. 1), 336.
Crape-ring of Saturn, 202.
Crucis κ, Cluster at, 318.
Curiosities of Comets, 248.
Cyclonic action in Sun-spots, 108.
Cygnus, Nebulous streams in, 339.
Dallmeyer on Dividing power, 293.
D’Arrest’s Comet, 240.
Dawes’s observations of Jupiter, 173.
—— observations of Saturn’s Crape-ring, 202.
——, On Dividing power, 292.
—— Solar Eyepiece, 92.
Definition in towns, 81.
Deimos, Outer Satellite of Mars, 165.
Democritus explains the Milky Way, 2.
Dennett announces the Red Spot on Jupiter, 173.
Denning’s Comet, 243.
Denza on the Meteors of Nov. 27, 272.
Derham, his list of Nebulæ, 327.
Description of Nebulæ and Clusters of Stars, 333.
Determination of the Sun’s rotation-period, 104.
Detonating Fireballs, 267.
Dewing of Mirrors, 62.
Diffraction-rings, 293.
Dimensions of Nebulæ and Scar-clusters, 332.
—— of Sun-spots, 94.
Disappearance of Saturn’s ring, 205.
Discordant observations of Saturn, 204.
Discoveries of Comets, 230.
—— of Nebulæ, 341.
Discovery of Neptune, 221.
—— of Planetoids, 167.
—— of Uranus, 215.
Distance of the stars, 299.
Distinction between Nebulæ and Star-clusters, 324.
Distribution of Nebulæ in R.A., 326.
Disturbances, Recurrent solar, 110.
Dividing power, 292.
Divisions in outer ring of Saturn, 201.
Doberck, Dr., On the Invention of the Telescope, 5.
——, On Comet-seeking, 258.
Dollond patents his Achromatic Telescope, 12.
——, His object-glasses, 16.
Donati’s Comet of 1858, 233.
Dörfel mountains, 131.
Double Comets:—
Biela’s, 238.
Brooks’s, 239.
—— Nebulæ, 332.
—— Stars, 300, 302.
Draco, planetary nebula in, 335.
Drawing, 73.
Drawings of Jupiter, 185.
Dumb-bell Nebula (M. 27), 337.
Duration of meteor-flights, 282.
Duration of Silver-on-glass films, 60.
Dynamometer, Berthon’s, 50.
Early observations of Jupiter, 172.
—— —— of nebulæ and star-clusters, 326.
—— —— of Neptune, 222.
—— —— of Saturn, 197.
—— —— of the Sun, 88.
—— —— of Uranus, 216.
—— —— of Venus, 147.
Earthshine on the Moon, 116.
Eccentric position of Saturn’s rings, 204.
Eclipses of Jupiter’s satellites, 189.
—— of the Moon, 118.
—— of the Sun, 97.
Elger’s lunar observations, 127, 131.
—— Drawings of lunar objects, 129, 130, 132.
Ellipse, 230.
—— on Jupiter, Gledhill’s, 173.
Elliptical nebulæ, 338.
Elongations of Mercury, 138.
—— of Venus, 145.
Encke’s Comet, 236.
—— division in Saturn’s ring, 202, 208.
English weather and Comet-seeking, 251.
Equatoreal spots on Jupiter, Bright, 175, 181.
—— ——, Dark, 181.
Equatoreals, Advantage of, 54.
Exceptional position of Sun-spots, 111.
Eyepiece, Field of, 50.
Eyepieces, 46.
——, Single-lens, 47.
Fabricius observes Sun-spots, 89.
Faculæ, Sudden outburst of, 108.
Faint objects, Observation of, 72.
Faintness of the markings on Venus, 150.
Falls of stone and iron, 266.
Faye’s Comet, 240.
Field of eyepiece, Diameter of, 50.
Figures, The Constellation, 290.
Fireball of Nov. 23, 1877, 267.
Fireballs, 267.
——, Heights of, 268.
First view of Mercury, Amateur’s, 139.
Formations, Lunar, 123.
Foucault parabolizes and silvers glass Speculæ, 15.
Fracastor, His remarks on lenses in 1538, 4.
Friendly Indulgences, 74.
Future, Past and, 84.
Future eclipses of the Moon, 118.
—— —— of the Sun, 98.
Galaxy, or Milky Way, The, 295.
Galilei and the invention of the Telescope, 2, 4, 5.
——, Discovery of Jupiter’s satellites, 187.
——, His first instrument and discoveries, 6, 7.
Galle observes Saturn’s crape-ring, 202.
—— observes Neptune, 222.
Geminids, 276.
Glass, Opera, 61.
Gledhill’s ellipse on Jupiter, 173.
Globe, Celestial, 63.
Globular clusters. 338.
—— ——, List of, 344.
Gore, Diameter of α Centauri, 299.
——, Dimensions of a Star-cluster, 332.
——, Stellar distribution, 294.
Greek alphabet, 287.
Gregory invents a reflecting-Telescope, 10.
Grimaldi, 129.
Grouping of periodical Comets, 241.
Groups of Stars, 316.
Grubb, Maker of a 4-foot Cassegrainian reflector, 14;
Performance of, 25.
——, Maker of a 27-inch refractor, 18;
Performance of, 27.
Hall, Chester More, invents achromatic Object-glass, 11.
Hall, Prof., discovers a white spot on Saturn, 199.
——, Observations of Saturn’s satellites, 213.
—— on the great Washington refractor, 26.
——, Remarks on large and small telescopes, 31.
Halley’s Comet, 236.
—— list of Nebulæ in 1716, 326.
Harriot, Early observer of Sun-spots, 89, 90.
Hartwig, Discovers a new Star in Andromeda, 315.
Heights of Fireballs, 268;
of Meteors, 277.
Heis, His labours in Meteoric astronomy, 262.
Hencke, Discoverer of Planetoids, 167.
Henry, Bros., make a 30-inch refractor, 18;
Performance of, 27.
—— observe the belts on Uranus, 218.
Herschel, Prof. A. S., observes meteors, 262.
Herschel, Sir J., Comet of 1861, 233.
——, Description of k Crucis, 318.
——, Disappearance of Saturn’s ring, 205.
——, Rediscovers Uranus, 219.
——, Satellites of Uranus, 220,
——, Texture of Comets, 229.
——, Thickness of Saturn’s ring, 205.
——, Trapezium of Orion, 318, 320.
Herschel, Sir W., and Cometary discovery, 231.
——, His discovery of Nebulæ, 327.
——, His discovery of nebulous Stars, 330.
——, His discovery of Uranus, 215;
of Satellites, 220.
——, His method of observing Sun-spots, 91.
——, His observations of Jupiter, 182.
——, His observations of Mercury, 142.
——, His observations of Saturn, 199.
——, His observations of Venus, 149.
——, Nucleus of Comet of 1811, 232.
—— observes Binary Stars, 300, 306.
——, Performance of 4-ft. reflector, 21.
——, Remarks on eyepieces, 47.
——, Rotation of Jupiter’s Satellites, 189.
——, Singular figure of Saturn, 196.
Herschel’s, Sir W., Telescopes, 12, 13, 39.
Hevelius, diameter of his object-glasses, 9.
Hind, Discoverer of a new Star in 1848, 314.
——, Discoverer of Planetoids, 167.
——, Discoverer of a variable Nebula, 328.
Hipparchus forms a Star-catalogue, 312.
Hoek on the origin of Comets, 243.
Hooke’s observations of Jupiter, 172.
Hough’s observations of Jupiter, 174, 182.
Howlett’s observations of Sun-spots, 101, 102.
Huygens on the invention of the Telescope, 2.
——, Discoveries on Saturn, 8, 198.
——, Length of his instruments, 8.
Huygens’s Negative eyepiece, 8, 46.
Hyginus, The rill or cleft of, 130.
Hyperbola, 230.
Identity of Meteors and Comets, 262.
Increasing number of Telescopes, 57.
Intra-Mercurial Planet, 137.
Jansen, Zachariah, Inventor of the Telescope, 4.
Johnson’s projections of Solar Eclipses, 99.
Juno, 168.
JUPITER, 170.
An interesting object, 170.
Brightness and position, 170.
Period &c., 171.
Belts and spots, 172.
Observations of Hooke, Cassini, and others, 172.
The “Ellipse” of 1869-70, 173.
The red spot, 173.
Rotation of red spot, 175.
Rotation of bright equatoreal spots, 175.
Rotation of dark spots in N. hemisphere, 175.
Rotation-period, 176.
Nature of the red spot, 177.
Bright equatoreal spots, 181.
Dark equatoreal spots, 181.
New belts, 182.
Changes on the planet, 182.
Further observations required, 183.
Occultations by the Moon, 185.
The four satellites, 187.
Their eclipses, occultations, and transits, 189.
The planet without visible satellites, 192.
Spots on the Satellites, 193.
Occultation of a Star by Jupiter, 193.
Keeler, White spots and canals on Mars, 348.
Kitchiner, The inutility of large Telescopes, 35.
——, Singular form of Saturn, 196.
Klein’s supposed new crater near Hyginus, 122.
Large and small telescopes compared, 20.
—— Comets, 231.
—— number of Comets, 228.
—— refractor intended for California, 36, 347.
Lassell, His large reflecting-telescopes, 14;
Their performance, 24.
—— discovers the satellite of Neptune, 224.
—— glimpses a belt on Uranus, 217.
Leander McCormick refractor, 26.
Learning the names of the Stars, 287.
Leibnitz mountains, 131.
Le Mairean or Herschelian telescope, 13.
Lenses, Cleaning, 59.
—— out of centre, 55.
Leonids, 276.
Lescarbault rediscovers Saturn, 350.
Le Verrier, Theoretical discoverer of Neptune, 222.
Lick, James, Founder of the Lick Observatory, 18.
Lick refractor, Performance of the, 27.
Light of Comets, Fluctuating, 245.
Limited means no obstacle, 51.
Lippersheim, Hans, Inventor of the telescope, 4, 5.
Lunar changes, 120.
—— formations, 123.
Lyræ α, 308.
Lyrids, 275.
Mädler’s observations of Lunar objects, 127, 131.
—— —— of Mars, 158.
—— —— of Venus, 149.
Magellanic clouds, 331.
Magnitudes of Stars, 294.
Marius, Simon, observes Jupiter’s satellites, 6.
—— observes Nebula in Andromeda, 326.
Markings on Mercury, Surface-, 142.
—— on Venus, 147.
—— ——, Faintness of, 150.
MARS, Appearance of, 155.
Period &c., 155.
Phase, 156.
Surface-configuration, 156.
Charts and nomenclature, 158.
Discovery of satellites and canal-shaped markings, 159.
Summary of observations, 160.
Rotation, 161.
Further observations required, 162.
Changes on the Planet, 163.
Satellites, 164.
Occultations by the Moon, 166.
Martin’s 4-foot reflector at Paris, 15;
Its performance, 25.
—— 29-inch refractor at Paris, 18.
Maunder on Sun-spots, 93.
Maxima and minima of Sun-spots, 100.
Means of measurement, 290.
MERCURY, 137.
Supposed planet Vulcan, 137.
Visibility, 138.
Period &c., 138.
Elongations, 138.
Amateur’s first view, 139.
Phases, 139.
Atmosphere, 139.
Telescopic observations, 140.
Schiaparelli’s results, 141.
Observations of Schröter and W. Herschel, 142.
Surface-markings, 142.
Transits across the Sun, 143.
Occultations, 144.
Messier, The Comet-hunter, 249.
Messier’s lists of Nebulæ, 327.
—— large Comet of 1769, 232.
METEORS AND METEORIC OBSERVATIONS, 260.
Ancient ideas, 260.
Meteoric apparitions, 261.
Radiation of Meteors, 262.
Identity of Meteors and Comets, 262.
Aerolites, 264.
Fireballs, 267.
Heights of Fireballs, 268.
Meteorite from Biela’s Comet, 270.
Differences of motion, 271.
Nomenclature of Meteor-systems, 271.
Meteor-storms, 271.
Telescopic Meteors, 272.
Meteor-showers, 274.
Varieties of Meteors, 276.
Meteor of Dec. 28, 1888, 277.
Average heights of Meteors, 277.
Computation of Meteor-heights, 278.
Meteoric observations, 279.
Meteors and terrestrial objects, 284.
—— and gales of wind, 285.
Method, 78.
Milky Way or Galaxy, 2, 295.
Minimum of Sun-spots, 347.
Mirrors, Dewing of, 62.
MOON, Attractive aspect of the, 113.
Diameter and distance, 114.
Crateriform aspect, 114.
Absence of air and water, 115.
Only one hemisphere visible, 115.
Earthshine, 116.
Telescopic observations, 116.
Eclipses, 118.
Physical changes, 120.
Active volcanoes, 120.
Crater Aristarchus, 120.
—— Linné, 121.
—— near Hyginus, 122.
General description of formations, 123.
Description of special objects, 125-132.
Objects near terminator, 133.
Occultation of Stars, 135.
Visibility of new and old Moon, 136.
Moonlight and planetary definition, 187.
Motion of light, 190.
Motion of Stars in the line of sight, 300.
—— of Sun-spots, Proper, 106.
Mounting of Telescopes, 45.
Naked-eye views of Comets, 248.
—— —— of Jupiter in daylight, 170.
—— —— of Jupiter’s satellites, 188.
—— —— of Mercury, 139.
—— —— of Sun-spots, 89.
—— —— of Uranus, 217.
—— —— of Venus in transit, 105.
—— —— of Vesta, 168.
Names of the Stars, Learning the, 287.
Nasmyth and Carpenter describe Plato, 126.
Nasmyth’s Telescopes, 16.
—— “Willow-leaves,” 101.
Nature of Cometary apparitions, 229.
—— of the red spot on Jupiter, 177.
NEBULÆ AND CLUSTERS OF STARS, 324.
Distinction, 324.
Large number visible, 324.
Varieties of form and grouping, 325.
Distribution in R.A., 326.
Early observations, 326.
Variable Nebulæ, 327.
Nebulous Stars, 330.
The Magellanic Clouds, 331.
Double Nebulæ, 332.
Real dimensions of Nebulæ and Clusters, 332.
Round Nebulæ and Clusters, 332.
Description of Nebulæ and Clusters, 333.
Great Nebula in Andromeda, 334.
—— —— in Orion, 334.
Planetary Nebulæ, 334.
Spiral Nebula, 335.
Crab Nebula in Taurus, 336.
Dumb-bell Nebula, 337.
Ring Nebula in Lyra, 337.
Elliptical Nebulæ, 338.
Globular Clusters, 338.
Further observations, 339.
Discovery of new Nebulæ, 341.
New Nebulæ discovered at Bristol, 342.
List of Clusters of Stars, 343.
—— of globular Clusters, 344.
—— of Nebulæ, 345.
Nebulous Stars, 330.
Neison, Lunar observations, 128, 129.
NEPTUNE, Discovery of, 221.
Observations in 1795, 222.
Period &c., 223.
Observations, 223.
Supposed ring, 223.
The satellite, 223.
New or temporary Stars, 312.
Newton, 128.
Newton, Sir Isaac, Experiments on Colours, 9.
——, His reflecting-telescope, 11.
——, On mountainous sites for telescopes, 347.
Noble, Occultation of Jupiter, 186.
——, Occultation of Saturn, 210.
—— on observations of Mercury and Venus, 348.
Nomenclature of Comets, 246.
—— of Lunar formations, 123.
—— of Mars, 158.
—— of Meteor-systems, 271.
Number of Comets visible, Large, 228.
—— of Nebulæ and Star-clusters, 324.
—— of Planetoids, 167.
—— of Stars, 293.
Observations of Neptune, 223.
—— required of the Sun, 97.
—— —— of the Moon, 116.
—— —— of Mercury, 143.
—— —— of Venus, 152.
—— —— of Mars, 162.
—— —— of Jupiter, 183.
—— —— of Saturn, 205.
—— —— of Uranus, 219.
—— —— of Comets, 243.
—— —— of Meteors, 279.
—— —— of Stars, 320.
—— —— of Nebulæ, 339.
——, Solar, 88.
Observatories, 64.
Observer’s aims, 42.
Observing, Open-air, 75.
Observing-seats, 53.
Occultations of Jupiter, 185.
—— of Jupiter’s satellites, 189, 190.
—— of Mars, 166.
—— of Mercury, 144.
—— of Regulus by Venus, 154.
—— of Saturn, 209.
—— of Star by Jupiter, 193.
—— of Venus, 153.
—— of Vesta, 169.
Olbers discovers Pallas and Vesta, 167.
——, His Comet of 1815, 235, 241.
——, Observer of Comets, 250.
Open-air observing, 75.
Opera-glass, 61.
Orbits of Comets, Differences in, 230.
Orion, Great Nebula in, 334.
——, The constellation, 289.
Orionids, 275.
Orionis β, 307.
—— θ, 318.
—— σ, 318.
Outbursts of Faculæ, 108.
Palisa, Discoverer of Planetoids, 167.
Palitzch, Discoverer of Halley’s Comet, 236.
Pallas, 168.
Parabola, 230.
Past and future work, 84.
Periodical Comets, 234.
—— ——, Grouping of, 241.
Periodicity of Jupiter’s markings, 184.
—— of Sun-spots, 100.
Perrotin observes the belts on Uranus, 218.
—— —— the canals on Mars, 27, 160.
—— on work with a 30-inch refractor, 347.
Perry on drawing Sun-spots, 93.
—— observes veiled Sun-spots, 110.
Persei β (Algol), 310.
—— χ, 317.
Perseids, 275.
——, Their shifting radiant-point, 283.
Perseverance, 79.
Petavius, 128.
Peters, Discoverer of Planetoids, 167.
Phase, Epochs of similar, 117.
—— of Jupiter, 172.
—— of Mars, 156.
Phases of Mercury, 139.
—— of Venus, 147.
Phobos, Inner Satellite of Mars, 165.
Photography, 82.
Photometric measures of Starlight, 295.
Physical aspects of Comets, 244.
—— changes on the Moon, 120.
Pickering on the canals of Mars, 348.
Planetary bodies on the Sun, 105.
—— conjunctions, 225.
—— Nebula, 334.
Planetoid, The 308th, 349.
PLANETOIDS, Number of, 167.
History of their discovery, 167.
Occultation of Vesta, 167.
Dimensions and brightness, 168.
Plato, 125.
Polaris, 308.
Pons, Discoverer of many Comets, 250.
Pons’s Comet of 1812, 241, 242, 245.
Pons-Winnecke’s Comet, 240.
Powers, Method of determining, 49.
——, Overstating, 49.
——, Requisite magnifying, 48.
Præsepe, 317.
Preparation of the observer, 66.
Princeton refractor, Performance of, 26.
Prizes for Cometary discoveries, 258.
Proctor on Amateur observers, 163.
—— on Sun-ejected Meteors, 349.
Projection of satellites of Jupiter, 190.
—— of Stars on the Moon, 135.
Prominences, Solar, 111.
Proper motion of spots on Jupiter, 173.
—— —— of Stars, 299.
—— —— of Sun-spots, 106.
Publications, Astronomical, 83.
Pulkowa, The 30-inch refractor at, 27.
Quadrantids, 274.
Radiation of Meteors, 262.
Ramsden’s positive eyepiece, 47.
Ranyard, Absorption of light by object-glasses, 37.
Recording Meteor-tracks, 280.
Records, 72.
Recurrent disturbances on the Sun, 110.
—— forms on the Sun, 111.
Red spot on Jupiter, Appearance of, 173;
Rotation of, 175;
Nature of, 177.
Refracting-lenses or burning-glasses, 3.
Refracting-telescope, 12.
Refractors and Reflectors, 39.
Rheita, Valley near, 131.
Rigel, 307.
Ring nebula in Lyra, 337.
—— of Neptune, Supposed, 223.
—— of Saturn, Division in the outer, 201.
—— ——, The Crape, 202.
Rings of Saturn, 201.
—— ——, Aspect of the, 204.
—— ——, Eccentric position of the, 204.
—— ——, Thickness, 205.
Roberts’s photographs of the Nebula in Andromeda, 334, 351.
—— —— of Nebulæ in Ursa Major, 338.
Rosse, Lord, Large reflecting-telescopes, 14;
Their performance, 21.
Rotation of Comets, Visible evidences of, 246.
—— of Jupiter, 176, 348.
—— of Mars, 161, 348.
—— of Mercury, 142.
—— of Saturn, 199.
—— of the Sun, 103.
—— of Uranus, 217.
—— of Venus, 149.
Round Nebulæ and Clusters, 332.
Safarik on Telescopic Meteors, 273.
Saros, The, 99.
Satellite of Neptune, 223.
—— of Venus, Alleged, 152.
Satellites of Jupiter, 187.
—— of Mars, 164.
—— of Saturn, 211.
—— of Uranus, 220.
SATURN, 195.
Apparent lustre, 195.
Period &c., 196.
“Square-shouldered” aspect, 196.
Early observations, 197.
His belts and spots, 199.
Rotation-period, 199.
The Rings, 201.
Divisions in outer ring, 201.
Crape-ring, 202.
Discordant observations, 204.
Eccentric position of rings, 204.
Aspect of the rings, 204.
Further observations, 205.
Occultations of Saturn, 209.
The Satellites, 211.
Transits of shadow of Titan, 213.
Occultations of Stars by Saturn, 214.
Scheiner’s early observations of Sun-spots, 89.
Schiaparelli, Observations of Mars, 159.
——, Observations of Mercury, 141.
—— associates Comets and Meteors, 264.
Schmidt announces change in a lunar crater, 121.
——, Discoverer of a new Star, 315.
Schröter’s observations of Mercury, 142.
—— —— of Saturn, 200.
—— —— of Venus, 148.
Scintillation of Stars, 297.
Scorpii α, 309.
Shadows cast by Faculæ, 109.
Short’s reflectors, 12.
Showers of Meteors, 274.
Sidereal work, 286.
Silver-on-glass films, Duration of, 60.
Sirius, 300, 307.
Small telescopes, 31.
—— —— and Mars, 160.
—— —— and Solar work, 90.
Solar Eclipse of Aug. 19, 1887, 98.
—— Eclipses visible in England, 98.
—— observations, 88.
—— Prominences, 111.
Southern Comets, Large, 233, 234.
Spiral Nebula, 335.
Spitaler’s Comet of 1890, 349.
“Square-shouldered” aspect of Saturn, 196.
Star-disks, 298.
Stars, Nebulous, 330.
——, Occultation of, 135.
—— visible through Comets, 246.
STARS, THE, 286.
Sidereal work, 286.
Greek Alphabet, 287.
Learning the names of the Stars, 287.
The constellation Orion, 289.
The constellation Figures, 290.
Means of Measurement, 290.
Dividing power, 292.
Number of Stars, 293.
Magnitudes, 294.
The Milky Way, 295.
Scintillation of Stars, 297.
Star-disks, 298.
Distance of the Stars, 299.
Proper motions of Stars, 299.
Double Stars and binary systems, 300.
List of Double Stars, 302-5.
α Canis Majoris, 307.
β Orionis, 307.
α Lyræ, 308.
α Ursæ Minoris, 308.
α Scorpii, 309.
Variable Stars, 309.
ο Ceti and β Persei, 310.
List of Variable Stars, 311.
New or temporary Stars, 312.
Description of temporary Stars, 312.
Star-colours, 315.
Groups of Stars, 316.
Coma Berenices, 317.
The Pleiades, 317.
Præsepe, 317.
χ Persei, 317.
κ Crucis, 318.
ζ Ursæ Majoris, 318.
σ Orionis, 318.
θ Orionis, 318.
Further Observations, 320.
“Stops,” Utility of, 58.
Storms, Meteor, 271.
Straight Wall, 130, 131.
Streak seen at Jask, Meteor-, 278.
Structure of Sun-spots, Crateriform, 101.
SUN, THE: Diameter and Distance, 87.
Solar observations, 88.
Spots on the Sun, 88.
Early observations, 88.
Small telescopes and solar work, 90.
Tinted glass, 91.
Solar diagonal, 92.
Drawing Sun-spots, 93.
Ascertaining dimensions, 94.
Sun-spot of June 19, 1889, 95.
Eclipses of the Sun, 97.
Periodicity of Spots, 100.
Crateriform Structure, 101.
“Willow Leaves,” 101.
Rotation of the Sun, 103.
Determining the Period, 104.
Planetary bodies in transit, 105.
Proper motion of Sun-spots, 106.
Rise and decay of spots, 106.
Black nuclei in the Umbræ, 106.
Bright objects near Sun, 107.
Cyclonic Action, 108.
Sudden outbursts of Faculæ, 108.
Shadows cast by Faculæ, 109.
Veiled Spots, 110.
Recurrent disturbances, 110.
Recurrent forms, 111.
Exceptional position of Spots, 111.
The Solar prominences, 111.
Sun-spots, 88, 347.
Superstitious ideas on Comets, 227.
Surface configuration of Mars, 156.
—— markings on Mercury, 142.
—— —— on Venus, 147.
Sweeping for Comets, 249.
—— for Nebulæ, 340.
Swift, Discoverer of Comets, 252.
——, —— of Nebulæ, 339.
Tails of Comets, 244.
Tarrant on Double Stars, 291.
Telescope, Invention and Development, 1.
Telescopes, Cheapness and increasing number of, 57.
——, Choice of, 38.
——, Large and small, 20.
——, Mounting of, 45.
——, Testing, 43.
Telescopic Comets, 256.
—— Meteors, 272.
—— Work, Attractions of, 85.
Tempel’s Comets, 241.
—— Nebula in the Pleiades, 329.
—— observation of Aristarchus, 120.
Temporary Stars, 312.
Tenuity of Comets, 229.
Terby’s White Spot on Saturn’s rings, 203.
Terminator, Moon’s age and Objects near, 133.
Testing Telescopes, 43.
Test-objects, 55.
Theophilus, 128.
Thornthwaite, Method of Solar observation, 92.
Tinted glass for Solar observation, 91.
Titan, Transit of, 213.
Total Eclipses of the Moon, 118.
—— —— of the Sun, 99.
Transits of Intra-Mercurial Planets, 105, 137.
—— of Jupiter’s satellites and their shadows, 189, 191.
—— of Mercury, 143.
—— of Venus, 153.
Trans-Neptunian planet, 224.
Tupman, Method of tabulating Meteors, 282.
——, Remarks on a Fireball, 267.
Tuttle’s Comet, 241.
Twilight on Venus, 151.
Twinkling of the Stars, 297.
Tycho, 127.
URANUS, Discovery, 215.
Mistaken for a Comet, 215.
True character revealed, 216.
Period &c., 217.
Observations, 217.
His belts, 218.
Further observations, 219.
The satellites, 220.
Ursæ Majoris ζ, 318.
Ursæ Minoris α, 308.
Utility of “stops,” 58.
Variable Nebulæ, 327, 351.
—— Stars, 309.
—— ——, List of, 311.
—— ——, Observations of, 321.
Variations in the light of Comets, 245.
Varieties of form and grouping in Nebulæ and Star-clusters, 325.
—— of Meteors, 276.
Vega, 301, 308.
Veiled Sun-spots, 110.
VENUS, Beauty and brilliancy of, 145.
Period &c., 146.
As a telescopic object, 146.
Surface-markings, 147.
Rotation-period, 149.
Faintness of the markings, 150.
Twilight, 151.
Alleged satellite, 152.
Further observations, 152.
Transits, 153.
Occultations, 153.
Vesta, 168;
Occultation of, 169.
Visibility of Mercury, 138.
Vision, 70.
Vulcan, Supposed planet, 137, 348.
Wargentin describes a Lunar Eclipse, 119.
Warner’s Comet-Prizes, 259.
Washington Refractor, The great, 25, 36.
Weather and Comet-seeking, English, 251.
Webb, Lunar observations, 127, 130, 131.
——, Markings on Mercury, 143.
Williams, Observations of canals of Mars, 160.
——, —— of Jupiter, 175, 177.
——, —— of Jupiter’s satellites, 191.
——, —— of Plato, 126.
“Willow-Leaves,” The Solar, 101.
Wind, Its influence on definition, 69.
With of Hereford, Maker of glass specula, 15.
Wolf on large and small Telescopes, 25, 35.
Working-lists, 68.
Young, Performance of 23-inch refractor, 26.
—— observes belts on Uranus, 217.
—— on the successes of small instruments, 34.
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