Astronomy of To-day - A Popular Introduction in Non-Technical Language

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Title: Astronomy of To-day - A Popular Introduction in Non-Technical Language
Author: Dolmage, Cecil Goodrich Julius, -1908
Language: English
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Libraries.)

Transcriber's Note

The punctuation and spelling from the original text have been faithfully
preserved. Only obvious typographical errors have been corrected. The
advertisement from the beginning of the book has been joined with the
other advertisements near the end of the book.

Greek words are spelled out and represented as [word]. Greek letters are
represented as [a] "for alpha".

ASTRONOMY OF TO-DAY

[Illustration: THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30TH, 1905.

The Corona; from a water-colour sketch, made at Burgos, in Spain, during
the total phase, by the French Artist, Mdlle. ANDRÉE MOCH.]

ASTRONOMY OF
TO-DAY

_A POPULAR INTRODUCTION IN
NON-TECHNICAL LANGUAGE_

CECIL G. DOLMAGE, M.A., LL.D., D.C.L.

Fellow of the Royal Astronomical Society; Member of
the British Astronomical Association; Member of
the Astronomical Society of the Pacific; Membre
de la Société Astronomique de France;
Membre de la Société Belge
d'Astronomie

With a Frontispiece in Colour
and 45 Illustrations & Diagrams

_THIRD EDITION_

LONDON
SEELEY AND CO. LIMITED
38 GREAT RUSSELL STREET
1910

PREFACE

The object of this book is to give an account of the science of
Astronomy, as it is known at the present day, in a manner acceptable to
the _general reader_.

It is too often supposed that it is impossible to acquire any useful
knowledge of Astronomy without much laborious study, and without
adventuring into quite a new world of thought. The reasoning applied to
the study of the celestial orbs is, however, of no different order from
that which is employed in the affairs of everyday life. The science of
mathematics is perhaps responsible for the idea that some kind of
difference does exist; but mathematical processes are, in effect, no
more than ordinary logic in concentrated form, the _shorthand of
reasoning_, so to speak. I have attempted in the following pages to take
the main facts and theories of Astronomy out of those mathematical forms
which repel the general reader, and to present them in the _ordinary
language of our workaday world_.

The few diagrams introduced are altogether supplementary, and are not
connected with the text by any wearying cross-references. Each diagram
is complete in itself, being intended to serve as a pictorial aid, in
case the wording of the text should not have perfectly conveyed the
desired meaning. The full page illustrations are also described as
adequately as possible at the foot of each.

As to the coloured frontispiece, this must be placed in a category by
itself. It is the work of the _artist_ as distinct from the scientist.

The book itself contains incidentally a good deal of matter concerned
with the Astronomy of the past, the introduction of which has been found
necessary in order to make clearer the Astronomy of our time.

It would be quite impossible for me to enumerate here the many sources
from which information has been drawn. But I acknowledge my especial
indebtedness to Professor F.R. Moulton's _Introduction to Astronomy_
(Macmillan, 1906), to the works on Eclipses of the late Rev. S.J.
Johnson and of Mr. W.T. Lynn, and to the excellent _Journals of the
British Astronomical Association_. Further, for those grand questions
concerned with the Stellar Universe at large, I owe a very deep debt to
the writings of the famous American astronomer, Professor Simon Newcomb,
and of our own countryman, Mr. John Ellard Gore; to the latter of whom I
am under an additional obligation for much valuable information
privately rendered.

In my search for suitable illustrations, I have been greatly aided by
the kindly advice of Mr. W. H. Wesley, the Assistant Secretary of the
Royal Astronomical Society. To those who have been so good as to permit
me to reproduce pictures and photographs, I desire to record my best
thanks as follows:--To the French Artist, Mdlle. Andrée Moch; to the
Astronomer Royal; to Sir David Gill, K.C.B., LL.D., F.R.S.; to the
Council of the Royal Astronomical Society; to Professor E.B. Frost,
Director of the Yerkes Observatory; to M.P. Puiseux, of the Paris
Observatory; to Dr. Max Wolf, of Heidelberg; to Professor Percival
Lowell; to the Rev. Theodore E.R. Phillips, M.A., F.R.A.S.; to Mr. W.H.
Wesley; to the Warner and Swasey Co., of Cleveland, Ohio, U.S.A.; to the
publishers of _Knowledge_, and to Messrs. Sampson, Low & Co. For
permission to reproduce the beautiful photograph of the Spiral Nebula in
Canes Venatici (Plate XXII.), I am indebted to the distinguished
astronomer, the late Dr. W.E. Wilson, D.Sc., F.R.S., whose untimely
death, I regret to state, occurred in the early part of this year.

Finally, my best thanks are due to Mr. John Ellard Gore, F.R.A.S.,
M.R.I.A., to Mr. W.H. Wesley, and to Mr. John Butler Burke, M.A., of
Cambridge, for their kindness in reading the proof-sheets.

CECIL G. DOLMAGE.

LONDON, S.W.,
_August 4, 1908._

PREFATORY NOTE TO THE SECOND EDITION

The author of this book lived only long enough to hear of the favour
with which it had been received, and to make a few corrections in view
of the second edition which it has so soon reached.

_December 1908._

CONTENTS

CHAPTER I
PAGE
THE ANCIENT VIEW 17

CHAPTER II
THE MODERN VIEW 20

CHAPTER III
THE SOLAR SYSTEM 29

CHAPTER IV
CELESTIAL MECHANISM 38

CHAPTER V
CELESTIAL DISTANCES 46

CHAPTER VI
CELESTIAL MEASUREMENT 55

CHAPTER VII
ECLIPSES AND KINDRED PHENOMENA 61

CHAPTER VIII
FAMOUS ECLIPSES OF THE SUN 83

CHAPTER IX
FAMOUS ECLIPSES OF THE MOON 101

CHAPTER X
THE GROWTH OF OBSERVATION 105

CHAPTER XI
SPECTRUM ANALYSIS 121

CHAPTER XII
THE SUN 127

CHAPTER XIII
THE SUN--_continued_ 134

CHAPTER XIV
THE INFERIOR PLANETS 146

CHAPTER XV
THE EARTH 158

CHAPTER XVI
THE MOON 183

CHAPTER XVII
THE SUPERIOR PLANETS 209

CHAPTER XVIII
THE SUPERIOR PLANETS--_continued_ 229

CHAPTER XIX
COMETS 247

CHAPTER XX
REMARKABLE COMETS 259

CHAPTER XXI
METEORS OR SHOOTING STARS 266

CHAPTER XXII
THE STARS 278

CHAPTER XXIII
THE STARS--_continued_ 287

CHAPTER XXIV
SYSTEMS OF STARS 300

CHAPTER XXV
THE STELLAR UNIVERSE 319

CHAPTER XXVI
THE STELLAR UNIVERSE--_continued_ 329

CHAPTER XXVII
THE BEGINNING OF THINGS 333

CHAPTER XXVIII
THE END OF THINGS 342

INDEX 351

LIST OF ILLUSTRATIONS

LIST OF PLATES

PLATE
THE TOTAL ECLIPSE OF THE SUN
OF AUGUST 30, 1905 _Frontispiece_

I. THE TOTAL ECLIPSE OF THE SUN
OF MAY 17, 1882 _To face page_ 96

II. GREAT TELESCOPE OF HEVELIUS " " 110

III. A TUBELESS, OR "AERIAL" TELESCOPE " " 112

IV. THE GREAT YERKES TELESCOPE " " 118

V. THE SUN, SHOWING SEVERAL
GROUPS OF SPOTS " " 134

VI. PHOTOGRAPH OF A SUNSPOT " " 136

VII. FORMS OF THE SOLAR CORONA
AT THE EPOCHS OF SUNSPOT
MAXIMUM AND SUNSPOT
MINIMUM RESPECTIVELY.
(A) THE TOTAL ECLIPSE OF
THE SUN OF DECEMBER 22, 1870.
(B) THE TOTAL ECLIPSE OF
THE SUN OF MAY 28, 1900 " " 142

VIII. THE MOON _To face page_ 196

IX. MAP OF THE MOON, SHOWING
THE PRINCIPAL "CRATERS,"
MOUNTAIN RANGES AND
"SEAS" " " 198

X. ONE OF THE MOST INTERESTING
REGIONS ON THE MOON " " 200

XI. THE MOON (SHOWING SYSTEMS
OF "RAYS") " " 204

XII. A MAP OF THE PLANET MARS " " 216

XIII. MINOR PLANET TRAILS " " 226

XIV. THE PLANET JUPITER " " 230

XV. THE PLANET SATURN " " 236

XVI. EARLY REPRESENTATIONS OF
SATURN " " 242

XVII. DONATI'S COMET " " 256

XVIII. DANIEL'S COMET OF 1907 " " 258

XIX. THE SKY AROUND THE NORTH
POLE " " 292

XX. ORION AND HIS NEIGHBOURS " " 296

XXI. THE GREAT GLOBULAR CLUSTER
IN THE SOUTHERN CONSTELLATION
OF CENTAURUS " " 306

XXII. SPIRAL NEBULA IN THE CONSTELLATION
OF CANES VENATICI " " 314

XXIII. THE GREAT NEBULA IN THE
CONSTELLATION OF ANDROMEDA _To face page_ 316

XXIV. THE GREAT NEBULA IN THE
CONSTELLATION OF ORION " " 318

LIST OF DIAGRAMS

FIG. PAGE
1. THE PTOLEMAIC IDEA OF THE UNIVERSE 19

2. THE COPERNICAN THEORY OF THE SOLAR
SYSTEM 21

3. TOTAL AND PARTIAL ECLIPSES OF THE MOON 64

4. TOTAL AND PARTIAL ECLIPSES OF THE SUN 67

5. "BAILY'S BEADS" 70

6. MAP OF THE WORLD ON MERCATOR'S PROJECTION,
SHOWING A PORTION OF THE PROGRESS
OF THE TOTAL SOLAR ECLIPSE OF
AUGUST 30, 1905, ACROSS THE SURFACE
OF THE EARTH 81

7. THE "RING WITH WINGS" 87

8. THE VARIOUS TYPES OF TELESCOPE 113

9. THE SOLAR SPECTRUM 123

10. A SECTION THROUGH THE SUN, SHOWING HOW
THE PROMINENCES RISE FROM THE
CHROMOSPHERE 131

11. ORBIT AND PHASES OF AN INFERIOR PLANET 148

12. THE "BLACK DROP" 153

13. SUMMER AND WINTER 176

14. ORBIT AND PHASES OF THE MOON 184

15. THE ROTATION OF THE MOON ON HER AXIS 187

16. LAPLACE'S "PERENNIAL FULL MOON" 191

17. ILLUSTRATING THE AUTHOR'S EXPLANATION OF
THE APPARENT ENLARGEMENT OF CELESTIAL
OBJECTS 195

18. SHOWING HOW THE TAIL OF A COMET IS
DIRECTED AWAY FROM THE SUN 248

19. THE COMET OF 1066, AS REPRESENTED IN THE
BAYEUX TAPESTRY 263

20. PASSAGE OF THE EARTH THROUGH THE THICKEST
PORTION OF A METEOR SWARM 269

ASTRONOMY OF TO-DAY

CHAPTER I

THE ANCIENT VIEW

It is never safe, as we know, to judge by appearances, and this is
perhaps more true of astronomy than of anything else.

For instance, the idea which one would most naturally form of the earth
and heaven is that the solid earth on which we live and move extends to
a great distance in every direction, and that the heaven is an immense
dome upon the inner surface of which the stars are fixed. Such must
needs have been the idea of the universe held by men in the earliest
times. In their view the earth was of paramount importance. The sun and
moon were mere lamps for the day and for the night; and these, if not
gods themselves, were at any rate under the charge of special deities,
whose task it was to guide their motions across the vaulted sky.

Little by little, however, this simple estimate of nature began to be
overturned. Difficult problems agitated the human mind. On what, for
instance, did the solid earth rest, and what prevented the vaulted
heaven from falling in upon men and crushing them out of existence?
Fantastic myths sprang from the vain attempts to solve these riddles.
The Hindoos, for example, imagined the earth as supported by four
elephants which stood upon the back of a gigantic tortoise, which, in
its turn, floated on the surface of an elemental ocean. The early
Western civilisations conceived the fable of the Titan Atlas, who, as a
punishment for revolt against the Olympian gods, was condemned to hold
up the expanse of sky for ever and ever.

Later on glimmerings of the true light began to break in upon men. The
Greek philosophers, who busied themselves much with such matters,
gradually became convinced that the earth was spherical in shape, that
is to say, round like a ball. In this opinion we now know that they were
right; but in their other important belief, viz. that the earth was
placed at the centre of all things, they were indeed very far from the
truth.

By the second century of the Christian era, the ideas of the early
philosophers had become hardened into a definite theory, which, though
it appears very incorrect to us to-day, nevertheless demands exceptional
notice from the fact that it was everywhere accepted as the true
explanation until so late as some four centuries ago. This theory of the
universe is known by the name of the Ptolemaic System, because it was
first set forth in definite terms by one of the most famous of the
astronomers of antiquity, Claudius Ptolemæus Pelusinensis (100-170
A.D.), better known as Ptolemy of Alexandria.

In his system the Earth occupied the centre; while around it circled in
order outwards the Moon, the planets Mercury and Venus, the Sun, and
then the planets Mars, Jupiter, and Saturn. Beyond these again revolved
the background of the heaven, upon which it was believed that the stars
were fixed--

"Stellis ardentibus aptum,"

as Virgil puts it (see Fig. 1).

[Illustration: FIG. 1.--The Ptolemaic idea of the Universe.]

The Ptolemaic system persisted unshaken for about fourteen hundred years
after the death of its author. Clearly men were flattered by the notion
that their earth was the most important body in nature, that it stood
still at the centre of the universe, and was the pivot upon which all
things revolved.

CHAPTER II

THE MODERN VIEW

It is still well under four hundred years since the modern, or
Copernican, theory of the universe supplanted the Ptolemaic, which had
held sway during so many centuries. In this new theory, propounded
towards the middle of the sixteenth century by Nicholas Copernicus
(1473-1543), a Prussian astronomer, the earth was dethroned from its
central position and considered merely as one of a number of planetary
bodies which revolve around the sun. As it is not a part of our purpose
to follow in detail the history of the science, it seems advisable to
begin by stating in a broad fashion the conception of the universe as
accepted and believed in to-day.

The Sun, the most important of the celestial bodies so far as we are
concerned, occupies the central position; not, however, in the whole
universe, but only in that limited portion which is known as the Solar
System. Around it, in the following order outwards, circle the planets
Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
(see Fig. 2, p. 21). At an immense distance beyond the solar system, and
scattered irregularly through the depth of space, lie the stars. The two
first-mentioned members of the solar system, Mercury and Venus, are
known as the Inferior Planets; and in their courses about the sun, they
always keep well inside the path along which our earth moves. The
remaining members (exclusive of the earth) are called Superior Planets,
and their paths lie all outside that of the earth.

[Illustration: FIG. 2.--The Copernican theory of the Solar System.]

The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been
known from all antiquity. Nothing then can bring home to us more
strongly the immense advance which has taken place in astronomy during
modern times than the fact that it is only 127 years since observation
of the skies first added a planet to that time-honoured number. It was
indeed on the 13th of March 1781, while engaged in observing the
constellation of the Twins, that the justly famous Sir William Herschel
caught sight of an object which he did not recognise as having met with
before. He at first took it for a comet, but observations of its
movements during a few days showed it to be a planet. This body, which
the power of the telescope alone had thus shown to belong to the solar
family, has since become known to science under the name of Uranus. By
its discovery the hitherto accepted limits of the solar system were at
once pushed out to twice their former extent, and the hope naturally
arose that other planets would quickly reveal themselves in the
immensities beyond.

For a number of years prior to Herschel's great discovery, it had been
noticed that the distances at which the then known planets circulated
appeared to be arranged in a somewhat orderly progression outwards from
the sun. This seeming plan, known to astronomers by the name of Bode's
Law, was closely confirmed by the distance of the new planet Uranus.
There still lay, however, a broad gap between the planets Mars and
Jupiter. Had another planet indeed circuited there, the solar system
would have presented an appearance of almost perfect order. But the void
between Mars and Jupiter was unfilled; the space in which one would
reasonably expect to find another planet circling was unaccountably
empty.

On the first day of the nineteenth century the mystery was however
explained, a body being discovered[1] which revolved in the space that
had hitherto been considered planetless. But it was a tiny globe hardly
worthy of the name of planet. In the following year a second body was
discovered revolving in the same space; but it was even smaller in size
than the first. During the ensuing five years two more of these little
planets were discovered. Then came a pause, no more such bodies being
added to the system until half-way through the century, when suddenly
the discovery of these so-called "minor planets" began anew. Since then
additions to this portion of our system have rained thick and fast. The
small bodies have received the name of Asteroids or Planetoids; and up
to the present time some six hundred of them are known to exist, all
revolving in the previously unfilled space between Mars and Jupiter.

In the year 1846 the outer boundary of the solar system was again
extended by the discovery that a great planet circulated beyond Uranus.
The new body, which received the name of Neptune, was brought to light
as the result of calculations made at the same time, though quite
independently, by the Cambridge mathematician Adams, and the French
astronomer Le Verrier. The discovery of Neptune differed, however, from
that of Uranus in the following respect. Uranus was found merely in the
course of ordinary telescopic survey of the heavens. The position of
Neptune, on the other hand, was predicted as the result of rigorous
mathematical investigations undertaken with the object of fixing the
position of an unseen and still more distant body, the attraction of
which, in passing by, was disturbing the position of Uranus in its
revolution around the sun. Adams actually completed his investigation
first; but a delay at Cambridge in examining that portion of the sky,
where he announced that the body ought just then to be, allowed France
to snatch the honour of discovery, and the new planet was found by the
observer Galle at Berlin, very near the place in the heavens which Le
Verrier had mathematically predicted for it.

Nearly fifty years later, that is to say, in our own time, another
important planetary discovery was made. One of the recent additions to
the numerous and constantly increasing family of the asteroids, a tiny
body brought to light in 1898, turned out after all not to be
circulating in the customary space between Mars and Jupiter, but
actually in that between our earth and Mars. This body is very small,
not more than about twenty miles across. It has received the name of
Eros (the Greek equivalent for Cupid), in allusion to its insignificant
size as compared with the other leading members of the system.

This completes the list of the planets which, so far, have revealed
themselves to us. Whether others exist time alone will show. Two or
three have been suspected to revolve beyond the path of Neptune; and it
has even been asserted, on more than one occasion, that a planet
circulates nearer to the sun than Mercury. This supposed body, to which
the name of "Vulcan" was provisionally given, is said to have been
"discovered" in 1859 by a French doctor named Lescarbault, of Orgères
near Orleans; but up to the present there has been no sufficient
evidence of its existence. The reason why such uncertainty should exist
upon this point is easy enough to understand, when we consider the
overpowering glare which fills our atmosphere all around the sun's place
in the sky. Mercury, the nearest known planet to the sun, is for this
reason always very difficult to see; and even when, in its course, it
gets sufficiently far from the sun to be left for a short time above the
horizon after sunset, it is by no means an easy object to observe on
account of the mists which usually hang about low down near the earth.
One opportunity, however, offers itself from time to time to solve the
riddle of an "intra-Mercurial" planet, that is to say, of a planet which
circulates within the path followed by Mercury. The opportunity in
question is furnished by a total eclipse of the sun; for when, during an
eclipse of that kind, the body of the moon for a few minutes entirely
hides the sun's face, and the dazzling glare is thus completely cut off,
astronomers are enabled to give an unimpeded, though all too hurried,
search to the region close around. A goodly number of total eclipses of
the sun have, however, come and gone since the days of Lescarbault, and
no planet, so far, has revealed itself in the intra-Mercurial space. It
seems, therefore, quite safe to affirm that no globe of sufficient size
to be seen by means of our modern telescopes circulates nearer to the
sun than the planet Mercury.

Next in importance to the planets, as permanent members of the solar
system, come the relatively small and secondary bodies known by the name
of Satellites. The name _satellite_ is derived from a Latin word
signifying _an attendant_; for the bodies so-called move along always in
close proximity to their respective "primaries," as the planets which
they accompany are technically termed.

The satellites cannot be considered as allotted with any particular
regularity among the various members of the system; several of the
planets, for instance, having a goodly number of these bodies
accompanying them, while others have but one or two, and some again have
none at all. Taking the planets in their order of distance outward from
the Sun, we find that neither Mercury nor Venus are provided with
satellites; the Earth has only one, viz. our neighbour the Moon; while
Mars has but two tiny ones, so small indeed that one might imagine them
to be merely asteroids, which had wandered out of their proper region
and attached themselves to that planet. For the rest, so far as we at
present know, Jupiter possesses seven,[2] Saturn ten, Uranus four, and
Neptune one. It is indeed possible, nay more, it is extremely probable,
that the two last-named planets have a greater number of these secondary
bodies revolving around them; but, unfortunately, the Uranian and
Neptunian systems are at such immense distances from us, that even the
magnificent telescopes of to-day can extract very little information
concerning them.

From the distribution of the satellites, the reader will notice that the
planets relatively near to the sun are provided with few or none, while
the more distant planets are richly endowed. The conclusion, therefore,
seems to be that nearness to the sun is in some way unfavourable either
to the production, or to the continued existence, of satellites.

A planet and its satellites form a repetition of the solar system on a
tiny scale. Just as the planets revolve around the sun, so do these
secondary bodies revolve around their primaries. When Galileo, in 1610,
turned his newly invented telescope upon Jupiter, he quickly recognised
in the four circling moons which met his gaze, a miniature edition of
the solar system.

Besides the planets and their satellites, there are two other classes of
bodies which claim membership of the solar system. These are Comets and
Meteors. Comets differ from the bodies which we have just been
describing in that they appear filmy and transparent, whereas the others
are solid and opaque. Again, the paths of the planets around the sun and
of the satellites around their primaries are not actually circles; they
are ovals, but their ovalness is not of a marked degree. The paths of
comets on the other hand are usually _very_ oval; so that in their
courses many of them pass out as far as the known limits of the solar
system, and even far beyond. It should be mentioned that nowadays the
tendency is to consider comets as permanent members of the system,
though this was formerly not by any means an article of faith with
astronomers.

Meteors are very small bodies, as a rule perhaps no larger than pebbles,
which move about unseen in space, and of which we do not become aware
until they arrive very close to the earth. They are then made visible to
us for a moment or two in consequence of being heated to a white heat by
the friction of rushing through the atmosphere, and are thus usually
turned into ashes and vapour long before they reach the surface of our
globe. Though occasionally a meteoric body survives the fiery ordeal,
and reaches the earth more or less in a solid state to bury itself deep
in the soil, the majority of these celestial visitants constitute no
source of danger whatever for us. Any one who will take the trouble to
gaze at the sky for a short time on a clear night, is fairly certain to
be rewarded with the view of a meteor. The impression received is as if
one of the stars had suddenly left its accustomed place, and dashed
across the heavens, leaving in its course a trail of light. It is for
this reason that meteors are popularly known under the name of "shooting
stars."

[1] By the Italian astronomer, Piazzi, at Palermo.

[2] Probably eight. (See note, page 232.)

CHAPTER III

THE SOLAR SYSTEM

We have seen, in the course of the last chapter, that the solar system
is composed as follows:--there is a central body, the sun, around which
revolve along stated paths a number of important bodies known as
planets. Certain of these planets, in their courses, carry along in
company still smaller bodies called satellites, which revolve around
them. With regard, however, to the remaining members of the system, viz.
the comets and the meteors, it is not advisable at this stage to add
more to what has been said in the preceding chapter. For the time being,
therefore, we will devote our attention merely to the sun, the planets,
and the satellites.

Of what shape then are these bodies? Of one shape, and that one alone
which appears to characterise all solid objects in the celestial spaces:
they are spherical, which means _round like a ball_.

Each of these spherical bodies rotates; that is to say, turns round and
round, as a top does when it is spinning. This rotation is said to take
place "upon an axis," a statement which may be explained as
follows:--Imagine a ball with a knitting-needle run right through its
centre. Then imagine this needle held pointing in one fixed direction
while the ball is turned round and round. Well, it is the same thing
with the earth. As it journeys about the sun, it keeps turning round and
round continually as if pivoted upon a mighty knitting needle
transfixing it from North Pole to South Pole. In reality, however, there
is no such material axis to regulate the constant direction of the
rotation, just as there are no actual supports to uphold the earth
itself in space. The causes which keep the celestial spheres poised, and
which control their motions, are far more wonderful than any mechanical
device.

At this juncture it will be well to emphasise the sharp distinction
between the terms _rotation_ and _revolution_. The term "rotation" is
invariably used by astronomers to signify the motion which a celestial
body has upon an axis; the term "revolution," on the other hand, is used
for the movement of one celestial body around another. Speaking of the
earth, for instance, we say, that it _rotates_ on its axis, and that it
_revolves_ around the sun.

So far, nothing has been said about the sizes of the members of our
system. Is there any stock size, any pattern according to which they may
be judged? None whatever! They vary enormously. Very much the largest of
all is the Sun, which is several hundred times larger than all the
planets and satellites of the system rolled together. Next comes Jupiter
and afterwards the other planets in the following order of
size:--Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury.
Very much smaller than any of these are the asteroids, of which Ceres,
the largest, is less than 500 miles in diameter. It is, by the way, a
strange fact that the zone of asteroids should mark the separation of
the small planets from the giant ones. The following table, giving
roughly the various diameters of the sun and the principal planets in
miles, will clearly illustrate the great discrepancy in size which
prevails in the system.

Sun 866,540 miles
Mercury 2,765 "
Venus 7,826 "
Earth 7,918 "
Mars 4,332 "

ZONE OF ASTEROIDS

Jupiter 87,380 "
Saturn 73,125 "
Uranus[3] 34,900 "
Neptune[3] 32,900 "

It does not seem possible to arrive at any generalisation from the above
data, except it be to state that there is a continuous increase in size
from Mercury to the earth, and a similar decrease in size from Jupiter
outwards. Were Mars greater than the earth, the planets could then with
truth be said to increase in size up to Jupiter, and then to decrease.
But the zone of asteroids, and the relative smallness of Mars, negative
any attempt to regard the dimensions of the planets as an orderly
sequence.

Next with respect to relative distance from the sun, Venus circulates
nearly twice as far from it as Mercury, the earth nearly three times as
far, and Mars nearly four times. After this, just as we found a sudden
increase in size, so do we meet with a sudden increase in distance.
Jupiter, for instance, is about thirteen times as far as Mercury, Saturn
about twenty-five times, Uranus about forty-nine times, and Neptune
about seventy-seven. (See Fig. 2, p. 21.)

It will thus be seen how enormously the solar system was enlarged in
extent by the discovery of the outermost planets. The finding of Uranus
plainly doubled its breadth; the finding of Neptune made it more than
half as broad again. Nothing indeed can better show the import of these
great discoveries than to take a pair of compasses and roughly set out
the above relative paths in a series of concentric circles upon a large
sheet of paper, and then to consider that the path of Saturn was the
supposed boundary of our solar system prior to the year 1781.

We have seen that the usual shape of celestial bodies themselves is
spherical. Of what form then are their paths, or _orbits_, as these are
called? One might be inclined at a venture to answer "circular," but
this is not the case. The orbits of the planets cannot be regarded as
true circles. They are ovals, or, to speak in technical language,
"ellipses." Their ovalness or "ellipticity" is, however, in each case
not by any means of the same degree. Some orbits--for instance, that of
the earth--differ only slightly from circles; while others--those of
Mars or Mercury, for example--are markedly elliptic. The orbit of the
tiny planet Eros is, however, far and away the most elliptic of all, as
we shall see when we come to deal with that little planet in detail.

It has been stated that the sun and planets are always rotating. The
various rates at which they do so will, however, be best appreciated by
a comparison with the rate at which the earth itself rotates.

But first of all, let us see what ground we have, if any, for asserting
that the earth rotates at all?

If we carefully watch the heavens we notice that the background of the
sky, with all the brilliant objects which sparkle in it, appears to turn
once round us in about twenty-four hours; and that the pivot upon which
this movement takes place is situated somewhere near what is known to us
as the _Pole Star_. This was one of the earliest facts noted with regard
to the sky; and to the men of old it therefore seems as if the heavens
and all therein were always revolving around the earth. It was natural
enough for them to take this view, for they had not the slightest idea
of the immense distance of the celestial bodies, and in the absence of
any knowledge of the kind they were inclined to imagine them
comparatively near. It was indeed only after the lapse of many
centuries, when men had at last realised the enormous gulf which
separated them from even the nearest object in the sky, that a more
reasonable opinion began to prevail. It was then seen that this
revolution of the heavens about the earth could be more easily and more
satisfactorily explained by supposing a mere rotation of the solid earth
about a fixed axis, pointed in the direction of the polar star. The
probability of such a rotation on the part of the earth itself was
further strengthened by the observations made with the telescope. When
the surfaces of the sun and planets were carefully studied these bodies
were seen to be rotating. This being the case, there could not surely
be much hesitation in granting that the earth rotated also; particularly
when it so simply explained the daily movement of the sky, and saved men
from the almost inconceivable notion that the whole stupendous vaulted
heaven was turning about them once in every twenty-four hours.

If the sun be regularly observed through a telescope, it will gradually
be gathered from the slow displacement of sunspots across its face,
their disappearance at one edge and their reappearance again at the
other edge, that it is rotating on an axis in a period of about
twenty-six days. The movement, too, of various well-known markings on
the surfaces of the planets Mars, Jupiter, and Saturn proves to us that
these bodies are rotating in periods, which are about twenty-four hours
for the first, and about ten hours for each of the other two. With
regard, however, to Uranus and Neptune there is much more uncertainty,
as these planets are at such great distances that even our best
telescopes give but a confused view of the markings which they display;
still a period of rotation of from ten to twelve hours appears to be
accepted for them. On the other hand the constant blaze of sunlight in
the neighbourhood of Mercury and Venus equally hampers astronomers in
this quest. The older telescopic observers considered that the rotation
periods of these two planets were about the same as that of the earth;
but of recent years the opinion has been gaining ground that they turn
round on their axes in exactly the same time as they revolve about the
sun. This question is, however, a very doubtful one, and will be again
referred to later on; but, putting it on one side, it will be seen from
what we have said above, that the rotation periods of the other planets
of our system are usually about twenty-four hours, or under. The fact
that the rotation period of the sun should run into _days_ need not seem
extraordinary when one considers its enormous size.

The periods taken by the various planets to revolve around the sun is
the next point which has to be considered. Here, too, it is well to
start with the earth's period of revolution as the standard, and to see
how the periods taken by the other planets compare with it.

The earth takes about 365-1/4 days to revolve around the sun. This
period of time is known to us as a "year." The following table shows in
days and years the periods taken by each of the other planets to make a
complete revolution round the sun:--

Mercury about 88 days.
Venus " 226 "
Mars " 1 year and 321 days.
Jupiter " 11 years and 313 days.
Saturn " 29 years and 167 days.
Uranus " 84 years and 7 days.
Neptune " 164 years and 284 days.

From these periods we gather an important fact, namely, that the nearer
a planet is to the sun the faster it revolves.

Compared with one of our years what a long time does an Uranian, or
Neptunian, "year" seem? For instance, if a "year" had commenced in
Neptune about the middle of the reign of George II., that "year" would
be only just coming to a close; for the planet is but now arriving back
to the position, with regard to the sun, which it then occupied. Uranus,
too, has only completed a little more than 1-1/2 of its "years" since
Herschel discovered it.

Having accepted the fact that the planets are revolving around the sun,
the next point to be inquired into is:--What are the positions of their
orbits, or paths, relatively to each other?

Suppose, for instance, the various planetary orbits to be represented by
a set of hoops of different sizes, placed one within the other, and the
sun by a small ball in the middle of the whole; in what positions will
these hoops have to be arranged so as to imitate exactly the true
condition of things?

First of all let us suppose the entire arrangement, ball and hoops, to
be on one level, so to speak. This may be easily compassed by imagining
the hoops as floating, one surrounding the other, with the ball in the
middle of all, upon the surface of still water. Such a set of objects
would be described in astronomical parlance as being _in the same
plane_. Suppose, on the other hand, that some of these floating hoops
are tilted with regard to the others, so that one half of a hoop rises
out of the water and the other half consequently sinks beneath the
surface. This indeed is the actual case with regard to the planetary
orbits. They do not by any means lie all exactly in the same plane. Each
one of them is tilted, or _inclined_, a little with respect to the plane
of the earth's orbit, which astronomers, for convenience, regard as the
_level_ of the solar system. This tilting, or "inclination," is, in the
larger planets, greatest for the orbit of Mercury, least for that of
Uranus. Mercury's orbit is inclined to that of the earth at an angle of
about 7°, that of Venus at a little over 3°, that of Saturn 2-1/2°;
while in those of Mars, Neptune, and Jupiter the inclination is less
than 2°. But greater than any of these is the inclination of the orbit
of the tiny planet Eros, viz. nearly 11°.

The systems of satellites revolving around their respective planets
being, as we have already pointed out, mere miniature editions of the
solar system, the considerations so far detailed, which regulate the
behaviour of the planets in their relations to the sun, will of
necessity apply to the satellites very closely. In one respect, however,
a system of satellites differs materially from a system of planets. The
central body around which planets are in motion is self-luminous,
whereas the planetary body around which a satellite revolves is not.
True, planets shine, and shine very brightly too; as, for instance,
Venus and Jupiter. But they do not give forth any light of their own, as
the sun does; they merely reflect the sunlight which they receive from
him. Putting this one fact aside, the analogy between the planetary
system and a satellite system is remarkable. The satellites are
spherical in form, and differ markedly in size; they rotate, so far as
we know, upon their axes in varying times; they revolve around their
governing planets in orbits, not circular, but elliptic; and these
orbits, furthermore, do not of necessity lie in the same plane. Last of
all the satellites revolve around their primaries at rates which are
directly comparable with those at which the planets revolve around the
sun, the rule in fact holding good that the nearer a satellite is to its
primary the faster it revolves.

[3] As there seems to be much difference of opinion concerning the
diameters of Uranus and Neptune, it should here be mentioned that the
above figures are taken from Professor F.R. Moulton's _Introduction to
Astronomy_ (1906). They are there stated to be given on the authority of
"Barnard's many measures at the Lick Observatory."

CHAPTER IV

CELESTIAL MECHANISM

As soon as we begin to inquire closely into the actual condition of the
various members of the solar system we are struck with a certain
distinction. We find that there are two quite different points of view
from which these bodies can be regarded. For instance, we may make our
estimates of them either as regards _volume_--that is to say, the mere
room which they take up; or as regards _mass_--that is to say, the
amount of matter which they contain.

Let us imagine two globes of equal volume; in other words, which take up
an equal amount of space. One of these globes, however, may be composed
of material much more tightly put together than in the other; or of
greater _density_, as the term goes. That globe is said to be the
greater of the two in mass. Were such a pair of globes to be weighed in
scales, one globe in each pan, we should see at once, by its weighing
down the other, which of the two was composed of the more tightly packed
materials; and we should, in astronomical parlance, say of this one that
it had the greater mass.

Volume being merely another word for size, the order of the members of
the solar system, with regard to their volumes, will be as follows,
beginning with the greatest:--the Sun, Jupiter, Saturn, Uranus,
Neptune, the Earth, Venus, Mars, and Mercury.

With regard to mass the same order strangely enough holds good. The
actual densities of the bodies in question are, however, very different.
The densest or closest packed body of all is the Earth, which is about
five and a half times as dense as if it were composed entirely of water.
Venus follows next, then Mars, and then Mercury. The remaining bodies,
on the other hand, are relatively loose in structure. Saturn is the
least dense of all, less so than water. The density of the Sun is a
little greater than that of water.

This method of estimating is, however, subject to a qualification. It
must be remembered that in speaking of the Sun, for instance, as being
only a little denser than water, we are merely treating the question
from the point of view of an average. Certain parts of it in fact will
be ever so much denser than water: those are the parts in the centre.
Other portions, for instance, the outside portions, will be very much
less dense. It will easily be understood that in all such bodies the
densest or most compressed portions are to be found towards the centre;
while the portions towards the exterior being less pressed upon, will be
less dense.

We now reach a very important point, the question of Gravitation.
_Gravitation_, or _gravity_, as it is often called, is the attractive
force which, for instance, causes objects to fall to the earth. Now it
seems rather strange that one should say that it is owing to a certain
force that things fall towards the earth. All things seem to us to fall
so of their own accord, as if it were quite natural, or rather most
unnatural if they did not. Why then require a "force" to make them fall?

The story goes that the great Sir Isaac Newton was set a-thinking on
this subject by seeing an apple fall from a tree to the earth. He then
carried the train of thought further; and, by studying the movements of
the moon, he reached the conclusion that a body even so far off as our
satellite would be drawn towards the earth in the same manner. This
being the case, one will naturally ask why the moon herself does not
fall in upon the earth. The answer is indeed found to be that the moon
is travelling round and round the earth at a certain rapid pace, and it
is this very same rapid pace which keeps her from falling in upon us.
Any one can test this simple fact for himself. If we tie a stone to the
end of a string, and keep whirling it round and round fast enough, there
will be a strong pull from the stone in an outward direction, and the
string will remain tight all the time that the stone is being whirled.
If, however, we gradually slacken the speed at which we are making the
stone whirl, a moment will come at length when the string will become
limp, and the stone will fall back towards our hand.

It seems, therefore, that there are two causes which maintain the stone
at a regular distance all the time it is being steadily whirled. One of
these is the continual pull inward towards our hand by means of the
string. The other is the continual pull away from us caused by the rate
at which the stone is travelling. When the rate of whirling is so
regulated that these pulls exactly balance each other, the stone travels
comfortably round and round, and shows no tendency either to fall back
upon our hand or to break the string and fly away into the air. It is
indeed precisely similar with regard to the moon. The continual pull of
the earth's gravitation takes the place of the string. If the moon were
to go round and round slower than it does, it would tend to fall in
towards the earth; if, on the other hand, it were to go faster, it would
tend to rush away into space.

The same kind of pull which the earth exerts upon the objects at its
surface, or upon its satellite, the moon, exists through space so far as
we know. Every particle of matter in the universe is found in fact to
attract every other particle. The moon, for instance, attracts the earth
also, but the controlling force is on the side of the much greater mass
of the earth. This force of gravity or attraction of gravitation, as it
is also called, is perfectly regular in its action. Its power depends
first of all exactly upon the mass of the body which exerts it. The
gravitational pull of the sun, for instance, reaches out to an enormous
distance, controlling perhaps, in their courses, unseen planets circling
far beyond the orbit of Neptune. Again, the strength with which the
force of gravity acts depends upon distance in a regularly diminishing
proportion. Thus, the nearer an object is to the earth, for instance,
the stronger is the gravitational pull which it gets from it; the
farther off it is, the weaker is this pull. If then the moon were to be
brought nearer to the earth, the gravitational pull of the latter would
become so much stronger that the moon's rate of motion would have also
to increase in due proportion to prevent her from being drawn into the
earth. Last of all, the point in a body from which the attraction of
gravitation acts, is not necessarily the centre of the body, but rather
what is known as its _centre of gravity_, that is to say, the balancing
point of all the matter which the body contains.

It should here be noted that the moon does not actually revolve around
the centre of gravity of the earth. What really happens is that both
orbs revolve around their _common_ centre of gravity, which is a point
within the body of the earth, and situated about three thousand miles
from its centre. In the same manner the planets and the sun revolve
around the centre of gravity of the solar system, which is a point
within the body of the sun.

The neatly poised movements of the planets around the sun, and of the
satellites around their respective planets, will therefore be readily
understood to result from a nice balance between gravitation and speed
of motion.

The mass of the earth is ascertained to be about eighty times that of
the moon. Our knowledge of the mass of a planet is learned from
comparing the revolutions of its satellite or satellites around it, with
those of the moon around the earth. We are thus enabled to deduce what
the mass of such a planet would be compared to the earth's mass; that is
to say, a study, for instance, of Jupiter's satellite system shows that
Jupiter must have a mass nearly three hundred and eighteen times that of
our earth. In the same manner we can argue out the mass of the sun from
the movements of the planets and other bodies of the system around it.
With regard, however, to Venus and Mercury, the problem is by no means
such an easy one, as these bodies have no satellites. For information in
this latter case we have to rely upon such uncertain evidence as, for
instance, the slight disturbances caused in the motion of the earth by
the attraction of these planets when they pass closest to us, or their
observed effect upon the motions of such comets as may happen to pass
near to them.

Mass and weight, though often spoken of as one and the same thing, are
by no means so. Mass, as we have seen, merely means the amount of matter
which a body contains. The weight of a body, on the other hand, depends
entirely upon the gravitational pull which it receives. The force of
gravity at the surface of the earth is, for instance, about six times as
great as that at the surface of the moon. All bodies, therefore, weigh
about six times as much on the earth as they would upon the moon; or,
rather, a body transferred to the moon's surface would weigh only about
one-sixth of what it did on the terrestrial surface. It will therefore
be seen that if a body of given _mass_ were to be placed upon planet
after planet in turn, its _weight_ would regularly alter according to
the force of gravity at each planet's surface.

Gravitation is indeed one of the greatest mysteries of nature. What it
is, the means by which it acts, or why such a force should exist at all,
are questions to which so far we have not had even the merest hint of an
answer. Its action across space appears to be instantaneous.

The intensity of gravitation is said in mathematical parlance "to vary
inversely with the square of the distance." This means that at _twice_
the distance the pull will become only _one-quarter_ as strong, and not
one-half as otherwise might be expected. At _four_ times the distance,
therefore, it will be _one-sixteenth_ as strong. At the earth's surface
a body is pulled by the earth's gravitation, or "falls," as we
ordinarily term it, through 16 feet in one _second_ of time; whereas at
the distance of the moon the attraction of the earth is so very much
weakened that a body would take as long as one _minute_ to fall through
the same space.

Newton's investigations showed that if a body were to be placed _at
rest_ in space entirely away from the attraction of any other body it
would remain always in a motionless condition, because there would
plainly be no reason why it should move in any one direction rather than
in another. And, similarly, if a body were to be projected in a certain
direction and at a certain speed, it would move always in the same
direction and at the same speed so long as it did not come within the
gravitational attraction of any other body.

The possibility of an interaction between the celestial orbs had
occurred to astronomers before the time of Newton; for instance, in the
ninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in
1553, and to Kepler, who suspected its existence from observation of the
tides. Horrox also, writing in 1635, spoke of the moon as moved by an
_emanation_ from the earth. But no one prior to Newton attempted to
examine the question from a mathematical standpoint.

Notwithstanding the acknowledged truth and far-reaching scope of the law
of gravitation--for we find its effects exemplified in every portion of
the universe--there are yet some minor movements which it does not
account for. For instance, there are small irregularities in the
movement of Mercury which cannot be explained by the influence of
possible intra-Mercurial planets, and similarly there are slight
unaccountable deviations in the motions of our neighbour the Moon.

CHAPTER V

CELESTIAL DISTANCES

Up to this we have merely taken a general view of the solar system--a
bird's-eye view, so to speak, from space.

In the course of our inquiry we noted in a rough way the _relative_
distances at which the various planets move around the sun. But we have
not yet stated what these distances _actually_ are, and it were
therefore well now to turn our attention to this important matter.

Each of us has a fair idea of what a mile is. It is a quarter of an
hour's sharp walk, for instance; or yonder village or building, we know,
lies such and such a number of miles away.

The measurements which have already been given of the diameters of the
various bodies of the solar system appear very great to us, who find
that a walk of a few miles at a time taxes our strength; but they are a
mere nothing when we consider the distances from the sun at which the
various planets revolve in their orbits.

The following table gives these distances in round numbers. As here
stated they are what are called "mean" distances; for, as the orbits are
oval, the planets vary in their distances from the sun, and we are
therefore obliged to strike a kind of average for each case:--

Mercury about 36,000,000 miles.
Venus " 67,200,000 "
Earth " 92,900,000 "
Mars " 141,500,000 "
Jupiter " 483,300,000 "
Saturn " 886,000,000 "
Uranus " 1,781,900,000 "
Neptune " 2,791,600,000 "

From the above it will be seen at a glance that we have entered upon a
still greater scale of distance than in dealing with the diameters of
the various bodies of the system. In that case the distances were
limited to thousands of miles; in this, however, we have to deal with
millions. A million being ten hundred thousand, it will be noticed that
even the diameter of the huge sun is well under a million miles.

How indeed are we to get a grasp of such distances, when those to which
we are ordinarily accustomed--the few miles' walk, the little stretch of
sea or land which we gaze upon around us--are so utterly minute in
comparison? The fact is, that though men may think that they can picture
in their minds such immense distances, they actually can not. In matters
like these we unconsciously employ a kind of convention, and we estimate
a thing as being two or three or more times the size of another. More
than this we are unable to do. For instance, our ordinary experience of
a mile enables us to judge, in a way, of a stretch of several miles,
such as one can take in with a glance; but in our estimation of a
thousand miles, or even of one hundred, we are driven back upon a mental
trick, so to speak.

In our attempts to realise such immense distances as those in the solar
system we are obliged to have recourse to analogies; to comparisons with
other and simpler facts, though this is at the best a mere self-cheating
device. The analogy which seems most suited to our purpose here, and one
which has often been employed by writers, is borrowed from the rate at
which an express train travels.

Let us imagine, for instance, that we possess an express train which is
capable of running anywhere, never stops, never requires fuel, and
always goes along at sixty miles an hour. Suppose we commence by
employing it to gauge the size of our own planet, the earth. Let us send
it on a trip around the equator, the span of which is about 24,000
miles. At its sixty-miles-an-hour rate of going, this journey will take
nearly 17 days. Next let us send it from the earth to the moon. This
distance, 240,000 miles, being ten times as great as the last, will of
course take ten times as long to cover, namely, 170 days; that is to
say, nearly half a year. Again, let us send it still further afield, to
the sun, for example. Here, however, it enters upon a journey which is
not to be measured in thousands of miles, as the others were, but in
millions. The distance from the earth to the sun, as we have seen in the
foregoing table, is about 93 millions of miles. Our express train would
take about 178 _years_ to traverse this.

Having arrived at the sun, let us suppose that our train makes a tour
right round it. This will take more than five years.

Supposing, finally, that our train were started from the sun, and made
to run straight out to the known boundaries of the solar system, that is
to say, as far as the orbit of Neptune, it would take over 5000 years to
traverse the distance.

That sixty miles an hour is a very great speed any one, I think, will
admit who has stood upon the platform of a country station while one of
the great mail trains has dashed past. But are not the immensities of
space appalling to contemplate, when one realises that a body moving
incessantly at such a rate would take so long as 10,000 years to
traverse merely the breadth of our solar system? Ten thousand years!
Just try to conceive it. Why, it is only a little more than half that
time since the Pyramids were built, and they mark for us the Dawn of
History. And since then half-a-dozen mighty empires have come and gone!

Having thus concluded our general survey of the appearance and
dimensions of the solar system, let us next inquire into its position
and size in relation to what we call the Universe.

A mere glance at the night sky, when it is free from clouds, shows us
that in every direction there are stars; and this holds good, no matter
what portion of the globe we visit. The same is really true of the sky
by day, though in that case we cannot actually see the stars, for their
light is quite overpowered by the dazzling light of the sun.

We thus reach the conclusion that our earth, that our solar system in
fact, lies plunged within the midst of a great tangle of stars. What
position, by the way, do we occupy in this mighty maze? Are we at the
centre, or anywhere near the centre, or where?

It has been indeed amply proved by astronomical research that the stars
are bodies giving off a light of their own, just as our sun does; that
they are in fact suns, and that our sun is merely one, perhaps indeed a
very unimportant member, of this great universe of stars. Each of these
stars, or suns, besides, may be the centre of a system similar to what
we call our solar system, comprising planets and satellites, comets and
meteors;--or perchance indeed some further variety of attendant bodies
of which we have no example in our tiny corner of space. But as to
whether one is right in a conjecture of this kind, there is up to the
present no proof whatever. No telescope has yet shown a planet in
attendance upon one of these distant suns; for such bodies, even if they
do exist, are entirely out of the range of our mightiest instruments. On
what then can we ground such an assumption? Merely upon analogy; upon
the common-sense deduction that as the stars have characteristics
similar to our particular star, the sun, it would seem unlikely that
ours should be the only such body in the whole of space which is
attended by a planetary system.

"The Stars," using that expression in its most general sense, do not lie
at one fixed distance from us, set here and there upon a background of
sky. There is in fact no background at all. The brilliant orbs are all
around us in space, at different distances from us and from each other;
and we can gaze between them out into the blackness of the void which,
perhaps, continues to extend unceasingly long after the very outposts of
the stellar universe has been left behind. Shall we then start our
imaginary express train once more, and send it out towards the nearest
of the stars? This would, however, be a useless experiment. Our
express-train method of gauging space would fail miserably in the
attempt to bring home to us the mighty gulf by which we are now faced.
Let us therefore halt for a moment and look back upon the orders of
distance with which we have been dealing. First of all we dealt with
thousands of miles. Next we saw how they shrank into insignificance when
we embarked upon millions. We found, indeed, that our sixty-mile-an-hour
train, rushing along without ceasing, would consume nearly the whole of
historical time in a journey from the sun to Neptune.

In the spaces beyond the solar system we are faced, however, by a new
order of distance. From sun to planets is measured in millions of miles,
but from sun to sun is measured in billions. But does the mere stating
of this fact convey anything? I fear not. For the word "billion" runs as
glibly off the tongue as "million," and both are so wholly unrealisable
by us that the actual difference between them might easily pass
unnoticed.

Let us, however, make a careful comparison. What is a million? It is a
thousand thousands. But what is a billion? It is a million millions.
Consider for a moment! A million of millions. That means a million, each
unit of which is again a million. In fact every separate "1" in this
million is itself a million. Here is a way of trying to realise this
gigantic number. A million seconds make only eleven and a half days and
nights. But a billion seconds will make actually more than thirty
thousand years!

Having accepted this, let us try and probe with our express train even a
little of the new gulf which now lies before us. At our old rate of
going it took almost two years to cover a million miles. To cover a
billion miles--that is to say, a million times this distance--would thus
take of course nearly two million years. Alpha Centauri, the nearest
star to our earth, is some twenty-five billions of miles away. Our
express train would thus take about fifty millions of years to reach it!

This shows how useless our illustration, appropriate though it seemed
for interplanetary space, becomes when applied to the interstellar
spaces. It merely gives us millions in return for billions; and so the
mind, driven in upon itself, whirls round and round like a squirrel in
its revolving cage. There is, however, a useful illustration still left
us, and it is the one which astronomers usually employ in dealing with
the distances of the stars. The illustration in question is taken from
the velocity of light.

Light travels at the tremendous speed of about 186,000 miles a second.
It therefore takes only about a second and a quarter to come to us from
the moon. It traverses the 93,000,000 of miles which separate us from
the sun in about eight minutes. It travels from the sun out to Neptune
in about four hours, which means that it would cross the solar system
from end to end in eight. To pass, however, across the distance which
separates us from Alpha Centauri it would take so long as about four
and a quarter years!

Astronomers, therefore, agree in estimating the distances of the stars
from the point of view of the time which light would take to pass from
them to our earth. They speak of that distance which light takes a year
to traverse as a "light year." According to this notation, Alpha
Centauri is spoken of as being about four and a quarter light years
distant from us.

Now as the rays of light coming from Alpha Centauri to us are chasing
one another incessantly across the gulf of space, and as each ray left
that star some four years before it reaches us, our view of the star
itself must therefore be always some four years old. Were then this star
to be suddenly removed from the universe at any moment, we should
continue to see it still in its place in the sky for some four years
more, after which it would suddenly disappear. The rays which had
already started upon their journey towards our earth must indeed
continue travelling, and reaching us in their turn until the last one
had arrived; after which no more would come.

We have drawn attention to Alpha Centauri as the nearest of the stars.
The majority of the others indeed are ever so much farther. We can only
hazard a guess at the time it takes for the rays from many of them to
reach our globe. Suppose, for instance, we see a sudden change in the
light of any of these remote stars, we are inclined to ask ourselves
when that change did actually occur. Was it in the days of Queen
Elizabeth, or at the time of the Norman Conquest; or was it when Rome
was at the height of her glory, or perhaps ages before that when the
Pyramids of Egypt were being built? Even the last of these suppositions
cannot be treated lightly. We have indeed no real knowledge of the
distance from us of those stars which our giant telescopes have brought
into view out of the depths of the celestial spaces.

CHAPTER VI

CELESTIAL MEASUREMENT

Had the telescope never been invented our knowledge of astronomy would
be trifling indeed.

Prior to the year 1610, when Galileo first turned the new instrument
upon the sky, all that men knew of the starry realms was gathered from
observation with their own eyes unaided by any artificial means. In such
researches they had been very much at a disadvantage. The sun and moon,
in their opinion, were no doubt the largest bodies in the heavens, for
the mere reason that they looked so! The mighty solar disturbances,
which are now such common-places to us, were then quite undreamed of.
The moon displayed a patchy surface, and that was all; her craters and
ring-mountains were surprises as yet in store for men. Nothing of course
was known about the surfaces of the planets. These objects had indeed no
particular characteristics to distinguish them from the great host of
the stars, except that they continually changed their positions in the
sky while the rest did not. The stars themselves were considered as
fixed inalterably upon the vault of heaven. The sun, moon, and planets
apparently moved about in the intermediate space, supported in their
courses by strange and fanciful devices. The idea of satellites was as
yet unknown. Comets were regarded as celestial portents, and meteors as
small conflagrations taking place in the upper air.

In the entire absence of any knowledge with regard to the actual sizes
and distances of the various celestial bodies, men naturally considered
them as small; and, concluding that they were comparatively near,
assigned to them in consequence a permanent connection with terrestrial
affairs. Thus arose the quaint and erroneous beliefs of astrology,
according to which the events which took place upon our earth were
considered to depend upon the various positions in which the planets,
for instance, found themselves from time to time.

It must, however, be acknowledged that the study of astrology,
fallacious though its conclusions were, indirectly performed a great
service to astronomy by reason of the accurate observations and diligent
study of the stars which it entailed.

We will now inquire into the means by which the distances and sizes of
the celestial orbs have been ascertained, and see how it was that the
ancients were so entirely in the dark in this matter.

There are two distinct methods of finding out the distance at which any
object happens to be situated from us.

One method is by actual measurement.

The other is by moving oneself a little to the right or left, and
observing whether the distant object appears in any degree altered in
position by our own change of place.

One of the best illustrations of this relative change of position which
objects undergo as a result of our own change of place, is to observe
the landscape from the window of a moving railway carriage. As we are
borne rapidly along we notice that the telegraph posts which are set
close to the line appear to fly past us in the contrary direction; the
trees, houses, and other things beyond go by too, but not so fast;
objects a good way off displace slowly; while some spire, or tall
landmark, in the far distance appears to remain unmoved during a
comparatively long time.

Actual change of position on our own part is found indeed to be
invariably accompanied by an apparent displacement of the objects about
us, such apparent displacement as a result of our own change of position
being known as "parallax." The dependence between the two is so
mathematically exact, that if we know the amount of our own change of
place, and if we observe the amount of the consequent displacement of
any object, we are enabled to calculate its precise distance from us.
Thus it comes to pass that distances can be measured without the
necessity of moving over them; and the breadth of a river, for instance,
or the distance from us of a ship at sea, can be found merely by such
means.

It is by the application of this principle to the wider field of the sky
that we are able to ascertain the distance of celestial bodies. We have
noted that it requires a goodly change of place on our own part to shift
the position in which some object in the far distance is seen by us. To
two persons separated by, say, a few hundred yards, a ship upon the
horizon will appear pretty much in the same direction. They would
require, in fact, to be much farther apart in order to displace it
sufficiently for the purpose of estimating their distance from it. It
is the same with regard to the moon. Two observers, standing upon our
earth, will require to be some thousands of miles apart in order to see
the position of our satellite sufficiently altered with regard to the
starry background, to give the necessary data upon which to ground their
calculations.

The change of position thus offered by one side of the earth's surface
at a time is, however, not sufficient to displace any but the nearest
celestial bodies. When we have occasion to go farther afield we have to
seek a greater change of place. This we can get as a consequence of the
earth's movement around the sun. Observations, taken several days apart,
will show the effect of the earth's change of place during the interval
upon the positions of the other bodies of our system. But when we desire
to sound the depths of space beyond, and to reach out to measure the
distance of the nearest star, we find ourselves at once thrown upon the
greatest change of place which we can possibly hope for; and this we get
during the long journey of many millions of miles which our earth
performs around the sun during the course of each year. But even this
last change of place, great as it seems in comparison with terrestrial
measurements, is insufficient to show anything more than the tiniest
displacements in a paltry forty-three out of the entire host of the
stars.

We can thus realise at what a disadvantage the ancients were. The
measuring instruments at their command were utterly inadequate to detect
such small displacements. It was reserved for the telescope to reveal
them; and even then it required the great telescopes of recent times to
show the slight changes in the position of the nearer stars, which were
caused by the earth's being at one time at one end of its orbit, and
some six months later at the other end--stations separated from each
other by a gulf of about one hundred and eighty-six millions of miles.

The actual distances of certain celestial bodies being thus
ascertainable, it becomes a matter of no great difficulty to determine
the actual sizes of the measurable ones. It is a matter of everyday
experience that the size which any object appears to have, depends
exactly upon the distance it is from us. The farther off it is the
smaller it looks; the nearer it is the bigger. If, then, an object which
lies at a known distance from us looks such and such a size, we can of
course ascertain its real dimensions. Take the moon, for instance. As we
have already shown, we are able to ascertain its distance. We observe
also that it looks a certain size. It is therefore only a matter of
calculation to find what its actual dimensions should be, in order that
it may look that size at that distance away. Similarly we can ascertain
the real dimensions of the sun. The planets, appearing to us as points
of light, seem at first to offer a difficulty; but, by means of the
telescope, we can bring them, as it were, so much nearer to us, that
their broad expanses may be seen. We fail, however, signally with regard
to the stars; for they are so very distant, and therefore such tiny
points of light, that our mightiest telescopes cannot magnify them
sufficiently to show any breadth of surface.

Instead of saying that an object looks a certain breadth across, such
as a yard or a foot, a statement which would really mean nothing,
astronomers speak of it as measuring a certain angle. Such angles are
estimated in what are called "degrees of arc"; each degree being divided
into sixty minutes, and each minute again into sixty seconds. Popularly
considered the moon and sun _look_ about the same size, or, as an
astronomer would put it, they measure about the same angle. This is an
angle, roughly, of thirty-two minutes of arc; that is to say, slightly
more than half a degree. The broad expanse of surface which a celestial
body shows to us, whether to the naked eye, as in the case of the sun
and moon, or in the telescope, as in the case of other members of our
system, is technically known as its "disc."

CHAPTER VII

ECLIPSES AND KINDRED PHENOMENA

Since some members of the solar system are nearer to us than others, and
all are again much nearer than any of the stars, it must often happen
that one celestial body will pass between us and another, and thus
intercept its light for a while. The moon, being the nearest object in
the universe, will, of course, during its motion across the sky,
temporarily blot out every one of the others which happen to lie in its
path. When it passes in this manner across the face of the sun, it is
said to _eclipse_ it. When it thus hides a planet or star, it is said to
_occult_ it. The reason why a separate term is used for what is merely a
case of obscuring light in exactly the same way, will be plain when one
considers that the disc of the sun is almost of the same apparent size
as that of the moon, and so the complete hiding of the sun can last but
a few minutes at the most; whereas a planet or a star looks so very
small in comparison, that it is always _entirely swallowed up for some
time_ when it passes behind the body of our satellite.

The sun, of course, occults planets and stars in exactly the same manner
as the moon does, but we cannot see these occultations on account of the
blaze of sunlight.

By reason of the small size which the planets look when viewed with the
naked eye, we are not able to note them in the act of passing over stars
and so blotting them out; but such occurrences may be seen in the
telescope, for the planetary bodies then display broad discs.

There is yet another occurrence of the same class which is known as a
_transit_. This takes place when an apparently small body passes across
the face of an apparently large one, the phenomenon being in fact the
exact reverse of an occultation. As there is no appreciable body nearer
to us than the moon, we can never see anything in transit across her
disc. But since the planets Venus and Mercury are both nearer to us than
the sun, they will occasionally be seen to pass across his face, and
thus we get the well-known phenomena called Transits of Venus and
Transits of Mercury.

As the satellites of Jupiter are continually revolving around him, they
will often pass behind or across his disc. Such occultations and
transits of satellites can be well observed in the telescope.

There is, however, a way in which the light of a celestial body may be
obscured without the necessity of its being hidden from us by one
nearer. It will no doubt be granted that any opaque object casts a
shadow when a strong light falls directly upon it. Thus the earth, under
the powerful light which is directed upon it from the sun, casts an
extensive shadow, though we are not aware of the existence of this
shadow until it falls upon something. The shadow which the earth casts
is indeed not noticeable to us until some celestial body passes into it.
As the sun is very large, and the earth in comparison very small, the
shadow thrown by the earth is comparatively short, and reaches out in
space for only about a million miles. There is no visible object except
the moon, which circulates within that distance from our globe, and
therefore she is the only body which can pass into this shadow. Whenever
such a thing happens, her surface at once becomes dark, for the reason
that she never emits any light of her own, but merely reflects that of
the sun. As the moon is continually revolving around the earth, one
would be inclined to imagine that once in every month, namely at what is
called _full moon_, when she is on the other side of the earth with
respect to the sun, she ought to pass through the shadow in question.
But this does not occur every time, because the moon's orbit is not
quite _upon the same plane_ with the earth's. It thus happens that time
after time the moon passes clear of the earth's shadow, sometimes above
it, and sometimes below it. It is indeed only at intervals of about six
months that the moon can be thus obscured. This darkening of her light
is known as an _eclipse of the moon_. It seems a great pity that custom
should oblige us to employ the one term "eclipse" for this and also for
the quite different occurrence, an eclipse of the sun; in which the
sun's face is hidden as a consequence of the moon's body coming directly
_between_ it and our eyes.

The popular mind seems always to have found it more difficult to grasp
the causes of an eclipse of the moon than an eclipse of the sun. As Mr.
J.E. Gore[4] puts it: "The darkening of the sun's light by the
interposition of the moon's body seems more obvious than the passing of
the moon through the earth's shadow."

Eclipses of the moon furnish striking spectacles, but really add little
to our knowledge. They exhibit, however, one of the most remarkable
evidences of the globular shape of our earth; for the outline of its
shadow when seen creeping over the moon's surface is always circular.

[Illustration: FIG. 3.--Total and Partial Eclipses of the Moon. The Moon
is here shown in two positions; i.e. _entirely_ plunged in the earth's
shadow and therefore totally eclipsed, and only _partly_ plunged in it
or partially eclipsed.]

_Eclipses of the Moon_, or Lunar Eclipses, as they are also called, are
of two kinds--_Total_, and _Partial_. In a total lunar eclipse the moon
passes entirely into the earth's shadow, and the whole of her surface is
consequently darkened. This darkening lasts for about two hours. In a
partial lunar eclipse, a portion only of the moon passes through the
shadow, and so only _part_ of her surface is darkened (see Fig. 3). A
very striking phenomenon during a total eclipse of the moon, is that the
darkening of the lunar surface is usually by no means so intense as one
would expect, when one considers that the sunlight at that time should
be _wholly_ cut off from it. The occasions indeed upon which the moon
has completely disappeared from view during the progress of a total
lunar eclipse are very rare. On the majority of these occasions she has
appeared of a coppery-red colour, while sometimes she has assumed an
ashen hue. The explanations of these variations of colour is to be found
in the then state of the atmosphere which surrounds our earth. When
those portions of our earth's atmosphere through which the sun's rays
have to filter on their way towards the moon are free from watery
vapour, the lunar surface will be tinged with a reddish light, such as
we ordinarily experience at sunset when our air is dry. The ashen colour
is the result of our atmosphere being laden with watery vapour, and is
similar to what we see at sunset when rain is about. Lastly, when the
air around the earth is thickly charged with cloud, no light at all can
pass; and on such occasions the moon disappears altogether for the time
being from the night sky.

_Eclipses of the Sun_, otherwise known as Solar Eclipses, are divided
into _Total_, _Partial_, and _Annular_. A total eclipse of the sun takes
place when the moon comes between the sun and the earth, in such a
manner that it cuts off the sunlight _entirely_ for the time being from
a _portion_ of the earth's surface. A person situated in the region in
question will, therefore, at that moment find the sun temporarily
blotted out from his view by the body of the moon. Since the moon is a
very much smaller body than the sun, and also very much the nearer to us
of the two, it will readily be understood that the portion of the earth
from which the sun is seen thus totally eclipsed will be of small
extent. In places not very distant from this region, the moon will
appear so much shifted in the sky that the sun will be seen only
partially eclipsed. The moon being in constant movement round the earth,
the portion of the earth's surface from which an eclipse is seen as
total will be always a comparatively narrow band lying roughly from west
to east. This band, known as the _track of totality_, can, at the
utmost, never be more than about 165 miles in width, and as a rule is
very much less. For about 2000 miles on either side of it the sun is
seen partially eclipsed. Outside these limits no eclipse of any kind is
visible, as from such regions the moon is not seen to come in the way of
the sun (see Fig. 4 (i.), p. 67).

It may occur to the reader that eclipses can also take place in the
course of which the positions, where the eclipse would ordinarily be
seen as total, will lie outside the surface of the earth. Such an
eclipse is thus not dignified with the name of total eclipse, but is
called a partial eclipse, because from the earth's surface the sun is
only seen _partly eclipsed at the utmost_ (see Fig. 4 (ii.), p. 67).

[Illustration: (i.) Total Eclipse of the Sun.]

[Illustration: (ii.) Partial Eclipse of the Sun.

FIG. 4.--Total and Partial Eclipses of the Sun. From the position A the
Sun cannot be seen, as it is entirely blotted out by the Moon. From B it
is seen partially blotted out, because the Moon is to a certain degree
in the way. From C no eclipse is seen, because the Moon does not come in
the way.

It is to be noted that in a Partial Eclipse of the Sun, the position A
lies _outside_ the surface of the Earth.]

An _Annular eclipse_ is an eclipse which just fails to become total for
yet another reason. We have pointed out that the orbits of the various
members of the solar system are not circular, but oval. Such oval
figures, it will be remembered, are technically known as ellipses. In an
elliptic orbit the controlling body is situated not in the middle of the
figure, but rather towards one of the ends; the actual point which it
occupies being known as the _focus_. The sun being at the focus of the
earth's orbit, it follows that the earth is, at times, a little nearer
to him than at others. The sun will therefore appear to us to vary a
little in size, looking sometimes slightly larger than at other times.
It is so, too, with the moon, at the focus of whose orbit the earth is
situated. She therefore also appears to us at times to vary slightly in
size. The result is that when the sun is eclipsed by the moon, and the
moon at the time appears the larger of the two, she is able to blot out
the sun completely, and so we can get a total eclipse. But when, on the
other hand, the sun appears the larger, the eclipse will not be quite
total, for a portion of the sun's disc will be seen protruding all
around the moon like a ring of light. This is what is known as an
annular eclipse, from the Latin word _annulus_, which means a ring. The
term is consecrated by long usage, but it seems an unfortunate one on
account of its similarity to the word "annual." The Germans speak of
this kind of eclipse as "ring-formed," which is certainly much more to
the point.

There can never be a year without an eclipse of the sun. Indeed there
must be always two such eclipses _at least_ during that period, though
there need be no eclipse of the moon at all. On the other hand, the
greatest number of eclipses which can ever take place during a year are
seven; that is to say, either five solar eclipses and two lunar, or four
solar and three lunar. This general statement refers merely to eclipses
in their broadest significance, and informs us in no way whether they
will be total or partial.

Of all the phenomena which arise from the hiding of any celestial body
by one nearer coming in the way, a total eclipse of the sun is far the
most important. It is, indeed, interesting to consider how much poorer
modern astronomy would be but for the extraordinary coincidence which
makes a total solar eclipse just possible. The sun is about 400 times
farther off from us than the moon, and enormously greater than her in
bulk. Yet the two are relatively so distanced from us as to look about
the same size. The result of this is that the moon, as has been seen,
can often blot out the sun entirely from our view for a short time. When
this takes place the great blaze of sunlight which ordinarily dazzles
our eyes is completely cut off, and we are thus enabled, unimpeded, to
note what is going on in the immediate vicinity of the sun itself.

In a total solar eclipse, the time which elapses from the moment when
the moon's disc first begins to impinge upon that of the sun at his
western edge until the eclipse becomes total, lasts about an hour.
During all this time the black lunar disc may be watched making its way
steadily across the solar face. Notwithstanding the gradual obscuration
of the sun, one does not notice much diminution of light until about
three-quarters of his disc are covered. Then a wan, unearthly appearance
begins to pervade all things, the temperature falls noticeably, and
nature seems to halt in expectation of the coming of something unusual.
The decreasing portion of sun becomes more and more narrow, until at
length it is reduced to a crescent-shaped strip of exceeding fineness.
Strange, ill-defined, flickering shadows (known as "Shadow Bands") may
at this moment be seen chasing each other across any white expanse such
as a wall, a building, or a sheet stretched upon the ground. The western
side of the sky has now assumed an appearance dark and lowering, as if a
rainstorm of great violence were approaching. This is caused by the
mighty mass of the lunar shadow sweeping rapidly along. It flies onward
at the terrific velocity of about half a mile a second.

If the gradually diminishing crescent of sun be now watched through a
telescope, the observer will notice that it does not eventually vanish
all at once, as he might have expected. Rather, it breaks up first of
all along its length into a series of brilliant dots, known as "Baily's
Beads." The reason of this phenomenon is perhaps not entirely agreed
upon, but the majority of astronomers incline to the opinion that the
so-called "beads" are merely the last remnants of sunlight peeping
between those lunar mountain peaks which happen at the moment to fringe
the advancing edge of the moon. The beads are no sooner formed than they
rapidly disappear one after the other, after which no portion of the
solar surface is left to view, and the eclipse is now total (see Fig.
5).

[Illustration: _In a total Eclipse_ _In an annular Eclipse_

FIG. 5.--"Baily's Beads."]

But with the disappearance of the sun there springs into view a new and
strange appearance, ordinarily unseen because of the blaze of sunlight.
It is a kind of aureole, or halo, pearly white in colour, which is seen
to surround the black disc of the moon. This white radiance is none
other than the celebrated phenomenon widely known as the _Solar Corona_.
It was once upon a time thought to belong to the moon, and to be perhaps
a lunar atmosphere illuminated by the sunlight shining through it from
behind. But the suddenness with which the moon always blots out stars
when occulting them, has amply proved that she possesses no atmosphere
worth speaking about. It is now, however, satisfactorily determined that
the corona belongs to the sun, for during the short time that it remains
in view the black body of the moon can be seen creeping across it.

All the time that the _total phase_ (as it is called) lasts, the corona
glows with its pale unearthly light, shedding upon the earth's surface
an illumination somewhat akin to full moonlight. Usually the planet
Venus and a few stars shine out the while in the darkened heaven.
Meantime around the observer animal and plant life behave as at
nightfall. Birds go to roost, bats fly out, worms come to the surface of
the ground, flowers close up. In the Norwegian eclipse of 1896 fish were
seen rising to the surface of the water. When the total phase at length
is over, and the moon in her progress across the sky has allowed the
brilliant disc of the sun to spring into view once more at the other
side, the corona disappears.

There is another famous accompaniment of the sun which partly reveals
itself during total solar eclipses. This is a layer of red flame which
closely envelops the body of the sun and lies between it and the corona.
This layer is known by the name of the _Chromosphere_. Just as at
ordinary times we cannot see the corona on account of the blaze of
sunlight, so are we likewise unable to see the chromosphere because of
the dazzling white light which shines through from the body of the sun
underneath and completely overpowers it. When, however, during a solar
eclipse, the lunar disc has entirely hidden the brilliant face of the
sun, we are still able for a few moments to see an edgewise portion of
the chromosphere in the form of a narrow red strip, fringing the
advancing border of the moon. Later on, just before the moon begins to
uncover the face of the sun from the other side, we may again get a view
of a strip of chromosphere.

The outer surface of the chromosphere is not by any means even. It is
rough and billowy, like the surface of a storm-tossed sea. Portions of
it, indeed, rise at times to such heights that they may be seen standing
out like blood-red points around the black disc of the moon, and remain
thus during a good part of the total phase. These projections are known
as the _Solar Prominences_. In the same way as the corona, the
chromosphere and prominences were for a time supposed to belong to the
moon. This, however, was soon found not to be the case, for the lunar
disc was noticed to creep slowly across them also.

The total phase, or "totality," as it is also called, lasts for
different lengths of time in different eclipses. It is usually of about
two or three minutes' duration, and at the utmost it can never last
longer than about eight minutes.

When totality is over and the corona has faded away, the moon's disc
creeps little by little from the face of the sun, light and heat returns
once more to the earth, and nature recovers gradually from the gloom in
which she has been plunged. About an hour after totality, the last
remnant of moon draws away from the solar disc, and the eclipse is
entirely at an end.

The corona, the chromosphere, and the prominences are the most important
of these accompaniments of the sun which a total eclipse reveals to us.
Our further consideration of them must, however, be reserved for a
subsequent chapter, in which the sun will be treated of at length.

Every one who has had the good fortune to see a total eclipse of the sun
will, the writer feels sure, agree with the verdict of Sir Norman
Lockyer that it is at once one of the "grandest and most awe-inspiring
sights" which man can witness. Needless to say, such an occurrence used
to cause great consternation in less civilised ages; and that it has not
in modern times quite parted with its terrors for some persons, is shown
by the fact that in Iowa, in the United States, a woman died from fright
during the eclipse of 1869.

To the serious observer of a total solar eclipse every instant is
extremely precious. Many distinct observations have to be crowded into a
time all too limited, and this in an eclipse-party necessitates constant
rehearsals in order that not a moment may be wasted when the longed-for
totality arrives. Such preparation is very necessary; for the rarity and
uncommon nature of a total eclipse of the sun, coupled with its
exceeding short duration, tends to flurry the mind, and to render it
slow to seize upon salient points of detail. And, even after every
precaution has been taken, weather possibilities remain to be reckoned
with, so that success is rather a lottery.

Above all things, therefore, a total solar eclipse is an occurrence for
the proper utilisation of which personal experience is of absolute
necessity. It was manifestly out of the question that such experience
could be gained by any individual in early times, as the imperfection
of astronomical theory and geographical knowledge rendered the
predicting of the exact position of the track of totality well-nigh
impossible. Thus chance alone would have enabled one in those days to
witness a total phase, and the probabilities, of course, were much
against a second such experience in the span of a life-time. And even in
more modern times, when the celestial motions had come to be better
understood, the difficulties of foreign travel still were in the way;
for it is, indeed, a notable fact that during many years following the
invention of the telescope the tracks were placed for the most part in
far-off regions of the earth, and Europe was visited by singularly few
total solar eclipses. Thus it came to pass that the building up of a
body of organised knowledge upon this subject was greatly delayed.

Nothing perhaps better shows the soundness of modern astronomical theory
than the almost exact agreement of the time predicted for an eclipse
with its actual occurrence. Similarly, by calculating backwards,
astronomers have discovered the times and seasons at which many ancient
eclipses took place, and valuable opportunities have thus arisen for
checking certain disputed dates in history.

It should not be omitted here that the ancients were actually able, _in
a rough way_, to predict eclipses. The Chaldean astronomers had indeed
noticed very early a curious circumstance, _i.e._ that eclipses tend to
repeat themselves after a lapse of slightly more than eighteen years.

In this connection it must, however, be pointed out, in the first
instance, that the eclipses which occur in any particular year are in
no way associated with those which occurred in the previous year. In
other words, the mere fact that an eclipse takes place upon a certain
day this year will not bring about a repetition of it at the same time
next year. However, the nicely balanced behaviour of the solar system,
an equilibrium resulting from æons of orbital ebb and flow, naturally
tends to make the members which compose that family repeat their ancient
combinations again and again; so that after definite lapses of time the
same order of things will _almost exactly_ recur. Thus, as a consequence
of their beautifully poised motions, the sun, the moon, and the earth
tend, after a period of 18 years and 10-1/3 days,[5] to occupy very
nearly the same positions with regard to each other. The result of this
is that, during each recurring period, the eclipses comprised within it
will be repeated in their order.

To give examples:--

The total solar eclipse of August 30, 1905, was a repetition of that of
August 19, 1887.

The partial solar eclipse of February 23, 1906, corresponded to that
which took place on February 11, 1888.

The annular eclipse of July 10, 1907, was a recurrence of that of June
28, 1889.

In this way we can go on until the eighteen year cycle has run out, and
we come upon a total solar eclipse predicted for September 10, 1923,
which will repeat the above-mentioned ones of 1905 and 1887; and so on
too with the others.

From mere observation alone, extending no doubt over many ages, those
time-honoured watchers of the sky, the early Chaldeans, had arrived at
this remarkable generalisation; and they used it for the rough
prediction of eclipses. To the period of recurrence they give the name
of "Saros."

And here we find ourselves led into one of the most interesting and
fascinating by-paths in astronomy, to which writers, as a rule, pay all
too little heed.

In order not to complicate matters unduly, the recurrence of solar
eclipses alone will first be dealt with. This limitation will, however,
not affect the arguments in the slightest, and it will be all the more
easy in consequence to show their application to the case of eclipses of
the moon.

The reader will perhaps have noticed that, with regard to the repetition
of an eclipse, it has been stated that the conditions which bring it on
at each recurrence are reproduced _almost exactly_. Here, then, lies the
_crux_ of the situation. For it is quite evident that were the
conditions _exactly_ reproduced, the recurrences of each eclipse would
go on for an indefinite period. For instance, if the lapse of a saros
period found the sun, moon, and earth again in the precise relative
situations which they had previously occupied, the recurrences of a
solar eclipse would tend to duplicate its forerunner with regard to the
position of the shadow upon the terrestrial surface. But the conditions
_not_ being exactly reproduced, the shadow-track does not pass across
the earth in quite the same regions. It is shifted a little, so to
speak; and each time the eclipse comes round it is found to be shifted a
little farther. Every solar eclipse has therefore a definite "life" of
its own upon the earth, lasting about 1150 years, or 64 saros returns,
and working its way little by little across our globe from north to
south, or from south to north, as the case may be. Let us take an
imaginary example. A _partial_ eclipse occurs, say, somewhere near the
North Pole, the edge of the "partial" shadow just grazing the earth, and
the "track of totality" being as yet cast into space. Here we have the
beginning of a series. At each saros recurrence the partial shadow
encroaches upon a greater extent of earth-surface. At length, in its
turn, the track of totality begins to impinge upon the earth. This track
streaks across our globe at each return of the eclipse, repeating itself
every time in a slightly more southerly latitude. South and south it
moves, passing in turn the Tropic of Cancer, the Equator, the Tropic of
Capricorn, until it reaches the South Pole; after which it touches the
earth no longer, but is cast into space. The rear portion of the partial
shadow, in its turn, grows less and less in extent; and it too in time
finally passes off. Our imaginary eclipse series is now no more--its
"life" has ended.

We have taken, as an example, an eclipse series moving from north to
south. We might have taken one moving from south to north, for they
progress in either direction.

From the description just given the reader might suppose that, if the
tracks of totality of an eclipse series were plotted upon a chart of the
world, they would lie one beneath another like a set of steps. This is,
however, _not_ the case, and the reason is easily found. It depends upon
the fact that the saros does not comprise an exact number of days, but
includes, as we have seen, one-third of a day in addition.

It will be granted, of course, that if the number of days was exact, the
_same_ parts of the earth would always be brought round by the axial
rotation _to front the sun_ at the moment of the recurrence of the
eclipse. But as there is still one-third of a day to complete the saros
period, the earth has yet to make one-third of a rotation upon its axis
before the eclipse takes place. Thus at every recurrence the track of
totality finds itself placed one-third of the earth's circumference to
the _westward_. Three of the recurrences will, of course, complete the
circuit of the globe; and so the fourth recurrence will duplicate the
one which preceded it, three saros returns, or 54 years and 1 month
before. This duplication, as we have already seen, will, however, be
situated in a latitude to the south or north of its predecessor,
according as the eclipse series is progressing in a southerly or
northerly direction.

Lastly, every eclipse series, after working its way across the earth,
will return again to go through the same process after some 12,000
years; so that, at the end of that great lapse of time, the entire
"life" of every eclipse should repeat itself, provided that the
conditions of the solar system have not altered appreciably during the
interval.

We are now in a position to consider this gradual southerly or
northerly progress of eclipse recurrences in its application to the case
of eclipses of the moon. It should be evident that, just as in solar
eclipses the lunar shadow is lowered or raised (as the case may be) each
time it strikes the terrestrial surface, so in lunar eclipses will the
body of the moon shift its place at each recurrence relatively to the
position of the earth's shadow. Every lunar eclipse, therefore, will
commence on our satellite's disc as a partial eclipse at the northern or
southern extremity, as the case may be. Let us take, as an example, an
imaginary series of eclipses of the moon progressing from north to
south. At each recurrence the partial phase will grow greater, its
boundary encroaching more and more to the southward, until eventually
the whole disc is enveloped by the shadow, and the eclipse becomes
total. It will then repeat itself as total during a number of
recurrences, until the entire breadth of the shadow has been passed
through, and the northern edge of the moon at length springs out into
sunlight. This illuminated portion will grow more and more extensive at
each succeeding return, the edge of the shadow appearing to recede from
it until it finally passes off at the south. Similarly, when a lunar
eclipse commences as partial at the south of the moon, the edge of the
shadow at each subsequent recurrence finds itself more and more to the
northward. In due course the total phase will supervene, and will
persist during a number of recurrences until the southerly trend of the
moon results in the uncovering of the lunar surface at the south. Thus,
as the boundary of the shadow is left more and more to the northward,
the illuminated portion on the southern side of the moon becomes at each
recurrence greater and the darkened portion on the northern side less,
until the shadow eventually passes off at the north.

The "life" of an eclipse of the moon happens, for certain reasons, to be
much shorter than that of an eclipse of the sun. It lasts during only
about 860 years, or 48 saros returns.

Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a
portion of the march of the total solar eclipse of August 30, 1905,
across the surface of the earth. The projection in question has been
employed because it is the one with which people are most familiar. This
eclipse began by striking the neighbourhood of the North Pole in the
guise of a partial eclipse during the latter part of the reign of Queen
Elizabeth, and became total on the earth for the first time on the 24th
of June 1797. Its next appearance was on the 6th of July 1815. It has
not been possible to show the tracks of totality of these two early
visitations on account of the distortion of the polar regions consequent
on the _fiction_ of Mercator's Projection. It is therefore made to
commence with the track of its third appearance, viz. on July 17, 1833.
In consequence of those variations in the apparent sizes of the sun and
moon, which result, as we have seen, from the variations in their
distances from the earth, this eclipse will change from a total into an
annular eclipse towards the end of the twenty-first century. By that
time the track will have passed to the southern side of the equator. The
track will eventually leave the earth near the South Pole about the
beginning of the twenty-sixth century, and the rear portion of the
partial shadow will in its turn be clear of the terrestrial surface by
about 2700 A.D., when the series comes to an end.

[Illustration: FIG. 6.--Map of the World on Mercator's Projection,
showing a portion of the progress of the Total Solar Eclipse of August
30, 1905, across the surface of the earth.]

[4] Astronomical Essays (p. 40), London, 1907.

[5] In some cases the periods between the dates of the corresponding
eclipses _appear_ to include a greater number of days than ten; but this
is easily explained when allowance is made for intervening _leap_ years
(in each of which an _extra_ day has of course been added), and also for
variations in local time.

CHAPTER VIII

FAMOUS ECLIPSES OF THE SUN

What is thought to be the earliest reference to an eclipse comes down to
us from the ancient Chinese records, and is over four thousand years
old. The eclipse in question was a solar one, and occurred, so far as
can be ascertained, during the twenty-second century B.C. The story runs
that the two state astronomers, Ho and Hi by name, being exceedingly
intoxicated, were unable to perform their required duties, which
consisted in superintending the customary rites of beating drums,
shooting arrows, and the like, in order to frighten away the mighty
dragon which it was believed was about to swallow up the Lord of Day.
This eclipse seems to have been only partial; nevertheless a great
turmoil ensued, and the two astronomers were put to death, no doubt with
the usual _celestial_ cruelty.

The next eclipse mentioned in the Chinese annals is also a solar
eclipse, and appears to have taken place more than a thousand years
later, namely in 776 B.C. Records of similar eclipses follow from the
same source; but as they are mere notes of the events, and do not enter
into any detail, they are of little interest. Curiously enough the
Chinese have taken practically no notice of eclipses of the moon, but
have left us a comparatively careful record of comets, which has been
of value to modern astronomy.

The earliest mention of a _total_ eclipse of the sun (for it should be
noted that the ancient Chinese eclipse above-mentioned was merely
partial) was deciphered in 1905, on a very ancient Babylonian tablet, by
Mr. L.W. King of the British Museum. This eclipse took place in the year
1063 B.C.

Assyrian tablets record three solar eclipses which occurred between
three and four hundred years later than this. The first of these was in
763 B.C.; the total phase being visible near Nineveh.

The next record of an eclipse of the sun comes to us from a Grecian
source. This eclipse took place in 585 B.C., and has been the subject of
much investigation. Herodotus, to whom we are indebted for the account,
tells us that it occurred during a battle in a war which had been waging
for some years between the Lydians and Medes. The sudden coming on of
darkness led to a termination of the contest, and peace was afterwards
made between the combatants. The historian goes on to state that the
eclipse had been foretold by Thales, who is looked upon as the Founder
of Grecian astronomy. This eclipse is in consequence known as the
"Eclipse of Thales." It would seem as if that philosopher were
acquainted with the Chaldean saros.

The next solar eclipse worthy of note was an annular one, and occurred
in 431 B.C., the first year of the Peloponnesian War. Plutarch relates
that the pilot of the ship, which was about to convey Pericles to the
Peloponnesus, was very much frightened by it; but Pericles calmed him by
holding up a cloak before his eyes, and saying that the only difference
between this and the eclipse was that something larger than the cloak
prevented his seeing the sun for the time being.

An eclipse of great historical interest is that known as the "Eclipse of
Agathocles," which occurred on the morning of the 15th of August, 310
B.C. Agathocles, Tyrant of Syracuse, had been blockaded in the harbour
of that town by the Carthaginian fleet, but effected the escape of his
squadron under cover of night, and sailed for Africa in order to invade
the enemy's territory. During the following day he and his vessels
experienced a total eclipse, in which "day wholly put on the appearance
of night, and the stars were seen in all parts of the sky."

A few solar eclipses are supposed to be referred to in early Roman
history, but their identity is very doubtful in comparison with those
which the Greeks have recorded. Additional doubt is cast upon them by
the fact that they are usually associated with famous events. The birth
and death of Romulus, and the Passage of the Rubicon by Julius Cæsar,
are stated indeed to have been accompanied by these marks of the
approval or disapproval of the gods!

Reference to our subject in the Bible is scanty. Amos viii. 9 is thought
to refer to the Nineveh eclipse of 763 B.C., to which allusion has
already been made; while the famous episode of Hezekiah and the shadow
on the dial of Ahaz has been connected with an eclipse which was partial
at Jerusalem in 689 B.C.

The first solar eclipse, recorded during the Christian Era, is known as
the "Eclipse of Phlegon," from the fact that we are indebted for the
account to a pagan writer of that name. This eclipse took place in A.D.
29, and the total phase was visible a little to the north of Palestine.
It has sometimes been confounded with the "darkness of the Crucifixion,"
which event took place near the date in question; but it is sufficient
here to say that the Crucifixion is well known to have occurred during
the Passover of the Jews, which is always celebrated at the _full_ moon,
whereas an eclipse of the sun can only take place at _new_ moon.

Dion Cassius, commenting on the Emperor Claudius about the year A.D. 45,
writes as follows:--

"As there was going to be an eclipse on his birthday, through fear of a
disturbance, as there had been other prodigies, he put forth a public
notice, not only that the obscuration would take place, and about the
time and magnitude of it, but also about the causes that produce such an
event."

This is a remarkable piece of information; for the Romans, an
essentially military nation, appear hitherto to have troubled themselves
very little about astronomical matters, and were content, as we have
seen, to look upon phenomena, like eclipses, as mere celestial
prodigies.

What is thought to be the first definite mention of the solar corona
occurs in a passage of Plutarch. The eclipse to which he refers is
probably one which took place in A.D. 71. He says that the obscuration
caused by the moon "has no time to last and no extensiveness, but some
light shows itself round the sun's circumference, which does not allow
the darkness to become deep and complete." No further reference to this
phenomenon occurs until near the end of the sixteenth century. It
should, however, be here mentioned that Mr. E.W. Maunder has pointed
out the probability[6] that we have a very ancient symbolic
representation of the corona in the "winged circle," "winged disc," or
"ring with wings," as it is variously called, which appears so often
upon Assyrian and Egyptian monuments, as the symbol of the Deity (Fig.
7).

[Illustration: FIG. 7.--The "Ring with Wings." The upper is the Assyrian
form of the symbol, the lower the Egyptian. (From _Knowledge_.) Compare
the form of the corona on Plate VII. (B), p. 142.]

The first solar eclipse recorded to have been seen in England is that of
A.D. 538, mention of which is found in the _Anglo-Saxon Chronicle_. The
track of totality did not, however, come near our islands, for only
two-thirds of the sun's disc were eclipsed at London.

In 840 a great eclipse took place in Europe, which was total for more
than five minutes across what is now Bavaria. Terror at this eclipse is
said to have hastened the death of Louis le Debonnaire, Emperor of the
West, who lay ill at Worms.

In 878--_temp._ King Alfred--an eclipse of the sun took place which was
total at London. From this until 1715 no other eclipse was total at
London itself; though this does not apply to other portions of England.

An eclipse, generally known as the "Eclipse of Stiklastad," is said to
have taken place in 1030, during the sea-fight in which Olaf of Norway
is supposed to have been slain. Longfellow, in his _Saga of King Olaf_,
has it that

"The Sun hung red
As a drop of blood,"

but, as in the case of most poets, the dramatic value of an eclipse
seems to have escaped his notice.

In the year 1140 there occurred a total eclipse of the sun, the last to
be visible in England for more than five centuries. Indeed there have
been only two such since--namely, those of 1715 and 1724, to which we
shall allude in due course. The eclipse of 1140 took place on the 20th
March, and is thus referred to in the _Anglo-Saxon Chronicle_:--

"In the Lent, the sun and the day darkened, about the noon-tide of the
day, when men were eating, and they lighted candles to eat by. That was
the 13th day before the calends of April. Men were very much struck with
wonder."

Several of the older historians speak of a "fearful eclipse" as having
taken place on the morning of the Battle of Crecy, 1346. Lingard, for
instance, in his _History of England_, has as follows:--

"Never, perhaps, were preparations for battle made under circumstances
so truly awful. On that very day the sun suffered a partial eclipse:
birds, in clouds, the precursors of a storm, flew screaming over the two
armies, and the rain fell in torrents, accompanied by incessant thunder
and lightning. About five in the afternoon the weather cleared up; the
sun in full splendour darted his rays in the eyes of the enemy."

Calculations, however, show that no eclipse of the sun took place in
Europe during that year. This error is found to have arisen from the
mistranslation of an obsolete French word _esclistre_ (lightning), which
is employed by Froissart in his description of the battle.

In 1598 an eclipse was total over Scotland and part of North Germany. It
was observed at Torgau by Jessenius, an Hungarian physician, who noticed
a bright light around the moon during the time of totality. This is said
to be the first reference to the corona since that of Plutarch, to which
we have already drawn attention.

Mention of Scotland recalls the fact that an unusual number of eclipses
happen to have been visible in that country, and the occult bent natural
to the Scottish character has traditionalised a few of them in such
terms as the "Black Hour" (an eclipse of 1433), "Black Saturday" (the
eclipse of 1598 which has been alluded to above), and "Mirk Monday"
(1652). The track of the last-named also passed over Carrickfergus in
Ireland, where it was observed by a certain Dr. Wybord, in whose account
the term "corona" is first employed. This eclipse is the last which has
been total in Scotland, and it is calculated that there will not be
another eclipse seen as total there until the twenty-second century.

An eclipse of the sun which took place on May 30, 1612, is recorded as
having been seen "through a tube." This probably refers to the then
recent invention--the telescope.

The eclipses which we have been describing are chiefly interesting from
an historical point of view. The old mystery and confusion to the
beholders seem to have lingered even into comparatively enlightened
times, for we see how late it is before the corona attracts definite
attention for the sake of itself alone.

It is not a far cry from notice of the corona to that of other
accompaniments of a solar eclipse. Thus the eclipse of 1706, the total
phase of which was visible in Switzerland, is of great interest; for it
was on this occasion that the famous red prominences seem first to have
been noted. A certain Captain Stannyan observed this eclipse from Berne
in Switzerland, and described it in a letter to Flamsteed, the then
Astronomer Royal. He says the sun's "getting out of his eclipse was
preceded by a blood-red streak of light from its left limb, which
continued not longer than six or seven seconds of time; then part of the
Sun's disc appeared all of a sudden, as bright as Venus was ever seen in
the night, nay brighter; and in that very instant gave a Light and
Shadow to things as strong as Moonlight uses to do." How little was then
expected of the sun is, however, shown by Flamsteed's words, when
communicating this information to the Royal Society:--

"The Captain is the first man I ever heard of that took notice of a Red
Streak of Light preceding the Emersion of the Sun's body from a total
Eclipse. And I take notice of it to you because it infers that _the Moon
has an atmosphere_; and its short continuance of only six or seven
seconds of time, tells us that _its height is not more than the five or
six hundredth part of her diameter_."

What a change has since come over the ideas of men! The sun has proved a
veritable mine of discovery, while the moon has yielded up nothing new.

The eclipse of 1715, the first total at London since that of 878, was
observed by the famous astronomer, Edmund Halley, from the rooms of the
Royal Society, then in Crane Court, Fleet Street. On this occasion both
the corona and a red projection were noted. Halley further makes
allusion to that curious phenomenon, which later on became celebrated
under the name of "Baily's beads." It was also on the occasion of this
eclipse that the _earliest recorded drawings of the corona_ were made.
Cambridge happened to be within the track of totality; and a certain
Professor Cotes of that University, who is responsible for one of the
drawings in question, forwarded them to Sir Isaac Newton together with a
letter describing his observations.

In 1724 there occurred an eclipse, the total phase of which was visible
from the south-west of England, but not from London. The weather was
unfavourable, and the eclipse consequently appears to have been seen by
only one person, a certain Dr. Stukeley, who observed it from Haraden
Hill near Salisbury Plain. This is the last eclipse of which the total
phase was seen in any part of England. The next will not be until June
29, 1927, and will be visible along a line across North Wales and
Lancashire. The discs of the sun and moon will just then be almost of
the same apparent size, and so totality will be of extremely short
duration; in fact only a few seconds. London itself will not see a
totality until the year 2151--a circumstance which need hardly distress
any of us personally!

It is only from the early part of the nineteenth century that serious
scientific attention to eclipses of the sun can be dated. An _annular_
eclipse, visible in 1836 in the south of Scotland, drew the careful
notice of Francis Baily of Jedburgh in Roxburghshire to that curious
phenomenon which we have already described, and which has ever since
been known by the name of "Baily's beads." Spurred by his observation,
the leading astronomers of the day determined to pay particular
attention to a total eclipse, which in the year 1842 was to be visible
in the south of France and the north of Italy. The public interest
aroused on this occasion was also very great, for the region across
which the track of totality was to pass was very populous, and inhabited
by races of a high degree of culture.

This eclipse occurred on the morning of the 8th July, and from it may be
dated that great enthusiasm with which total eclipses of the sun have
ever since been received. Airy, our then Astronomer Royal, observed it
from Turin; Arago, the celebrated director of the Paris Observatory,
from Perpignan in the south of France; Francis Baily from Pavia; and Sir
John Herschel from Milan. The corona and three large red prominences
were not only well observed by the astronomers, but drew tremendous
applause from the watching multitudes.

The success of the observations made during this eclipse prompted
astronomers to pay similar attention to that of July 28, 1851, the total
phase of which was to be visible in the south of Norway and Sweden, and
across the east of Prussia. This eclipse was also a success, and it was
now ascertained that the red prominences belonged to the sun and not to
the moon; for the lunar disc, as it moved onward, was seen to cover and
to uncover them in turn. It was also noted that these prominences were
merely uprushes from a layer of glowing gaseous matter, which was seen
closely to envelop the sun.

The total eclipse of July 18, 1860, was observed in Spain, and
photography was for the first time _systematically_ employed in its
observation.[7] In the photographs taken the stationary appearance of
both the corona and prominences with respect to the moving moon,
definitely confirmed the view already put forward that they were actual
appendages of the sun.

The eclipse of August 18, 1868, the total phase of which lasted nearly
six minutes, was visible in India, and drew thither a large concourse of
astronomers. In this eclipse the spectroscope came to the front, and
showed that both the prominences, and the chromospheric layer from which
they rise, are composed of glowing vapours--chief among which is the
vapour of hydrogen. The direct result of the observations made on this
occasion was the spectroscopic method of examining prominences at any
time in full daylight, and without a total eclipse. This method, which
has given such an immense impetus to the study of the sun, was the
outcome of independent and simultaneous investigation on the part of the
French astronomer, the late M. Janssen, and the English astronomer,
Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscent
of the discovery of Neptune. The principles on which the method was
founded seem, however, to have occurred to Dr. (now Sir William) Huggins
some time previously.

The eclipse of December 22, 1870, was total for a little more than two
minutes, and its track passed across the Mediterranean. M. Janssen, of
whom mention has just been made, escaped in a balloon from then besieged
Paris, taking his instruments with him, and made his way to Oran, in
Algeria, in order to observe it; but his expectations were disappointed
by cloudy weather. The expedition sent out from England had the
misfortune to be shipwrecked off the coast of Sicily. But the occasion
was redeemed by a memorable observation made by the American astronomer,
the late Professor Young, which revealed the existence of what is now
known as the "Reversing Layer." This is a shallow layer of gases which
lies immediately beneath the chromosphere. An illustration of the
corona, as it was seen during the above eclipse, will be found on Plate
VII. (A), p. 142.

In the eclipse of December 12, 1871, total across Southern India, the
photographs of the corona obtained by Mr. Davis, assistant to Lord
Lindsay (now the Earl of Crawford), displayed a wealth of detail
hitherto unapproached.

The eclipse of July 29, 1878, total across the western states of North
America, was a remarkable success, and a magnificent view of the corona
was obtained by the well-known American astronomer and physicist, the
late Professor Langley, from the summit of Pike's Peak, Colorado, over
14,000 feet above the level of the sea. The coronal streamers were seen
to extend to a much greater distance at this altitude than at points
less elevated, and the corona itself remained visible during more than
four minutes after the end of totality. It was, however, not entirely a
question of altitude; the coronal streamers were actually very much
longer on this occasion than in most of the eclipses which had
previously been observed.

The eclipse of May 17, 1882, observed in Upper Egypt, is notable from
the fact that, in one of the photographs taken by Dr. Schuster at Sohag,
a bright comet appeared near the outer limit of the corona (see Plate
I., p. 96). The comet in question had not been seen before the eclipse,
and was never seen afterwards. This is the third occasion on which
attention has been drawn to a comet _merely_ by a total eclipse. The
first is mentioned by Seneca; and the second by Philostorgius, in an
account of an eclipse observed at Constantinople in A.D. 418. A fourth
case of the kind occurred in 1893, when faint evidences of one of these
filmy objects were found on photographs of the corona taken by the
American astronomer, Professor Schaeberle, during the total eclipse of
April 16 of that year.

The eclipse of May 6, 1883, had a totality of over five minutes, but
the central track unfortunately passed across the Pacific Ocean, and the
sole point of land available for observing it from was one of the
Marquesas Group, Caroline Island, a coral atoll seven and a half miles
long by one and a half broad. Nevertheless astronomers did not hesitate
to take up their posts upon that little spot, and were rewarded with
good weather.

The next eclipse of importance was that of April 16, 1893. It stretched
from Chili across South America and the Atlantic Ocean to the West Coast
of Africa, and, as the weather was fine, many good results were
obtained. Photographs were taken at both ends of the track, and these
showed that the appearance of the corona remained unchanged during the
interval of time occupied by the passage of the shadow across the earth.
It was on the occasion of this eclipse that Professor Schaeberle found
upon his photographs those traces of the presence of a comet, to which
allusion has already been made.

Extensive preparations were made to observe the eclipse of August 9,
1896. Totality lasted from two to three minutes, and the track stretched
from Norway to Japan. Bad weather disappointed the observers, with the
exception of those taken to Nova Zembla by Sir George Baden Powell in
his yacht _Otaria_.

The eclipse of January 22, 1898, across India _viâ_ Bombay and Benares,
was favoured with good weather, and is notable for a photograph obtained
by Mrs. E.W. Maunder, which showed a ray of the corona extending to a
most unusual distance.

[Illustration: PLATE I. THE TOTAL ECLIPSE OF THE SUN OF MAY 17TH, 1882

A comet is here shown in the immediate neighbourhood of the corona.

Drawn by Mr. W.H. Wesley from the photographs.

(Page 95)]

Of very great influence in the growth of our knowledge with regard to
the sun, is the remarkable piece of good fortune by which the countries
around the Mediterranean, so easy of access, have been favoured with a
comparatively large number of total eclipses during the past sixty
years. Tracks of totality have, for instance, traversed the Spanish
peninsula on no less than five occasions during that period. Two of
these are among the most notable eclipses of recent years, namely, those
of May 28, 1900, and of August 30, 1905. In the former the track of
totality stretched from the western seaboard of Mexico, through the
Southern States of America, and across the Atlantic Ocean, after which
it passed over Portugal and Spain into North Africa. The total phase
lasted for about a minute and a half, and the eclipse was well observed
from a great many points along the line. A representation of the corona,
as it appeared on this occasion, will be found on Plate VII. (B), p.
142.

The track of the other eclipse to which we have alluded, _i.e._ that of
August 30, 1905, crossed Spain about 200 miles to the northward of that
of 1900. It stretched from Winnipeg in Canada, through Labrador, and
over the Atlantic; then traversing Spain, it passed across the Balearic
Islands, North Africa, and Egypt, and ended in Arabia (see Fig. 6, p.
81). Much was to be expected from a comparison between the photographs
taken in Labrador and Egypt on the question as to whether the corona
would show any alteration in shape during the time that the shadow was
traversing the intervening space--some 6000 miles. The duration of the
total phase in this eclipse was nearly four minutes. Bad weather,
however, interfered a good deal with the observations. It was not
possible, for instance, to do anything at all in Labrador. In Spain the
weather conditions were by no means favourable; though at Burgos, where
an immense number of people had assembled, the total phase was,
fortunately, well seen. On the whole, the best results were obtained at
Guelma in Algeria. The corona on the occasion of this eclipse was a very
fine one, and some magnificent groups of prominences were plainly
visible to the naked eye (see the Frontispiece).

The next total eclipse after that of 1905 was one which occurred on
January 14, 1907. It passed across Central Asia and Siberia, and had a
totality lasting two and a half minutes at most; but it was not observed
as the weather was extremely bad, a circumstance not surprising with
regard to those regions at that time of year.

The eclipse of January 3, 1908, passed across the Pacific Ocean. Only
two small coral islands--Hull Island in the Phoenix Group, and Flint
Island about 400 miles north of Tahiti--lay in the track. Two
expeditions set out to observe it, _i.e._ a combined American party from
the Lick Observatory and the Smithsonian Institution of Washington, and
a private one from England under Mr. F.K. McClean. As Hull Island
afforded few facilities, both parties installed their instruments on
Flint Island, although it was very little better. The duration of the
total phase was fairly long--about four minutes, and the sun very
favourably placed, being nearly overhead. Heavy rain and clouds,
however, marred observation during the first minute of totality, but the
remaining three minutes were successfully utilised, good photographs of
the corona being obtained.

The next few years to come are unfortunately by no means favourable
from the point of view of the eclipse observer. An eclipse will take
place on June 17, 1909, the track stretching from Greenland across the
North Polar regions into Siberia. The geographical situation is,
however, a very awkward one, and totality will be extremely short--only
six seconds in Greenland and twenty-three seconds in Siberia.

The eclipse of May 9, 1910, will be visible in Tasmania. Totality will
last so long as four minutes, but the sun will be at the time much too
low in the sky for good observation.

The eclipse of the following year, April 28, 1911, will also be
confined, roughly speaking, to the same quarter of the earth, the track
passing across the old convict settlement of Norfolk Island, and then
out into the Pacific.

The eclipse of April 17, 1912, will stretch from Portugal, through
France and Belgium into North Germany. It will, however, be of
practically no service to astronomy. Totality, for instance, will last
for only three seconds in Portugal; and, though Paris lies in the
central track, the eclipse, which begins as barely total, will have
changed into an _annular_ one by the time it passes over that city.

The first really favourable eclipse in the near future will be that of
August 21, 1914. Its track will stretch from Greenland across Norway,
Sweden, and Russia. This eclipse is a return, after one saros, of the
eclipse of August 9, 1896.

The last solar eclipse which we will touch upon is that predicted for
June 29, 1927. It has been already alluded to as the first of those in
the future to be _total_ in England. The central line will stretch from
Wales in a north-easterly direction. Stonyhurst Observatory, in
Lancashire, will lie in the track; but totality there will be very
short, only about twenty seconds in duration.

[6] _Knowledge_, vol. xx. p. 9, January 1897.

[7] The _first photographic representation of the corona_ had, however,
been made during the eclipse of 1851. This was a daguerreotype taken by
Dr. Busch at Königsberg in Prussia.

CHAPTER IX

FAMOUS ECLIPSES OF THE MOON

The earliest lunar eclipse, of which we have any trustworthy
information, was a total one which took place on the 19th March, 721
B.C., and was observed from Babylon. For our knowledge of this eclipse
we are indebted to Ptolemy, the astronomer, who copied it, along with
two others, from the records of the reign of the Chaldean king,
Merodach-Baladan.

The next eclipse of the moon worth noting was a total one, which took
place some three hundred years later, namely, in 425 B.C. This eclipse
was observed at Athens, and is mentioned by Aristophanes in his play,
_The Clouds_.

Plutarch relates that a total eclipse of the moon, which occurred in 413
B.C., so greatly frightened Nicias, the general of the Athenians, then
warring in Sicily, as to cause a delay in his retreat from Syracuse
which led to the destruction of his whole army.

Seven years later--namely, in 406 B.C., the twenty-sixth year of the
Peloponnesian War--there took place another total lunar eclipse of which
mention is made by Xenophon.

Omitting a number of other eclipses alluded to by ancient writers, we
come to one recorded by Josephus as having occurred a little before the
death of Herod the Great. It is probable that the eclipse in question
was the total lunar one, which calculation shows to have taken place on
the 15th September 5 B.C., and to have been visible in Western Asia.
This is very important, for we are thus enabled to fix that year as the
date of the birth of Christ, for Herod is known to have died in the
early part of the year following the Nativity.

In those accounts of total lunar eclipses, which have come down to us
from the Dark and Middle Ages, the colour of the moon is nearly always
likened to "blood." On the other hand, in an account of the eclipse of
January 23, A.D. 753, our satellite is described as "covered with a
horrid black shield." We thus have examples of the two distinct
appearances alluded to in Chapter VII., _i.e._ when the moon appears of
a coppery-red colour, and when it is entirely darkened.

It appears, indeed, that, in the majority of lunar eclipses on record,
the moon has appeared of a ruddy, or rather of a coppery hue, and the
details on its surface have been thus rendered visible. One of the best
examples of a _bright_ eclipse of this kind is that of the 19th March
1848, when the illumination of our satellite was so great that many
persons could not believe that an eclipse was actually taking place. A
certain Mr. Foster, who observed this eclipse from Bruges, states that
the markings on the lunar disc were almost as visible as on an "ordinary
dull moonlight night." He goes on to say that the British Consul at
Ghent, not knowing that there had been any eclipse, wrote to him for an
explanation of the red colour of the moon on that evening.

Out of the _dark_ eclipses recorded, perhaps the best example is that
of May 18, 1761, observed by Wargentin at Stockholm. On this occasion
the lunar disc is said to have disappeared so completely, that it could
not be discovered even with the telescope. Another such instance is the
eclipse of June 10, 1816, observed from London. The summer of that year
was particularly wet--a point worthy of notice in connection with the
theory that these different appearances are due to the varying state of
our earth's atmosphere.

Sometimes, indeed, it has happened that an eclipse of the moon has
partaken of both appearances, part of the disc being visible and part
invisible. An instance of this occurred in the eclipse of July 12, 1870,
when the late Rev. S.J. Johnson, one of the leading authorities on
eclipses, who observed it, states that he found one-half the moon's
surface quite invisible, both with the naked eye and with the telescope.

In addition to the examples given above, there are three total lunar
eclipses which deserve especial mention.

1. A.D. 755, November 23. During the progress of this eclipse the moon
occulted the star Aldebaran in the constellation of Taurus.

2. A.D. 1493, April 2. This is the celebrated eclipse which is said to
have so well served the purposes of Christopher Columbus. Certain
natives having refused to supply him with provisions when in sore
straits, he announced to them that the moon would be darkened as a sign
of the anger of heaven. When the event duly came to pass, the savages
were so terrified that they brought him provisions as much as he needed.

3. A.D. 1610, July 6. The eclipse in question is notable as having been
seen through the telescope, then a recent invention. It was without
doubt the first so observed, but unfortunately the name of the observer
has not come down to us.

CHAPTER X

THE GROWTH OF OBSERVATION

The earliest astronomical observations must have been made in the Dawn
of Historic Time by the men who tended their flocks upon the great
plains. As they watched the clear night sky they no doubt soon noticed
that, with the exception of the moon and those brilliant wandering
objects known to us as the planets, the individual stars in the heaven
remained apparently fixed with reference to each other. These seemingly
changeless points of light came in time to be regarded as sign-posts to
guide the wanderer across the trackless desert, or the voyager upon the
wide sea.

Just as when looking into the red coals of a fire, or when watching the
clouds, our imagination conjures up strange and grotesque forms, so did
the men of old see in the grouping of the stars the outlines of weird
and curious shapes. Fed with mythological lore, they imagined these to
be rough representations of ancient heroes and fabled beasts, whom they
supposed to have been elevated to the heavens as a reward for great
deeds done upon the earth. We know these groupings of stars to-day under
the name of the Constellations. Looking up at them we find it extremely
difficult to fit in the majority with the figures which the ancients
believed them to represent. Nevertheless, astronomy has accepted the
arrangement, for want of a better method of fixing the leading stars in
the memory.

Our early ancestors lived the greater part of their lives in the open
air, and so came to pay more attention in general to the heavenly orbs
than we do. Their clock and their calendar was, so to speak, in the
celestial vault. They regulated their hours, their days, and their
nights by the changing positions of the sun, the moon, and the stars;
and recognised the periods of seed-time and harvest, of calm and stormy
weather, by the rising or setting of certain well-known constellations.
Students of the classics will recall many allusions to this, especially
in the Odes of Horace.

As time went on and civilisation progressed, men soon devised measuring
instruments, by means of which they could note the positions of the
celestial bodies in the sky with respect to each other; and, from
observations thus made, they constructed charts of the stars. The
earliest complete survey of this kind, of which we have a record, is the
great Catalogue of stars which was made, in the second century B.C., by
the celebrated Greek astronomer, Hipparchus, and in which he is said to
have noted down about 1080 stars.

It is unnecessary to follow in detail the tedious progress of
astronomical discovery prior to the advent of the telescope. Certain it
is that, as time went on, the measuring instruments to which we have
alluded had become greatly improved; but, had they even been perfect,
they would have been utterly inadequate to reveal those minute
displacements, from which we have learned the actual distance of the
nearest of the celestial orbs. From the early times, therefore, until
the mediæval period of our own era, astronomy grew up upon a faulty
basis, for the earth ever seemed so much the largest body in the
universe, that it continued from century to century to be regarded as
the very centre of things.

To the Arabians is due the credit of having kept alive the study of the
stars during the dark ages of European history. They erected some fine
observatories, notably in Spain and in the neighbourhood of Bagdad.
Following them, some of the Oriental peoples embraced the science in
earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for
instance, a great observatory at Samarcand in Central Asia. The Mongol
emperors of India also established large astronomical instruments in the
chief cities of their empire. When the revival of learning took place in
the West, the Europeans came to the front once more in science, and
rapidly forged ahead of those who had so assiduously kept alight the
lamp of knowledge through the long centuries.

The dethronement of the older theories by the Copernican system, in
which the earth was relegated to its true place, was fortunately soon
followed by an invention of immense import, the invention of the
Telescope. It is to this instrument, indeed, that we are indebted for
our knowledge of the actual scale of the celestial distances. It
penetrated the depths of space; it brought the distant orbs so near,
that men could note the detail on the planets, or measure the small
changes in their positions in the sky which resulted from the movement
of our own globe.

It was in the year 1609 that the telescope was first constructed. A
year or so previous to this a spectacle-maker of Middleburgh in Holland,
one Hans Lippershey, had, it appears, hit upon the fact that distant
objects, when viewed through certain glass lenses suitably arranged,
looked nearer.[8] News of this discovery reached the ears of Galileo
Galilei, of Florence, the foremost philosopher of the day, and he at
once applied his great scientific attainments to the construction of an
instrument based upon this principle. The result was what was called an
"optick tube," which magnified distant objects some few times. It was
not much larger than what we nowadays contemptuously refer to as a
"spy-glass," yet its employment upon the leading celestial objects
instantly sent astronomical science onward with a bound. In rapid
succession Galileo announced world-moving discoveries; large spots upon
the face of the sun; crater-like mountains upon the moon; four
subordinate bodies, or satellites, circling around the planet Jupiter;
and a strange appearance in connection with Saturn, which later
telescopic observers found to be a broad flat ring encircling that
planet. And more important still, the magnified image of Venus showed
itself in the telescope at certain periods in crescent and other forms;
a result which Copernicus is said to have announced should of necessity
follow if his system were the true one.

The discoveries made with the telescope produced, as time went on, a
great alteration in the notions of men with regard to the universe at
large. It must have been, indeed, a revelation to find that those points
of light which they called the planets, were, after all, globes of a
size comparable with the earth, and peopled perchance with sentient
beings. Even to us, who have been accustomed since our early youth to
such an idea, it still requires a certain stretch of imagination to
enlarge, say, the Bright Star of Eve, into a body similar in size to our
earth. The reader will perhaps recollect Tennyson's allusion to this in
_Locksley Hall, Sixty Years After_:--

"Hesper--Venus--were we native to that splendour or in Mars,
We should see the Globe we groan in, fairest of their evening stars.

"Could we dream of wars and carnage, craft and madness, lust and spite,
Roaring London, raving Paris, in that point of peaceful light?"

The form of instrument as devised by Galileo is called the Refracting
Telescope, or "Refractor." As we know it to-day it is the same in
principle as his "optick tube," but it is not quite the same in
construction. The early _object-glass_, or large glass at the end, was a
single convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is,
on the other hand, composed of two lenses fitted together. The attempts
to construct large telescopes of the Galilean type met in course of time
with a great difficulty. The magnified image of the object observed was
not quite pure; its edges, indeed, were fringed with rainbow-like
colours. This defect was found to be aggravated with increase in the
size of object-glasses. A method was, however, discovered of
diminishing this colouration, or _chromatic aberration_ as it is called
from the Greek word [chrôma] (_chroma_), which means colour, viz. by
making telescopes of great length and only a few inches in width. But
the remedy was, in a way, worse than the disease; for telescopes thus
became of such huge proportions as to be too unwieldy for use. Attempts
were made to evade this unwieldiness by constructing them with skeleton
tubes (see Plate II., p. 110), or, indeed, even without tubes at all;
the object-glass in the tubeless or "aerial" telescope being fixed at
the top of a high post, and the _eye-piece_, that small lens or
combination of lenses, which the eye looks directly into, being kept in
line with it by means of a string and manoeuvred about near the ground
(Plate III., p. 112). The idea of a telescope without a tube may appear
a contradiction in terms; but it is not really so, for the tube adds
nothing to the magnifying power of the instrument, and is, in fact, no
more than a mere device for keeping the object-glass and eye-piece in a
straight line, and for preventing the observer from being hindered by
stray lights in his neighbourhood. It goes without saying, of course,
that the image of a celestial object will be more clear and defined when
examined in the darkness of a tube.

The ancients, though they knew nothing of telescopes, had, however,
found out the merit of a tube in this respect; for they employed simple
tubes, blackened on the inside, in order to obtain a clearer view of
distant objects. It is said that Julius Cæsar, before crossing the
Channel, surveyed the opposite coast of Britain through a tube of this
kind.

[Illustration: PLATE II. GREAT TELESCOPE OF HEVELIUS

This instrument, 150 feet in length, with a _skeleton_ tube, was
constructed by the celebrated seventeenth century astronomer, Hevelius
of Danzig. From an illustration in the _Machina Celestis_.

(Page 110)]

A few of the most famous of the immensely long telescopes above alluded
to are worthy of mention. One of these, 123 feet in length, was
presented to the Royal Society of London by the Dutch astronomer
Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in
length (see Plate II., p. 110). Bradley used a tubeless one 212 feet
long to measure the diameter of Venus in 1722; while one of 600 feet is
said to have been constructed, but to have proved quite unworkable!

Such difficulties, however, produced their natural result. They set men
at work to devise another kind of telescope. In the new form, called the
Reflecting Telescope, or "Reflector," the light coming from the object
under observation was _reflected_ into the eye-piece from the surface of
a highly polished concave metallic mirror, or _speculum_, as it was
called. It is to Sir Isaac Newton that the world is indebted for the
reflecting telescope in its best form. That philosopher had set himself
to investigate the causes of the rainbow-like, or prismatic colours
which for a long time had been such a source of annoyance to telescopic
observers; and he pointed out that, as the colours were produced in the
passage of the rays of light _through_ the glass, they would be entirely
absent if the light were reflected from the _surface_ of a mirror
instead.

The reflecting telescope, however, had in turn certain drawbacks of its
own. A mirror, for instance, can plainly never be polished to such a
high degree as to reflect as much light as a piece of transparent glass
will let through. Further, the position of the eye-piece is by no means
so convenient. It cannot, of course, be pointed directly towards the
mirror, for the observer would then have to place his head right in the
way of the light coming from the celestial object, and would thus, of
course, cut it off. In order to obviate this difficulty, the following
device was employed by Newton in his telescope, of which he constructed
his first example in 1668. A small, flat mirror was fixed by thin wires
in the centre of the tube of the telescope, and near to its open end. It
was set slant-wise, so that it reflected the rays of light directly into
the eye-piece, which was screwed into a hole at the side of the tube
(see Fig. 8, p. 113, "Newtonian").

Although the Newtonian form of telescope had the immense advantage of
doing away with the prismatic colours, yet it wasted a great deal of
light; for the objection in this respect with regard to loss of light by
reflection from the large mirror applied, of course, to the small mirror
also. In addition, the position of the "flat," as the small mirror is
called, had the further effect of excluding from the great mirror a
certain proportion of light. But the reflector had the advantage, on the
other hand, of costing less to make than the refractor, as it was not
necessary to procure flawless glass for the purpose. A disc of a certain
metallic composition, an alloy of copper and tin, known in consequence
as _speculum metal_, had merely to be cast; and this had to be ground
and polished _upon one side only_, whereas a lens has to be thus treated
_upon both its sides_. It was, therefore, possible to make a much larger
instrument at a great deal less labour and expense.

[Illustration: PLATE III. A TUBELESS, OR "AERIAL" TELESCOPE

From an illustration in the _Opera Varia_ of Christian Huyghens.

(Page 110)]

[Illustration: FIG. 8.--The various types of Telescope. All the above
telescopes are _pointed_ in the same direction; that is to say, the rays
of light from the object are coming from the left-hand side.]

We have given the Newtonian form as an example of the principle of the
reflecting telescope. A somewhat similar instrument had, however, been
projected, though not actually constructed, by James Gregory a few years
earlier than Newton's, _i.e._ in 1663. In this form of reflector, known
as the "Gregorian" telescope, a hole was made in the big concave mirror;
and a small mirror, also concave, which faced it at a certain distance,
received the reflected rays, and reflected them back again through the
hole in question into the eye-piece, which was fixed just behind (see
Fig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimental
advantage of being _pointed directly at the object_. The hole in the big
mirror did not cause any loss of light, for the central portion in which
it was made was anyway unable to receive light through the small mirror
being directly in front of it. An adaptation of the Gregorian was the
"Cassegrainian" telescope, devised by Cassegrain in 1672, which differed
from it chiefly in the small mirror being convex instead of concave (see
Fig. 8, p. 113, "Cassegrainian"). These _direct-view_ forms of the
reflecting telescope were much in vogue about the middle of the
eighteenth century, when many beautiful examples of Gregorians were made
by the famous optician, James Short, of Edinburgh.

An adaptation of the Newtonian type of telescope is known as the
"Herschelian," from being the kind favoured by Sir William Herschel. It
is, however, only suitable in immense instruments, such as Herschel was
in the habit of employing. In this form the object-glass is set at a
slight slant, so that the light coming from the object is reflected
straight into the eye-piece, which is fixed facing it in the side of the
tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an
advantage over the other forms of reflector through the saving of light
consequent on doing away with the _second_ reflection. There is,
however, the objection that the slant of the object-glass is productive
of some distortion in the appearance of the object observed; but this
slant is of necessity slight when the length of the telescope is very
great.

The principle of this type of telescope had been described to the
French Academy of Sciences as early as 1728 by Le Maire, but no one
availed himself of the idea until 1776, when Herschel tried it. At
first, however, he rejected it; but in 1786 he seems to have found that
it suited the huge instruments which he was then making. Herschel's
largest telescope, constructed in 1789, was about four feet in diameter
and forty feet in length. It is generally spoken of as the "Forty-foot
Telescope," though all other instruments have been known by their
_diameters_, rather than by their lengths.

To return to the refracting telescope. A solution of the colour
difficulty was arrived at in 1729 (two years after Newton's death) by an
Essex gentleman named Chester Moor Hall. He discovered that by making a
double object-glass, composed of an outer convex lens and an inner
concave lens, made respectively of different kinds of glass, _i.e._
_crown_ glass and _flint_ glass, the troublesome colour effects could
be, _to a very great extent_, removed. Hall's investigations appear to
have been rather of an academic nature; and, although he is believed to
have constructed a small telescope upon these lines, yet he seems to
have kept the matter so much to himself that it was not until the year
1758 that the first example of the new instrument was given to the
world. This was done by John Dollond, founder of the well-known optical
firm of Dollond, of Ludgate Hill, London, who had, quite independently,
re-discovered the principle.

This "Achromatic" telescope, or telescope "free from colour effects," is
the kind ordinarily in use at present, whether for astronomical or for
terrestrial purposes (see Fig. 8, p. 113, "Achromatic"). The expense of
making large instruments of this type is very great, for, in the
object-glass alone, no less than _four_ surfaces have to be ground and
polished to the required curves; and, usually, the two lenses of which
it is composed have to fit quite close together.

With the object of evading the expense referred to, and of securing
_complete_ freedom from colour effects, telescopes have even been made,
the object-glasses of which were composed of various transparent liquids
placed between thin lenses; but leakages, and currents set up within
them by changes of temperature, have defeated the ingenuity of those who
devised these substitutes.

The solution of the colour difficulty by means of Dollond's achromatic
refractor has not, however, ousted the reflecting telescope in its best,
or Newtonian form, for which great concave mirrors made of glass,
covered with a thin coating of silver and highly polished, have been
used since about 1870 instead of metal mirrors. They are very much
lighter in weight and cheaper to make than the old specula; and though
the silvering, needless to say, deteriorates with time, it can be
renewed at a comparatively trifling cost. Also these mirrors reflect
much more light, and give a clearer view, than did the old metallic
ones.

When an object is viewed through the type of astronomical telescope
ordinarily in use, it is seen _upside down_. This is, however, a matter
of very small moment in dealing with celestial objects; for, as they are
usually round, it is really not of much consequence which part we regard
as top and which as bottom. Such an inversion would, of course, be most
inconvenient when viewing terrestrial objects. In order to observe the
latter we therefore employ what is called a terrestrial telescope, which
is merely a refractor with some extra lenses added in the eye portion
for the purpose of turning the inverted image the right way up again.
These extra lenses, needless to say, absorb a certain amount of light;
wherefore it is better in astronomical observation to save light by
doing away with them, and putting up with the slight inconvenience of
seeing the object inverted.

This inversion of images by the astronomical telescope must be specially
borne in mind with regard to the photographs of the moon in Chapter XVI.

In the year 1825 the largest achromatic refractor in existence was one
of nine and a half inches in diameter constructed by Fraunhofer for the
Observatory of Dorpat in Russia. The largest refractors in the world
to-day are in the United States, _i.e._ the forty-inch of the Yerkes
Observatory (see Plate IV., p. 118), and the thirty-six inch of the
Lick. The object-glasses of these and of the thirty-inch telescope of
the Observatory of Pulkowa, in Russia, were made by the great optical
house of Alvan Clark & Sons, of Cambridge, Massachusetts, U.S.A. The
tubes and other portions of the Yerkes and Lick telescopes were,
however, constructed by the Warner and Swasey Co., of Cleveland, Ohio.

The largest reflector, and so the largest telescope in the world, is
still the six-foot erected by the late Lord Rosse at Parsonstown in
Ireland, and completed in the year 1845. It is about fifty-six feet in
length. Next come two of five feet, with mirrors of silver on glass;
one of them made by the late Dr. Common, of Ealing, and the other by the
American astronomer, Professor G.W. Ritchey. The latter of these is
installed in the Solar Observatory belonging to Carnegie Institution of
Washington, which is situated on Mount Wilson in California. The former
is now at the Harvard College Observatory, and is considered by
Professor Moulton to be probably the most efficient reflector in use at
present. Another large reflector is the three-foot made by Dr. Common.
It came into the possession of Mr. Crossley of Halifax, who presented it
to the Lick Observatory, where it is now known as the "Crossley
Reflector."

Although to the house of Clark belongs, as we have seen, the credit of
constructing the object-glasses of the largest refracting telescopes of
our time, it has nevertheless keen competitors in Sir Howard Grubb, of
Dublin, and such well-known firms as Cooke of York and Steinheil of
Munich. In the four-foot reflector, made in 1870 for the Observatory of
Melbourne by the firm of Grubb, the Cassegrainian principle was
employed.

With regard to the various merits of refractors and reflectors much
might be said. Each kind of instrument has, indeed, its special
advantages; though perhaps, on the whole, the most perfect type of
telescope is the achromatic refractor.

[Illustration: PLATE IV. THE GREAT YERKES TELESCOPE

Great telescope at the Yerkes Observatory of the University of Chicago,
Williams Bay, Wisconsin, U.S.A. It was erected in 1896-7, and is the
largest refracting telescope in the world. Diameter of object-glass, 40
inches; length of telescope, about 60 feet. The object-glass was made by
the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other
portions of the instrument by the Warner and Swasey Co., of Cleveland,
Ohio.

(Page 117)]

In connection with telescopes certain devices have from time to time
been introduced, but these merely aim at the _convenience_ of the
observer and do not supplant the broad principles upon which are based
the various types of instrument above described. Such, for instance, are
the "Siderostat," and another form of it called the "Coelostat," in
which a plane mirror is made to revolve in a certain manner, so as to
reflect those portions of the sky which are to be observed, into the
tube of a telescope kept fixed. Such too are the "Equatorial Coudé" of
the late M. Loewy, Director of the Paris Observatory, and the
"Sheepshanks Telescope" of the Observatory of Cambridge, in which a
telescope is separated into two portions, the eye-piece portion being
fixed upon a downward slant, and the object-glass portion jointed to it
at an angle and pointed up at the sky. In these two instruments (which,
by the way, differ materially) an arrangement of slanting mirrors in the
tubes directs the journey of the rays of light from the object-glass to
the eye-piece. The observer can thus sit at the eye-end of his telescope
in the warmth and comfort of his room, and observe the stars in the same
unconstrained manner as if he were merely looking down into a
microscope.

Needless to say, devices such as these are subject to the drawback that
the mirrors employed sap a certain proportion of the rays of light. It
will be remembered that we made allusion to loss of light in this way,
when pointing out the advantage in light grasp of the Herschelian form
of telescope, where only _one_ reflection takes place, over the
Newtonian in which there are _two_.

It is an interesting question as to whether telescopes can be made much
larger. The American astronomer, Professor G.E. Hale, concludes that the
limit of refractors is about five feet in diameter, but he thinks that
reflectors as large as nine feet in diameter might now be made. As
regards refractors there are several strong reasons against augmenting
their proportions. First of all comes the great cost. Secondly, since
the lenses are held in position merely round their rims, they will bend
by their weight in the centres if they are made much larger. On the
other hand, attempts to obviate this, by making the lenses thicker,
would cause a decrease in the amount of light let through.

But perhaps the greatest stumbling-block to the construction of larger
telescopes is the fact that the unsteadiness of the air will be
increasingly magnified. And further, the larger the tubes become, the
more difficult will it be to keep the air within them at one constant
temperature throughout their lengths.

It would, indeed, seem as if telescopes are not destined greatly to
increase in size, but that the means of observation will break out in
some new direction, as it has already done in the case of photography
and the spectroscope. The direct use of the eye is gradually giving
place to indirect methods. We are, in fact, now _feeling_ rather than
seeing our way about the universe. Up to the present, for instance, we
have not the slightest proof that life exists elsewhere than upon our
earth. But who shall say that the twentieth century has not that in
store for us, by which the presence of life in other orbs may be
perceived through some form of vibration transmitted across illimitable
space? There is no use speaking of the impossible or the inconceivable.
After the extraordinary revelations of the spectroscope--nay, after the
astounding discovery of Röntgen--the word impossible should be cast
aside, and inconceivability cease to be regarded as any criterion.

[8] The principle upon which the telescope is based appears to have been
known _theoretically_ for a long time previous to this. The monk Roger
Bacon, who lived in the thirteenth century, describes it very clearly;
and several writers of the sixteenth century have also dealt with the
idea. Even Lippershey's claims to a practical solution of the question
were hotly contested at the time by two of his own countrymen, _i.e._ a
certain Jacob Metius, and another spectacle-maker of Middleburgh, named
Jansen.

CHAPTER XI

SPECTRUM ANALYSIS

If white light (that of the sun, for instance) be passed through a glass
prism, namely, a piece of glass of triangular shape, it will issue from
it in rainbow-tinted colours. It is a common experience with any of us
to notice this when the sunlight shines through cut-glass, as in the
pendant of a chandelier, or in the stopper of a wine-decanter.

The same effect may be produced when light passes through water. The
Rainbow, which we all know so well, is merely the result of the sunlight
passing through drops of falling rain.

White light is composed of rays of various colours. Red, orange, yellow,
green, blue, indigo, and violet, taken all together, go, in fact, to
make up that effect which we call white.

It is in the course of the _refraction_, or bending of a beam of light,
when it passes in certain conditions through a transparent and denser
medium, such as glass or water, that the constituent rays are sorted out
and spread in a row according to their various colours. This production
of colour takes place usually near the edges of a lens; and, as will be
recollected, proved very obnoxious to the users of the old form of
refracting telescope.

It is, indeed, a strange irony of fate that this very same production
of colour, which so hindered astronomy in the past, should have aided it
in recent years to a remarkable degree. If sunlight, for instance, be
admitted through a narrow slit before it falls upon a glass prism, it
will issue from the latter in the form of a band of variegated colour,
each colour blending insensibly with the next. The colours arrange
themselves always in the order which we have mentioned. This seeming
band is, in reality, an array of countless coloured images of the
original slit ranged side by side; the colour of each image being the
slightest possible shade different from that next to it. This strip of
colour when produced by sunlight is called the "Solar Spectrum" (see
Fig. 9, p. 123). A similar strip, or _spectrum_, will be produced by any
other light; but the appearance of the strip, with regard to
preponderance of particular colours, will depend upon the character of
that light. Electric light and gas light yield spectra not unlike that
of sunlight; but that of gas is less rich in blue and violet than that
of the sun.

The Spectroscope, an instrument devised for the examination of spectra,
is, in its simplest form, composed of a small tube with a narrow slit
and prism at one end, and an eye-piece at the other. If we drop ordinary
table salt into the flame of a gas light, the flame becomes strongly
yellow. If, then, we observe this yellow flame with the spectroscope, we
find that its spectrum consists almost entirely of two bright yellow
transverse lines. Chemically considered ordinary table salt is sodium
chloride; that is to say, a compound of the metal sodium and the gas
chlorine. Now if other compounds of sodium be experimented with in the
same manner, it will soon be found that these two yellow lines are
characteristic of sodium when turned into vapour by great heat. In the
same manner it can be ascertained that every element, when heated to a
condition of vapour, gives as its spectrum a set of lines peculiar to
itself. Thus the spectroscope enables us to find out the composition of
substances when they are reduced to vapour in the laboratory.

[Illustration: FIG. 9.--The Solar Spectrum.]

In order to increase the power of a spectroscope, it is necessary to
add to the number of prisms. Each extra prism has the effect of
lengthening the coloured strip still more, so that lines, which at first
appeared to be single merely through being crowded together, are
eventually drawn apart and become separately distinguishable.

On this principle it has gradually been determined that the sun is
composed of elements similar to those which go to make up our earth.
Further, the composition of the stars can be ascertained in the same
manner; and we find them formed on a like pattern, though with certain
elements in greater or less proportion as the case may be. It is in
consequence of our thus definitely ascertaining that the stars are
self-luminous, and of a sun-like character, that we are enabled to speak
of them as _suns_, or to call the sun a _star_.

In endeavouring to discover the elements of which the planets and
satellites of our system are composed, we, however, find ourselves
baffled, for the simple reason that these bodies emit no real light of
their own. The light which reaches us from them, being merely reflected
sunlight, gives only the ordinary solar spectrum when examined with the
spectroscope. But in certain cases we find that the solar spectrum thus
viewed shows traces of being weakened, or rather of suffering
absorption; and it is concluded that this may be due to the sunlight
having had to pass through an atmosphere on its way to and from the
surface of the planet from which it is reflected to us.

Since the sun is found to be composed of elements similar to those which
go to make up our earth, we need not be disheartened at this failure of
the spectroscope to inform us of the composition of the planets and
satellites. We are justified, indeed, in assuming that more or less the
same constituents run through our solar system; and that the elements of
which these bodies are composed are similar to those which are found
upon our earth and in the sun.

The spectroscope supplies us with even more information. It tells us,
indeed, whether the sun-like body which we are observing is moving away
from us or towards us. A certain slight shifting of the lines towards
the red or violet end of the spectrum respectively, is found to follow
such movement. This method of observation is known by the name of
_Doppler's Method_,[9] and by it we are enabled to confirm the evidence
which the sunspots give us of the rotation of the sun; for we find thus
that one edge of that body is continually approaching us, and the other
edge is continually receding from us. Also, we can ascertain in the same
manner that certain of the stars are moving towards us, and certain of
them away from us.

[9] The idea, initiated by Christian Doppler at Prague in 1842, was
originally applied to sound. The approach or recession of a source from
which sound is coming is invariably accompanied by alterations of pitch,
as the reader has no doubt noticed when a whistling railway-engine has
approached him or receded from him. It is to Sir William Huggins,
however, that we are indebted for the application of the principle to
spectroscopy. This he gave experimental proof of in the year 1868.

CHAPTER XII

THE SUN

The sun is the chief member of our system. It controls the motions of
the planets by its immense gravitative power. Besides this it is the
most important body in the entire universe, so far as we are concerned;
for it pours out continually that flood of light and heat, without which
life, as we know it, would quickly become extinct upon our globe.

Light and heat, though not precisely the same thing, may be regarded,
however, as next-door neighbours. The light rays are those which
directly affect the eye and are comprised in the visible spectrum. We
_feel_ the heat rays, the chief of which are beyond the red portion of
the spectrum. They may be investigated with the _bolometer_, an
instrument invented by the late Professor Langley. Chemical rays--for
instance, those radiations which affect the photographic plate--are for
the most part also outside the visible spectrum. They are, however, at
the other end of it, namely, beyond the violet.

Such a scale of radiations may be compared to the keyboard of an
imaginary piano, the sound from only one of whose octaves is audible to
us.

The brightest light we know on the earth is dull compared with the light
of the sun. It would, indeed, look quite dark if held up against it.

It is extremely difficult to arrive at a precise notion of the
temperature of the body of the sun. However, it is far in excess of any
temperature which we can obtain here, even in the most powerful electric
furnace.

A rough idea of the solar heat may be gathered from the calculation that
if the sun's surface were coated all over with a layer of ice 4000 feet
thick, it would melt through this completely in one hour.

The sun cannot be a hot body merely cooling; for the rate at which it is
at present giving off heat could not in such circumstances be kept up,
according to Professor Moulton, for more than 3000 years. Further, it is
not a mere burning mass, like a coal fire, for instance; as in that case
about a thousand years would show a certain drop in temperature. No
perceptible diminution of solar heat having taken place within historic
experience, so far as can be ascertained, we are driven to seek some
more abstruse explanation.

The theory which seems to have received most acceptance is that put
forward by Helmholtz in 1854. His idea was that gravitation produces
continual contraction, or falling in of the outer parts of the sun; and
that this falling in, in its turn, generates enough heat to compensate
for what is being given off. The calculations of Helmholtz showed that a
contraction of about 100 feet a year from the surface towards the centre
would suffice for the purpose. In recent years, however, this estimate
has been extended to about 180 feet. Nevertheless, even with this
increased figure, the shrinkage required is so slight in comparison with
the immense girth of the sun, that it would take a continual
contraction at this rate for about 6000 years, to show even in our
finest telescopes that any change in the size of that body was taking
place at all. Upon this assumption of continuous contraction, a time
should, however, eventually be reached when the sun will have shrunk to
such a degree of solidity, that it will not be able to shrink any
further. Then, the loss of heat not being made up for any longer, the
body of the sun should begin to grow cold. But we need not be distressed
on this account; for it will take some 10,000,000 years, according to
the above theory, before the solar orb becomes too cold to support life
upon our earth.

Since the discovery of radium it has, on the other hand, been suggested,
and not unreasonably, that radio-active matter may possibly play an
important part in keeping up the heat of the sun. But the body of
scientific opinion appears to consider the theory of contraction as a
result of gravitation, which has been outlined above, to be of itself
quite a sound explanation. Indeed, the late Lord Kelvin is said to have
held to the last that it was amply sufficient to account for the
underground heat of the earth, the heat of the sun, and that of all the
stars in the universe.

One great difficulty in forming theories with regard to the sun, is the
fact that the temperature and gravitation there are enormously in excess
of anything we meet with upon our earth. The force of gravity at the
sun's surface is, indeed, about twenty-seven times that at the surface
of our globe.

The earth's atmosphere appears to absorb about one-half of the
radiations which come to us from the sun. This absorptive effect is very
noticeable when the solar orb is low down in our sky, for its light and
heat are then clearly much reduced. Of the light rays, the blue ones are
the most easily absorbed in this way; which explains why the sun looks
red when near the horizon. It has then, of course, to shine through a
much greater thickness of atmosphere than when high up in the heavens.

What astonishes one most about the solar radiation, is the immense
amount of it that is apparently wasted into space in comparison with
what falls directly upon the bodies of the solar system. Only about the
one-hundred-millionth is caught by all the planets together. What
becomes of the rest we cannot tell.

That brilliant white body of the sun, which we see, is enveloped by
several layers of gases and vaporous matter, in the same manner as our
globe is enveloped by its atmosphere (see Fig. 10, p. 131). These are
transparent, just as our atmosphere is transparent; and so we see the
white bright body of the sun right through them.

This white bright portion is called the _Photosphere_. From it comes
most of that light and heat which we see and feel. We do not know what
lies under the photosphere, but, no doubt, the more solid portions of
the sun are there situated. Just above the photosphere, and lying close
upon it, is a veil of smoke-like haze.

Next upon this is what is known as the _Reversing Layer_, which is
between 500 and 1000 miles in thickness. It is cooler than the
underlying photosphere, and is composed of glowing gases. Many of the
elements which go to make up our earth are present in the reversing
layer in the form of vapour.

The _Chromosphere_, of which especial mention has already been made in
dealing with eclipses of the sun, is another layer lying immediately
upon the last one. It is between 5000 and 10,000 miles in thickness.
Like the reversing layer, it is composed of glowing gases, chief among
which is the vapour of hydrogen. The colour of the chromosphere is, in
reality, a brilliant scarlet; but, as we have already said, the
intensely white light of the photosphere shines through it from behind,
and entirely overpowers its redness. The upper portion of the
chromosphere is in violent agitation, like the waves of a stormy sea,
and from it rise those red prominences which, it will be recollected,
are such a notable feature in total solar eclipses.

[Illustration: FIG. 10.--A section through the Sun, showing how the
prominences rise from the chromosphere.]

The _Corona_ lies next in order outside the chromosphere, and is, so
far as we know, the outermost of the accompaniments of the sun. This
halo of pearly-white light is irregular in outline, and fades away into
the surrounding sky. It extends outwards from the sun to several
millions of miles. As has been stated, we can never see the corona
unless, when during a total solar eclipse, the moon has, for the time
being, hidden the brilliant photosphere completely from our view.

The solar spectrum is really composed of three separate spectra
commingled, _i.e._ those of the photosphere, of the reversing layer, and
of the chromosphere respectively.

If, therefore, the photosphere could be entirely removed, or covered up,
we should see only the spectra of those layers which lie upon it. Such a
state of things actually occurs in a total eclipse of the sun. When the
moon's body has crept across the solar disc, and hidden the last piece
of photosphere, the solar spectrum suddenly becomes what is technically
called "reversed,"--the dark lines crossing it changing into bright
lines. This occurs because a strip of those layers which lie immediately
upon the photosphere remains still uncovered. The lower of these layers
has therefore been called the "reversing layer," for want of a better
name. After a second or two this reversed spectrum mostly vanishes, and
an altered spectrum is left to view. Taking into consideration the rate
at which the moon is moving across the face of the sun, and the very
short time during which the spectrum of the reversing layer lasts, the
thickness of that layer is estimated to be not more than a few hundred
miles. In the same way the last of the three spectra--namely, that of
the chromosphere--remains visible for such a time as allows us to
estimate its depth at about ten times that of the reversing layer, or
several thousand miles.

When the chromosphere, in its turn during a total eclipse, has been
covered by the moon, the corona alone is left. This has a distinct
spectrum of its own also; wherein is seen a strange line in the green
portion, which does not tally with that of any element we are acquainted
with upon the earth. This unknown element has received for the time
being the name of "Coronium."

CHAPTER XIII

THE SUN--_continued_

The various parts of the Sun will now be treated of in detail.

I. PHOTOSPHERE.

The photosphere, or "light-sphere," from the Greek [phôs] (_phos_),
which means _light_, is, as we have already said, the innermost portion
of the sun which can be seen. Examined through a good telescope it shows
a finely mottled structure, as of brilliant granules, somewhat like rice
grains, with small dark spaces lying in between them. It has been
supposed that we have here the process of some system of circulation by
which the sun keeps sending forth its radiations. In the bright granules
we perhaps see masses of intensely heated matter, rising from the
interior of the sun. The dark interspaces may represent matter which has
become cooled and darkened through having parted with its heat and
light, and is falling back again into the solar furnace.

The _sun spots_, so familiar to every one nowadays, are dark patches
which are often seen to break out in the photosphere (see Plate V., p.
134). They last during various periods of time; sometimes only for a few
days, sometimes so long as a month or more. A spot is usually composed
of a dark central portion called the _umbra_, and a less dark fringe
around this called the _penumbra_ (see Plate VI., p. 136). The umbra
ordinarily has the appearance of a deep hole in the photosphere; but,
that it is a hole at all, has by no means been definitely proved.

[Illustration: PLATE V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS

From a photograph taken at the Royal Observatory, Greenwich. The
cross-lines seen on the disc are in no way connected with the Sun, but
belong to the telescope through which the photograph was taken.

(Page 134)]

Sun spots are, as a rule, some thousands of miles across. The umbra of
a good-sized spot could indeed engulf at once many bodies the size of
our earth.

Sun spots do not usually appear singly, but in groups. The total area of
a group of this kind may be of immense extent; even so great as to cover
the one-hundredth part of the whole surface of the sun. Very large
spots, when such are present, may be seen without any telescope; either
through a piece of smoked glass, or merely with the naked eye when the
air is misty, or the sun low on the horizon.

The umbra of a spot is not actually dark. It only appears so in contrast
with the brilliant photosphere around.

Spots form, grow to a large size in comparatively short periods of time,
and then quickly disappear. They seem to shrink away as a consequence of
the photosphere closing in upon them.

That the sun is rotating upon an axis, is shown by the continual change
of position of all spots in one constant direction across his disc. The
time in which a spot is carried completely round depends, however, upon
the position which it occupies upon the sun's surface. A spot situated
near the equator of the sun goes round once in about twenty-five days.
The further a spot is situated from this equator, the longer it takes.
About twenty-seven days is the time taken by a spot situated midway
between the equator and the solar poles. Spots occur to the north of
the sun's equator, as well as to the south; though, since regular
observations have been made--that is to say, during the past fifty years
or so--they appear to have broken out a little more frequently in the
southern parts.

From these considerations it will be seen that the sun does not rotate
as the earth does, but that different portions appear to move at
different speeds. Whether in the neighbourhood of the solar poles the
time of rotation exceeds twenty-seven days we are unable to ascertain,
for spots are not seen in those regions. No explanation has yet been
given of this peculiar rotation; and the most we can say on the subject
is that the sun is not by any means a solid body.

_Faculæ_ (Latin, little torches) are brilliant patches which appear here
and there upon the sun's surface, and are in some way associated with
spots. Their displacement, too, across the solar face confirms the
evidence which the spots give us of the sun's rotation.

Our proofs of this rotation are still further strengthened by the
Doppler spectroscopic method of observation alluded to in Chapter XI. As
was then stated, one edge of the sun is thus found to be continually
approaching us, and the other side continually receding from us. The
varying rates of rotation, which the spots and faculæ give us, are duly
confirmed by this method.

[Illustration: PLATE VI. PHOTOGRAPH OF A SUNSPOT

This fine picture was taken by the late M. Janssen. The granular
structure of the Sun's surface is here well represented. (From
_Knowledge_.)

(Page 135)]

The first attempt to bring some regularity into the question of
sunspots was the discovery by Schwabe, in 1852, that they were subject
to a regular variation. As a matter of fact they wax and wane in their
number, and the total area which they cover, in the course of a period,
or cycle, of on an average about 11-1/4 years; being at one part of this
period large and abundant, and at another few and small. This period of
11-1/4 years is known as the sun spot cycle. No explanation has yet been
given of the curious round of change, but the period in question seems
to govern most of the phenomena connected with the sun.

II. REVERSING LAYER.

This is a layer of relatively cool gases lying immediately upon the
photosphere. We never see it directly; and the only proof we have of its
presence is that remarkable reversal of the spectrum already described,
when during an instant or two in a total eclipse, the advancing edge of
the moon, having just hidden the brilliant photosphere, is moving across
the fine strip which the layer then presents edgewise towards us. The
fleeting moments during which this reversed spectrum lasts, informs us
that the layer is comparatively shallow; little more indeed than about
500 miles in depth.

The spectrum of the reversing layer, or "flash spectrum," as it is
sometimes called on account of the instantaneous character with which
the change takes place, was, as we have seen, first noticed by Young in
1870; and has been successfully photographed since then during several
eclipses. The layer itself appears to be in a fairly quiescent state; a
marked contrast to the seething photosphere beneath, and the agitated
chromosphere above.

III. THE CHROMOSPHERE.

The Chromosphere--so called from the Greek [chrôma] (_chroma_), which
signifies _colour_--is a layer of gases lying immediately upon the
preceding one. Its thickness is, however, plainly much the greater of
the two; for whereas the reversing layer is only revealed to us
_indirectly_ by the spectroscope, a portion of the chromosphere may
clearly be _seen_ in a total eclipse in the form of a strip of scarlet
light. The time which the moon's edge takes to traverse it tells us that
it must be about ten times as deep as the reversing layer, namely, from
5000 to 10,000 miles in depth. Its spectrum shows that it is composed
chiefly of hydrogen, calcium and helium, in the state of vapour. Its red
colour is mainly due to glowing hydrogen. The element helium, which it
also contains, has received its appellation from [hêlios] (_helios_),
the Greek name for the sun; because, at the time when it first attracted
attention, there appeared to be no element corresponding to it upon our
earth, and it was consequently imagined to be confined to the sun alone.
Sir William Ramsay, however, discovered it to be also a terrestrial
element in 1895, and since then it has come into much prominence as one
of the products given off by radium.

Taking into consideration the excessive force of gravity on the sun, one
would expect to find the chromosphere and reversing layer growing
gradually thicker in the direction of the photosphere. This, however, is
not the case. Both these layers are strangely enough of the same
densities all through; which makes it suspected that, in these regions,
the force of gravity may be counteracted by some other force or forces,
exerting a powerful pressure outwards from the sun.

IV. THE PROMINENCES.

We have already seen, in dealing with total eclipses, that the exterior
surface of the chromosphere is agitated like a stormy sea, and from it
billows of flame are tossed up to gigantic heights. These flaming jets
are known under the name of prominences, because they were first noticed
in the form of brilliant points projecting from behind the rim of the
moon when the sun was totally eclipsed. Prominences are of two kinds,
_eruptive_ and _quiescent_. The eruptive prominences spurt up directly
from the chromosphere with immense speeds, and change their shape with
great rapidity. Quiescent prominences, on the other hand, have a form
somewhat like trees, and alter their shape but slowly. In the eruptive
prominences glowing masses of gas are shot up to altitudes sometimes as
high as 300,000 miles,[10] with velocities even so great as from 500 to
600 miles a second. It has been noticed that the eruptive prominences
are mostly found in those portions of the sun where spots usually
appear, namely, in the regions near the solar equator. The quiescent
prominences, on the other hand, are confined, as a rule, to the
neighbourhood of the sun's poles.

Prominences were at first never visible except during total eclipses of
the sun. But in the year 1868, as we have already seen, a method of
employing the spectroscope was devised, by means of which they could be
observed and studied at any time, without the necessity of waiting for
an eclipse.

A still further development of the spectroscope, the
_Spectroheliograph_, an instrument invented almost simultaneously by
Professor Hale and the French astronomer, M. Deslandres, permits of
photographs being taken of the sun, with the light emanating from _only
one_ of its glowing gases at a time. For instance, we can thus obtain a
record of what the glowing hydrogen alone is doing on the solar body at
any particular moment. With this instrument it is also possible to
obtain a series of photographs, showing what is taking place upon the
sun at various levels. This is very useful in connection with the study
of the spots; for we are, in consequence, enabled to gather more
evidence on the subject of their actual form than is given us by their
highly foreshortened appearances when observed directly in the
telescope.

V. CORONA. (Latin, _a Crown_.)

This marvellous halo of pearly-white light, which displays itself to our
view only during the total phase of an eclipse of the sun, is by no
means a layer like those other envelopments of the sun of which we have
just been treating. It appears, on the other hand, to be composed of
filmy matter, radiating outwards in every direction, and fading away
gradually into space. Its structure is noted to bear a strong
resemblance to the tails of comets, or the streamers of the aurora
borealis.

Our knowledge concerning the corona has, however, advanced very slowly.
We have not, so far, been as fortunate with regard to it as with regard
to the prominences; and, for all we can gather concerning it, we are
still entirely dependent upon the changes and chances of total solar
eclipses. All attempts, in fact, to apply the spectroscopic method, so
as to observe the corona at leisure in full sunlight in the way in which
the prominences can be observed, have up to the present met with
failure.

The general form under which the corona appears to our eyes varies
markedly at different eclipses. Sometimes its streamers are many, and
radiate all round; at other times they are confined only to the middle
portions of the sun, and are very elongated, with short feathery-looking
wisps adorning the solar poles. It is noticed that this change of shape
varies in close accordance with that 11-1/4 year period during which the
sun spots wax and wane; the many-streamered regular type corresponding
to the time of great sunspot activity, while the irregular type with the
long streamers is present only when the spots are few (see Plate VII.,
p. 142). Streamers have often been noted to issue from those regions of
the sun where active prominences are at the moment in existence; but it
cannot be laid down that this is always the case.

No hypothesis has yet been formulated which will account for the
structure of the corona, or for its variation in shape. The great
difficulty with regard to theorising upon this subject, is the fact
that we see so much of the corona under conditions of marked
foreshortening. Assuming, what indeed seems natural, that the rays of
which it is composed issue in every direction from the solar body, in a
manner which may be roughly imitated by sticking pins all over a ball;
it is plainly impossible to form any definite idea concerning streamers,
which actually may owe most of the shape they present to us, to the
mixing up of multitudes of rays at all kinds of angles to the line of
sight. In a word, we have to try and form an opinion concerning an
arrangement which, broadly speaking, is _spherical_, but which, on
account of its distance, must needs appear to us as absolutely _flat_.

The most known about the composition of the corona is that it is made up
of particles of matter, mingled with a glowing gas. It is an element in
the composition of this gas which, as has been stated, is not found to
tally with any known terrestrial element, and has, therefore, received
the name of coronium for want of a better designation.

One definite conclusion appears to be reached with regard to the corona,
_i.e._ that the matter of which it is composed, must be exceedingly
rarefied; as it is not found, for instance, to retard appreciably the
speed of comets, on occasions when these bodies pass very close to the
sun. A calculation has indeed been made which would tend to show that
the particles composing the coronal matter, are separated from each
other by a distance of perhaps between two and three yards! The density
of the corona is found not to increase inwards towards the sun. This is
what has already been noted with regard to the layers lying beneath it.
Powerful forces, acting in opposition to gravity, must hold sway here
also.

[Illustration: (A.) THE TOTAL ECLIPSE OF THE SUN OF DECEMBER 22ND, 1870

Drawn by Mr. W.H. Wesley from a photograph taken at Syracuse by Mr.
Brothers. This is the type of corona seen at the time of _greatest_
sunspot activity. The coronas of 1882 (Plate I., p. 96) and of 1905
(Frontispiece) are of the same type.

(B.) THE TOTAL ECLIPSE OF THE SUN OF MAY 28TH, 1900

Drawn by Mr. W.H. Wesley from photographs taken by Mr. E.W. Maunder.
This is the type of corona seen when the sunspots are _least_ active.
Compare the "Ring with Wings," Fig. 7, p. 87.

PLATE VII. FORMS OF THE SOLAR CORONA AT THE EPOCHS OF SUNSPOT MAXIMUM
AND SUNSPOT MINIMUM, RESPECTIVELY

(Page 141)]

The 11-1/4 year period, during which the sun spots vary in number and
size, appears to govern the activities of the sun much in the same way
that our year does the changing seasonal conditions of our earth. Not
only, as we have seen, does the corona vary its shape in accordance with
the said period, but the activity of the prominences, and of the faculæ,
follow suit. Further, this constant round of ebb and flow is not
confined to the sun itself, but, strangely enough, affects the earth
also. The displays of the aurora borealis, which we experience here,
coincide closely with it, as does also the varying state of the earth's
magnetism. The connection may be still better appreciated when a great
spot, or group of spots, has made its appearance upon the sun. It has,
for example, often been noted that when the solar rotation carries a
spot, or group of spots, across the middle of the visible surface of the
sun, our magnetic and electrical arrangements are disturbed for the time
being. The magnetic needles in our observatories are, for instance, seen
to oscillate violently, telegraphic communication is for a while upset,
and magnificent displays of the aurora borealis illumine our night
skies. Mr. E.W. Maunder, of Greenwich Observatory, who has made a very
careful investigation of this subject, suspects that, when elongated
coronal streamers are whirled round in our direction by the solar
rotation, powerful magnetic impulses may be projected upon us at the
moments when such streamers are pointing towards the earth.

Some interesting investigations with regard to sunspots have recently
been published by Mrs. E.W. Maunder. In an able paper, communicated to
the Royal Astronomical Society on May 10, 1907, she reviews the
Greenwich Observatory statistics dealing with the number and extent of
the spots which have appeared during the period from 1889 to 1901--a
whole sunspot cycle. From a detailed study of the dates in question, she
finds that the number of those spots which are formed on the side of the
sun turned away from us, and die out upon the side turned towards us, is
much greater than the number of those which are formed on the side
turned towards us and die out upon the side turned away. It used, for
instance, to be considered that the influence of a planet might
_produce_ sunspots; but these investigations make it look rather as if
some influence on the part of the earth tends, on the contrary, to
_extinguish_ them. Mrs. Maunder, so far, prefers to call the influence
thus traced an _apparent_ influence only, for, as she very fairly points
out, it seems difficult to attribute a real influence in this matter to
the earth, which is so small a thing in comparison not only with the
sun, but even with many individual spots.

The above investigation was to a certain degree anticipated by Mr. Henry
Corder in 1895; but Mrs. Maunder's researches cover a much longer
period, and the conclusions deduced are of a wider and more defined
nature.

With regard to its chemical composition, the spectroscope shows us that
thirty-nine of the elements which are found upon our earth are also to
be found in the sun. Of these the best known are hydrogen, oxygen,
helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and
lead. Some elements of the metallic order have, however, not been found
there, as, for instance, gold and mercury; while a few of the other
class of element, such as nitrogen, chlorine, and sulphur, are also
absent. It must not, indeed, be concluded that the elements apparently
missing do not exist at all in the solar body. Gold and mercury have, in
consequence of their great atomic weight, perhaps sunk away into the
centre. Again, the fact that we cannot find traces of certain other
elements, is no real proof of their entire absence. Some of them may,
for instance, be resolved into even simpler forms, under the unusual
conditions which exist in the sun; and so we are unable to trace them
with the spectroscope, the experience of which rests on laboratory
experiments conducted, at best, in conditions which obtain upon the
earth.

[10] On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford
University, noted a prominence which rose to a height of 324,600 miles.

CHAPTER XIV

THE INFERIOR PLANETS

Starting from the centre of the solar system, the first body we meet
with is the planet Mercury. It circulates at an average distance from
the sun of about thirty-six millions of miles. The next body to it is
the planet Venus, at about sixty-seven millions of miles, namely, about
double the distance of Mercury from the sun. Since our earth comes next
again, astronomers call those planets which circulate within its orbit,
_i.e._ Mercury and Venus, the Inferior Planets, while those which
circulate outside it they call the Superior Planets.[11]

In studying the inferior planets, the circumstances in which we make our
observations are so very similar with regard to each, that it is best to
take them together. Let us begin by considering the various positions of
an inferior planet, as seen from the earth, during the course of its
journeys round the sun. When furthest from us it is at the other side of
the sun, and cannot then be seen owing to the blaze of light. As it
continues its journey it passes to the left of the sun, and is then
sufficiently away from the glare to be plainly seen. It next draws in
again towards the sun, and is once more lost to view in the blaze at
the time of its passing nearest to us. Then it gradually comes out to
view on the right hand, separates from the sun up to a certain distance
as before, and again recedes beyond the sun, and is for the time being
once more lost to view.

To these various positions technical names are given. When the inferior
planet is on the far side of the sun from us, it is said to be in
_Superior Conjunction_. When it has drawn as far as it can to the left
hand, and is then as east as possible of the sun, it is said to be at
its _Greatest Eastern Elongation_. Again, when it is passing nearest to
us, it is said to be in _Inferior Conjunction_; and, finally, when it
has drawn as far as it can to the right hand, it is spoken of as being
at its _Greatest Western Elongation_ (see Fig. 11, p. 148).

The continual variation in the distance of an interior planet from us,
during its revolution around the sun, will of course be productive of
great alterations in its apparent size. At superior conjunction it
ought, being then farthest away, to show the smallest disc; while at
inferior conjunction, being the nearest, it should look much larger.
When at greatest elongation, whether eastern or western, it should
naturally present an appearance midway in size between the two.

[Illustration: Various positions, and illumination by the Sun, of an
Inferior Planet in the course of its orbit.

Corresponding views of the same situations of an Inferior Planet as seen
from the Earth, showing consequent phases and alterations in apparent
size.

FIG. 11.--Orbit and Phases of an Inferior Planet.]

From the above considerations one would be inclined to assume that the
best time for studying the surface of an interior planet with the
telescope is when it is at inferior conjunction, or, nearest to us. But
that this is not the case will at once appear if we consider that the
sunlight is then falling upon the side away from us, leaving the side
which is towards us unillumined. In superior conjunction, on the other
hand, the light falls full upon the side of the planet facing us; but
the disc is then so small-looking, and our view besides is so dazzled by
the proximity of the sun, that observations are of little avail. In the
elongations, however, the sunlight comes from the side, and so we see
one half of the planet lit up; the right half at eastern elongation, and
the left half at western elongation. Piecing together the results given
us at these more favourable views, we are enabled, bit by bit, to gather
some small knowledge concerning the surface of an inferior planet.

From these considerations it will be seen at once that the inferior
planets show various phases comparable to the waxing and waning of our
moon in its monthly round. Superior conjunction is, in fact, similar to
full moon, and inferior conjunction to new moon; while the eastern and
western elongations may be compared respectively to the moon's first and
last quarters. It will be recollected how, when these phases were first
seen by the early telescopic observers, the Copernican theory was felt
to be immensely strengthened; for it had been pointed out that if this
system were the correct one, the planets Venus and Mercury, were it
possible to see them more distinctly, would of necessity present phases
like these when viewed from the earth. It should here be noted that the
telescope was not invented until nearly seventy years after the death of
Copernicus.

The apparent swing of an inferior planet from side to side of the sun,
at one time on the east side, then passing into and lost in the sun's
rays to appear once more on the west side, is the explanation of what is
meant when we speak of an _evening_ or a _morning star_. An inferior
planet is called an evening star when it is at its eastern elongation,
that is to say, on the left-hand of the sun; for, being then on the
eastern side, it will set after the sun sets, as both sink in their turn
below the western horizon at the close of day. Similarly, when such a
planet is at its western elongation, that is to say, to the right-hand
of the sun, it will go in advance of him, and so will rise above the
eastern horizon before the sun rises, receiving therefore the
designation of morning star. In very early times, however, before any
definite ideas had been come to with regard to the celestial motions, it
was generally believed that the morning and evening stars were quite
distinct bodies. Thus Venus, when a morning star, was known to the
ancients under the name of Phosphorus, or Lucifer; whereas they called
it Hesperus when it was an evening star.

Since an inferior planet circulates between us and the sun, one would be
inclined to expect that such a body, each time it passed on the side
nearest to the earth, should be seen as a black spot against the bright
solar disc. Now this would most certainly be the case were the orbit of
an inferior planet in the same plane with the orbit of the earth. But we
have already seen how the orbits in the solar system, whether those of
planets or of satellites, are by no means in the one plane; and that it
is for this very reason that the moon is able to pass time after time in
the direction of the sun, at the epoch known as new moon, and yet not to
eclipse him save after the lapse of several such passages. Transits,
then, as the passages of an inferior planet across the sun's disc are
called, take place, for the same reason, only after certain regular
lapses of time; and, as regards the circumstances of their occurrence,
are on a par with eclipses of the sun. The latter, however, happen much
more frequently, because the moon passes in the neighbourhood of the
sun, roughly speaking, once a month, whereas Venus comes to each
inferior conjunction at intervals so long apart as a year and a half,
and Mercury only about every four months. From this it will be further
gathered that transits of Mercury take place much oftener than transits
of Venus.

Until recent years _Transits of Venus_ were phenomena of great
importance to astronomers, for they furnished the best means then
available of calculating the distance of the sun from the earth. This
was arrived at through comparing the amount of apparent displacement in
the planet's path across the solar disc, when the transit was observed
from widely separated stations on the earth's surface. The last transit
of Venus took place in 1882, and there will not be another until the
year 2004.

_Transits of Mercury_, on the other hand, are not of much scientific
importance. They are of no interest as a popular spectacle; for the
dimensions of the planet are so small, that it can be seen only with the
aid of a telescope when it is in the act of crossing the sun's disc. The
last transit of Mercury took place on November 14, 1907, and there will
be another on November 6, 1914.

The first person known to have observed a transit of an inferior planet
was the celebrated French philosopher, Gassendi. This was the transit of
Mercury which took place on the 7th of December 1631.

The first time a transit of Venus was ever seen, so far as is known, was
on the 24th of November 1639. The observer was a certain Jeremiah
Horrox, curate of Hoole, near Preston, in Lancashire. The transit in
question commenced shortly before sunset, and his observations in
consequence were limited to only about half-an-hour. Horrox happened to
have a great friend, one William Crabtree, of Manchester, whom he had
advised by letter to be on the look out for the phenomenon. The weather
in Crabtree's neighbourhood was cloudy, with the result that he only got
a view of the transit for about ten minutes before the sun set.

That this transit was observed at all is due entirely to the remarkable
ability of Horrox. According to the calculations of the great Kepler, no
transit could take place that year (1639), as the planet would just pass
clear of the lower edge of the sun. Horrox, however, not being satisfied
with this, worked the question out for himself, and came to the
conclusion that the planet would _actually_ traverse the lower portion
of the sun's disc. The event, as we have seen, proved him to be quite in
the right. Horrox is said to have been a veritable prodigy of
astronomical skill; and had he lived longer would, no doubt, have become
very famous. Unfortunately he died about two years after his celebrated
transit, in his _twenty-second_ year only, according to the accounts.
His friend Crabtree, who was then also a young man, is said to have been
killed at the battle of Naseby in 1645.

There is an interesting phenomenon in connection with transits which is
known as the "Black Drop." When an inferior planet has just made its way
on to the face of the sun, it is usually seen to remain for a short time
as if attached to the sun's edge by what looks like a dark ligament (see
Fig. 12, p. 153). This gives to the planet for the time being an
elongated appearance, something like that of a pear; but when the
ligament, which all the while keeps getting thinner and thinner, has at
last broken, the black body of the planet is seen to stand out round
against the solar disc.

[Illustration: FIG. 12.--The "Black Drop."]

This appearance may be roughly compared to the manner in which a drop of
liquid (or, preferably, of some glutinous substance) tends for a while
to adhere to an object from which it is falling.

When the planet is in turn making its way off the face of the sun, the
ligament is again seen to form and to attach it to the sun's edge before
its due time.

The phenomenon of the black drop, or ligament, is entirely an illusion,
and, broadly speaking, of an optical origin. Something very similar will
be noticed if one brings one's thumb and forefinger _slowly_ together
against a very bright background.

This peculiar phenomenon has proved one of the greatest drawbacks to the
proper observation of transits, for it is quite impossible to note the
exact instant of the planet's entrance upon and departure from the solar
disc in conditions such as these.

The black drop seems to bear a family resemblance, so to speak, to the
phenomenon of Baily's beads. In the latter instance the lunar peaks, as
they approach the sun's edge, appear to lengthen out in a similar manner
and bridge the intervening space before their time, thus giving
prominence to an effect which otherwise should scarcely be noticeable.

The last transit of Mercury, which, as has been already stated, took
place on November 14, 1907, was not successfully observed by astronomers
in England, on account of the cloudiness of the weather. In France,
however, Professor Moye, of Montpellier, saw it under good conditions,
and mentions that the black drop remained very conspicuous for fully a
minute. The transit was also observed in the United States, the reports
from which speak of the black drop as very "troublesome."

Before leaving the subject of transits it should be mentioned that it
was in the capacity of commander of an expedition to Otaheite, in the
Pacific, to observe the transit of Venus of June 3, 1769, that Captain
Cook embarked upon the first of his celebrated voyages.

In studying the surfaces of Venus and Mercury with the telescope,
observers are, needless to say, very much hindered by the proximity of
the sun. Venus, when at the greatest elongations, certainly draws some
distance out of the glare; but her surface is, even then, so dazzlingly
bright, that the markings upon it are difficult to see. Mercury, on the
other hand, is much duller in contrast, but the disc it shows in the
telescope is exceedingly small; and, in addition, when that planet is
left above the horizon for a short time after sunset, as necessarily
happens after certain intervals, the mists near the earth's surface
render observation of it very difficult.

Until about twenty-five years ago, it was generally believed that both
these planets rotated on their axes in about twenty-four hours, a
notion, no doubt, originally founded upon an unconscious desire to bring
them into some conformity with our earth. But Schiaparelli, observing in
Italy, and Percival Lowell, in the clear skies of Arizona and Mexico,
have lately come to the conclusion that both planets rotate upon their
axes in the same time as they revolve in their orbits,[12] the result
being that they turn one face ever towards the sun in the same manner
that the moon turns one face ever towards the earth--a curious state of
things, which will be dealt with more fully when we come to treat of our
satellite.

The marked difference in the brightness between the two planets has
already been alluded to. The surface of Venus is, indeed, about five
times as bright as that of Mercury. The actual brightness of Mercury is
about equivalent to that of our moon, and astronomers are, therefore,
inclined to think that it may resemble her in having a very rugged
surface and practically no atmosphere. This probable lack of atmosphere
is further corroborated by two circumstances. One of these is that when
Mercury is just about to transit the face of the sun, no ring of
diffused light is seen to encircle its disc as would be the case if it
possessed an atmosphere. Such a lack of atmosphere is, indeed, only to
be expected from what is known as the _Kinetic Theory of Gases_.
According to this theory, which is based upon the behaviour of various
kinds of gas, it is found that these elements tend to escape into space
from the surface of bodies whose force of gravitation is weak. Hydrogen
gas, for example, tends to fly away from our earth, as any one may see
for himself when a balloon rises into the air. The gravitation of the
earth seems, however, powerful enough to hold down other gases, as, for
instance, those of which the air is chiefly composed, namely, oxygen and
nitrogen. In due accordance with the Kinetic theory, we find the moon
and Mercury, which are much about the same size, destitute of
atmospheres. Mars, too, whose diameter is only about double that of the
moon, has very little atmosphere. We find, on the other hand, that
Venus, which is about the same size as our earth, clearly possesses an
atmosphere, as just before the planet is in transit across the sun, the
outline of its dark body is seen to be surrounded by a bright ring of
light.

The results of telescopic observation show that more markings are
visible on Mercury than on Venus. The intense brilliancy of Venus is,
indeed, about the same as that of our white clouds when the sun is
shining directly upon them. It has, therefore, been supposed that the
planet is thickly enveloped in cloud, and that we do not ever see any
part of its surface, except perchance the summit of some lofty mountain
projecting through the fleecy mass.

With regard to the great brilliancy of Venus, it may be mentioned that
she has frequently been seen in England, with the naked eye in full
sunshine, when at the time of her greatest brightness. The writer has
seen her thus at noonday. Needless to say, the sky at the moment was
intensely blue and clear.

The orbit of Mercury is very oval, and much more so than that of any
other planet. The consequence is that, when Mercury is nearest to the
sun, the heat which it receives is twice as great as when it is farthest
away. The orbit of Venus, on the other hand, is in marked contrast with
that of Mercury, and is, besides, more nearly of a circular shape than
that of any of the other planets. Venus, therefore, always keeps about
the same distance from the sun, and so the heat which she receives
during the course of her year can only be subject to very slight
variations.

[11] In employing the terms Inferior and Superior the writer bows to
astronomical custom, though he cannot help feeling that, in the
circumstances, Interior and Exterior would be much more appropriate.

[12] This question is, however, uncertain, for some very recent
spectroscopic observations of Venus seem to show a rotation period of
about twenty-four hours.

CHAPTER XV

THE EARTH

We have already seen (in Chapter I.) how, in very early times, men
naturally enough considered the earth to be a flat plane extending to a
very great distance in every direction; but that, as years went on,
certain of the Greek philosophers suspected it to be a sphere. One or
two of the latter are, indeed, said to have further believed in its
rotation about an axis, and even in its revolution around the sun; but,
as the ideas in question were founded upon fancy, rather than upon any
direct evidence, they did not generally attract attention. The small
effect, therefore, which these theories had upon astronomy, may well be
gathered from the fact that in the Ptolemaic system the earth was
considered as fixed and at the centre of things; and this belief, as we
have seen, continued unaltered down to the days of Copernicus. It was,
indeed, quite impossible to be certain of the real shape of the earth or
the reality of its motions until knowledge became more extended and
scientific instruments much greater in precision.

We will now consider in detail a few of the more obvious arguments which
can be put forward to show that our earth is a sphere.

If, for instance, the earth were a plane surface, a ship sailing away
from us over the sea would appear to grow smaller and smaller as it
receded into the distance, becoming eventually a tiny speck, and fading
gradually from our view. This, however, is not at all what actually
takes place. As we watch a vessel receding, its hull appears bit by bit
to slip gently down over the horizon, leaving the masts alone visible.
Then, in their turn, the masts are seen to slip down in the same manner,
until eventually every trace of the vessel is gone. On the other hand,
when a ship comes into view, the masts are the first portions to appear.
They gradually rise up from below the horizon, and the hull follows in
its turn, until the whole vessel is visible. Again, when one is upon a
ship at sea, a set of masts will often be seen sticking up alone above
the horizon, and these may shorten and gradually disappear from view
without the body of the ship to which they belong becoming visible at
all. Since one knows from experience that there is no _edge_ at the
horizon over which a vessel can drop down, the appearance which we have
been describing can only be explained by supposing that the surface of
the earth is always curving gradually in every direction.

The distance at which what is known as the _horizon_ lies away from us
depends entirely upon the height above the earth's surface where we
happen at the moment to be. A ship which has appeared to sink below the
horizon for a person standing on the beach, will be found to come back
again into view if he at once ascends a high hill. Experiment shows that
the horizon line lies at about three miles away for a person standing at
the water's edge. The curving of the earth's surface is found, indeed,
to be at the rate of eight inches in every mile. Now it can be
ascertained, by calculation, that a body curving at this rate in every
direction must be a globe about 8000 miles in diameter.

Again, the fact that, if not stopped by such insuperable obstacles as
the polar ice and snow, those who travel continually in any one
direction upon the earth's surface always find themselves back again at
the regions from which they originally set out, is additional ground for
concluding that the earth is a globe.

We can find still further evidence. For instance, in an eclipse of the
moon the earth's shadow, when seen creeping across the moon's face, is
noted to be _always_ circular in shape. One cannot imagine how such a
thing could take place unless the earth were a sphere.

Also, it is found from observation that the sun, the planets, and the
satellites are, all of them, round. This roundness cannot be the
roundness of a flat plate, for instance, for then the objects in
question would sometimes present their thin sides to our view. It
happens, also, that upon the discs which these bodies show, we see
certain markings shifting along continually in one direction, to
disappear at one side and to reappear again at the other. Such bodies
must, indeed, be spheres in rotation.

The crescent and other phases, shown by the moon and the inferior
planets, should further impress the truth of the matter upon us, as such
appearances can only be caused by the sunlight falling from various
directions upon the surfaces of spherical bodies.

Another proof, perhaps indeed the weightiest of all, is the continuous
manner in which the stars overhead give place to others as one travels
about the surface of the earth. When in northern regions the Pole Star
and its neighbours--the stars composing the Plough, for instance--are
over our heads. As one journeys south these gradually sink towards the
northern horizon, while other stars take their place, and yet others are
uncovered to view from the south. The regularity with which these
changes occur shows that every point on the earth's surface faces a
different direction of the sky, and such an arrangement would only be
possible if the earth were a sphere. The celebrated Greek philosopher,
Aristotle, is known to have believed in the globular shape of the earth,
and it was by this very argument that he had convinced himself that it
was so.

The idea of the sphericity of the earth does not appear, however, to
have been generally accepted until the voyages of the great navigators
showed that it could be sailed round.

The next point we have to consider is the rotation of the earth about
its axis. From the earliest times men noticed that the sky and
everything in it appeared to revolve around the earth in one fixed
direction, namely, towards what is called the West, and that it made one
complete revolution in the period of time which we know as twenty-four
hours. The stars were seen to come up, one after another, from below the
eastern horizon, to mount the sky, and then to sink in turn below the
western horizon. The sun was seen to perform exactly the same journey,
and the moon, too, whenever she was visible. One or two of the ancient
Greek philosophers perceived that this might be explained, either by a
movement of the entire heavens around the earth, or by a turning motion
on the part of the earth itself. Of these diverse explanations, that
which supposed an actual movement of the heavens appealed to them the
most, for they could hardly conceive that the earth should continually
rotate and men not be aware of its movement. The question may be
compared to what we experience when borne along in a railway train. We
see the telegraph posts and the trees and buildings near the line fly
past us one after another in the contrary direction. Either these must
be moving, or we must be moving; and as we happen to _know_ that it is,
indeed, we who are moving, there can be no question therefore about the
matter. But it would not be at all so easy to be sure of this movement
were one unable to see the objects close at hand displacing themselves.
For instance, if one is shut up in a railway carriage at night with the
blinds down, there is really nothing to show that one is moving, except
the jolting of the train. And even then it is hard to be sure in which
direction one is actually travelling.

The way we are situated upon the earth is therefore as follows. There
are no other bodies sufficiently near to be seen flying past us in turn;
our earth spins without a jolt; we and all things around us, including
the atmosphere itself, are borne along together with precisely the same
impetus, just as all the objects scattered about a railway carriage
share in the forward movement of the train. Such being the case, what
wonder that we are unconscious of the earth's rotation, of which we
should know nothing at all, were it not for that slow displacement of
the distant objects in the heavens, as we are borne past them in turn.

If the night sky be watched, it will be soon found that its apparent
turning movement seems to take place around a certain point, which
appears as if fixed. This point is known as the north pole of the
heavens; and a rather bright star, which is situated very close to this
hub of movement, is in consequence called the Pole Star. For the
dwellers in southern latitudes there is also a point in their sky which
appears to remain similarly fixed, and this is known as the south pole
of the heavens. Since, however, the heavens do not turn round at all,
but the earth does, it will easily be seen that these apparently
stationary regions in the sky are really the points towards which the
axis of the earth is directed. The positions on the earth's surface
itself, known as the North and South Poles, are merely the places where
the earth's axis, if there were actually such a thing, would be expected
to jut out. The north pole of the earth will thus be situated exactly
beneath the north pole of the heavens, and the south pole of the earth
exactly beneath the south pole of the heavens.

We have seen that the earth rotates upon its imaginary axis once in
about every twenty-four hours. This means that everything upon the
surface of the earth is carried round once during that time. The
measurement around the earth's equator is about 24,000 miles; and,
therefore, an object situated at the equator must be carried round
through a distance of about 24,000 miles in each twenty-four hours.
Everything at the equator is thus moving along at the rapid rate of
about 1000 miles an hour, or between sixteen and seventeen times as
fast as an express train. If, however, one were to take measurements
around the earth parallel to the equator, one would find these
measurements becoming less and less, according as the poles were
approached. It is plain, therefore, that the speed with which any point
moves, in consequence of the earth's rotation, will be greatest at the
equator, and less and less in the direction of the poles; while at the
poles themselves there will be practically no movement, and objects
there situated will merely turn round.

The considerations above set forth, with regard to the different speeds
at which different portions of a rotating globe will necessarily be
moving, is the foundation of an interesting experiment, which gives us
further evidence of the rotation of our earth. The measurement around
the earth at any distance below the surface, say, for instance, at the
depth of a mile, will clearly be less than a similar measurement at the
surface itself. The speed of a point at the bottom of a mine, which
results from the actual rotation of the earth, must therefore be less
than the speed of a point at the surface overhead. This can be
definitely proved by dropping a heavy object down a mine shaft. The
object, which starts with the greater speed of the surface, will, when
it reaches the bottom of the mine, be found, as might be indeed
expected, to be a little ahead (_i.e._ to the east) of the point which
originally lay exactly underneath it. The distance by which the object
gains upon this point is, however, very small. In our latitudes it
amounts to about an inch in a fall of 500 feet.

The great speed at which, as we have seen, the equatorial regions of
the earth are moving, should result in giving to the matter there
situated a certain tendency to fly outwards. Sir Isaac Newton was the
first to appreciate this point, and he concluded from it that the earth
must be _bulged_ a little all round the equator. This is, indeed, found
to be the case, the diameter at the equator being nearly twenty-seven
miles greater than it is from pole to pole. The reader will, no doubt,
be here reminded of the familiar comparison in geographies between the
shape of the earth and that of an orange.

In this connection it is interesting to consider that, were the earth to
rotate seventeen times as fast as it does (_i.e._ in one hour
twenty-five minutes, instead of twenty-four hours), bodies at the
equator would have such a strong tendency to fly outwards that the force
of terrestrial gravity acting upon them would just be counterpoised, and
they would virtually have _no weight_. And, further, were the earth to
rotate a little faster still, objects lying loose upon its surface would
be shot off into space.

The earth is, therefore, what is technically known as an _oblate
spheroid_; that is, a body of spherical shape flattened at the poles. It
follows of course from this, that objects at the polar regions are
slightly nearer to the earth's centre than objects at the equatorial
regions. We have already seen that gravitation acts from the central
parts of a body, and that its force is greater the nearer are those
central parts. The result of this upon our earth will plainly be that
objects in the polar regions will be pulled with a slightly stronger
pull, and will therefore _weigh_ a trifle more than objects in the
equatorial regions. This is, indeed, found by actual experiment to be
the case. As an example of the difference in question, Professor Young,
in his _Manual of Astronomy_, points out that a man who weighs 190
pounds at the equator would weigh 191 at the pole. In such an experiment
the weighing would, however, have to be made with a _spring balance_,
and _not with scales_; for, in the latter case, the "weights" used would
alter in their weight in exactly the same degree as the objects to be
weighed.

It used to be thought that the earth was composed of a relatively thin
crust, with a molten interior. Scientific men now believe, on the other
hand, that such a condition cannot after all prevail, and that the earth
must be more or less solid all through, except perhaps in certain
isolated places where collections of molten matter may exist.

The _atmosphere_, or air which we breathe, is in the form of a layer of
limited depth which closely envelops the earth. Actually, it is a
mixture of several gases, the most important being nitrogen and oxygen,
which between them practically make up the air, for the proportion of
the other gases, the chief of which is carbonic acid gas, is exceedingly
small.

It is hard to picture our earth, as we know it, without this atmosphere.
Deprived of it, men at once would die; but even if they could be made to
go on living without it by any miraculous means, they would be like unto
deaf beings, for they would never hear any sound. What we call _sounds_
are merely vibrations set up in the air, which travel along and strike
upon the drum of the ear.

The atmosphere is densest near the surface of the earth, and becomes
less and less dense away from it, as a result of diminishing pressure of
air from above. The greater portion of it is accumulated within four or
five miles of the earth's surface.

It is impossible to determine exactly at what distance from the earth's
surface the air ceases altogether, for it grows continually more and
more rarefied. There are, however, two distinct methods of ascertaining
the distance beyond which it can be said practically not to exist. One
of these methods we get from twilight. Twilight is, in fact, merely
light reflected to us from those upper regions of the air, which still
continue to be illuminated by the sun after it has disappeared from our
view below the horizon. The time during which twilight lasts, shows us
that the atmosphere must be at least fifty miles high.

But the most satisfactory method of ascertaining the height to which the
atmosphere extends is from the observation of meteors. It is found that
these bodies become ignited, by the friction of passing into the
atmosphere, at a height of about 100 miles above the surface of the
earth. We thus gather that the atmosphere has a certain degree of
density even at this height. It may, indeed, extend as far as about 150
miles.

The layer of atmosphere surrounding our earth acts somewhat in the
manner of the glass covering of a greenhouse, bottling in the sun's
rays, and thus storing up their warmth for our benefit. Were this not
so, the heat which we get from the sun would, after falling upon the
earth, be quickly radiated again into space.

It is owing to the unsteadiness of the air that stars are seen to
twinkle. A night when this takes place, though it may please the average
person, is worse than useless to the astronomer, for the unsteadiness is
greatly magnified in the telescope. This twinkling is, no doubt, in a
great measure responsible for the conventional "points" with which Art
has elected to embellish stars, and which, of course, have no existence
in fact.

The phenomena of _Refraction_,[13] namely, that bending which rays of
light undergo, when passing _slant-wise_ from a rare into a dense
transparent medium, are very marked with regard to the atmosphere. The
denser the medium into which such rays pass, the greater is this bending
found to be. Since the layer of air around us becomes denser and denser
towards the surface of the earth, it will readily be granted that the
rays of light reaching our eyes from a celestial object, will suffer the
greater bending the lower the object happens to be in the sky. Celestial
objects, unless situated directly overhead, are thus not seen in their
true places, and when nearest to the horizon are most out of place. The
bending alluded to is upwards. Thus the sun and the moon, for instance,
when we see them resting upon the horizon, are actually _entirely_
beneath it.

When the sun, too, is sinking towards the horizon, the lower edge of its
disc will, for the above reason, look somewhat more raised than the
upper. The result is a certain appearance of flattening; which may
plainly be seen by any one who watches the orb at setting.

In observations to determine the exact positions of celestial objects
correction has to be made for the effects of refraction, according to
the apparent elevation of these objects in the sky. Such effects are
least when the objects in question are directly overhead, for then the
rays of light, coming from them to the eye, enter the atmosphere
perpendicularly, and not at any slant.

A very curious effect, due to refraction, has occasionally been observed
during a total eclipse of the moon. To produce an eclipse of this kind,
_the earth must, of course, lie directly between the sun and the moon_.
Therefore, when we see the shadow creeping over the moon's surface, the
sun should actually be well below the horizon. But when a lunar eclipse
happens to come on just about sunset, the sun, although really sunk
below the horizon, appears still above it through refraction, and the
eclipsed moon, situated, of course, exactly opposite to it in the sky,
is also lifted up above the horizon by the same cause. Pliny, writing in
the first century of the Christian era, describes an eclipse of this
kind, and refers to it as a "prodigy." The phenomenon is known as a
"horizontal eclipse." It was, no doubt, partly owing to it that the
ancients took so long to decide that an eclipse of the moon was really
caused by the shadow cast by the earth. Plutarch, indeed, remarks that
it was easy enough to understand that a solar eclipse was caused by the
interposition of the moon, but that one could not imagine by the
interposition _of what body_ the moon itself could be eclipsed.

In that apparent movement of the heavens about the earth, which men now
know to be caused by the mere rotation of the earth itself, a slight
change is observed to be continually taking place. The stars, indeed,
are always found to be gradually drawing westward, _i.e._ towards the
sun, and losing themselves one after the other in the blaze of his
light, only to reappear, however, on the other side of him after a
certain lapse of time. This is equivalent to saying that the sun itself
seems always creeping slowly _eastward_ in the heaven. The rate at which
this appears to take place is such that the sun finds itself back again
to its original position, with regard to the starry background, at the
end of a year's time. In other words, the sun seems to make a complete
tour of the heavens in the course of a year. Here, however, we have
another illusion, just as the daily movement of the sky around the earth
was an illusion. The truth indeed is, that this apparent movement of the
sun eastward among the stars during a year, arises merely from a
_continuous displacement of his position_ caused by an actual motion of
the earth itself around him in that very time. In a word, it is the
earth which really moves around the sun, and not the sun around the
earth.

The stress laid upon this fundamental point by Copernicus, marks the
separation of the modern from the ancient view. Not that Copernicus,
indeed, had obtained any real proof that the earth is merely a planet
revolving around the sun; but it seemed to his profound intellect that a
movement of this kind on the part of our globe was the more likely
explanation of the celestial riddle. The idea was not new; for, as we
have already seen, certain of the ancient Greeks (Aristarchus of Samos,
for example) had held such a view; but their notions on the subject were
very fanciful, and unsupported by any good argument.

What Copernicus, however, really seems to have done was to _insist_ upon
the idea that the sun occupied the _centre_, as being more consonant
with common sense. No doubt, he was led to take up this position by the
fact that the sun appeared entirely of a different character from the
other members of the system. The one body in the scheme, which performed
the important function of dispenser of light and heat, would indeed be
more likely to occupy a position apart from the rest; and what position
more appropriate for its purposes than the centre!

But here Copernicus only partially solved the difficult question. He
unfortunately still clung to an ancient belief, which as yet remained
unquestioned; _i.e._ the great virtue, one might almost say, the
_divineness_, of circular motion. The ancients had been hag-ridden, so
to speak, by the circle; and it appeared to them that such a perfectly
formed curve was alone fitted for the celestial motions. Ptolemy
employed it throughout his system. According to him the "planets" (which
included, under the ancient view, both the sun and the moon), moved
around the earth in circles; but, as their changing positions in the sky
could not be altogether accounted for in this way, it was further
supposed that they performed additional circular movements, around
peculiarly placed centres, during the course of their orbital
revolutions. Thus the Ptolemaic system grew to be extremely
complicated; for astronomers did not hesitate to add new circular
movements whenever the celestial positions calculated for the planets
were found not to tally with the positions observed. In this manner,
indeed, they succeeded in doctoring the theory, so that it fairly
satisfied the observations made with the rough instruments of
pre-telescopic times.

Although Copernicus performed the immense service to astronomy of boldly
directing general attention to the central position of the sun, he
unfortunately took over for the new scheme the circular machinery of the
Ptolemaic system. It therefore remained for the famous Kepler, who lived
about a century after him, to find the complete solution. Just as
Copernicus, for instance, had broken free from tradition with regard to
the place of the sun; so did Kepler, in turn, break free from the spell
of circular motion, and thus set the coping-stone to the new
astronomical edifice. This astronomer showed, in fact, that if the paths
of the planets around the sun, and of the moon around the earth, were
not circles, but _ellipses_, the movements of these bodies about the sky
could be correctly accounted for. The extreme simplicity of such an
arrangement was far more acceptable than the bewildering intricacy of
movement required by the Ptolemaic theory. The Copernican system, as
amended by Kepler, therefore carried the day; and was further
strengthened, as we have already seen, by the telescopic observations of
Galileo and the researches of Newton into the effects of gravitation.

And here a word on the circle, now fallen from its high estate. The
ancients were in error in supposing that it stood entirely apart--the
curve of curves. As a matter of fact it is merely _a special kind of
ellipse_. To put it paradoxically, it is an ellipse which has no
ellipticity, an oval without any ovalness!

Notwithstanding all this, astronomy had to wait yet a long time for a
definite proof of the revolution of the earth around the sun. The
leading argument advanced by Aristotle, against the reality of any
movement of the earth, still held good up to about seventy years ago.
That philosopher had pointed out that the earth could not move about in
space to any great extent, or the stars would be found to alter their
apparent places in the sky, a thing which had never been observed to
happen. Centuries ran on, and instruments became more and more perfect,
yet no displacements of stars were noted. In accepting the Copernican
theory men were therefore obliged to suppose these objects as
immeasurably distant. At length, however, between the years 1835 and
1840, it was discovered by the Prussian astronomer, Bessel, that a star
known as 61 Cygni--that is to say, the star marked in celestial atlases
as No. 61 in the constellation of the Swan--appeared, during the course
of a year, to perform a tiny circle in the heavens, such as would result
from a movement on our own part around the sun. Since then about
forty-three stars have been found to show minute displacements of a
similar kind, which cannot be accounted for upon any other supposition
than that of a continuous revolution of the earth around the sun. The
triumph of the Copernican system is now at last supreme.

If the axis of the earth stood "straight up," so to speak, while the
earth revolved in its orbit, the sun would plainly keep always on a
level with the equator. This is equivalent to stating that, in such
circumstances, a person at the equator would see it rise each morning
exactly in the east, pass through the _zenith_, that is, the point
directly overhead of him, at midday, and set in the evening due in the
west. As this would go on unchangingly at the equator every day
throughout the year, it should be clear that, at any particular place
upon the earth, the sun would in these conditions always be seen to move
in an unvarying manner across the sky at a certain altitude depending
upon the latitude of the place. Thus the more north one went upon the
earth's surface, the more southerly in the sky would the sun's path lie;
while at the north pole itself, the sun would always run round and round
the horizon. Similarly, the more south one went from the equator the
more northerly would the path of the sun lie, while at the south pole it
would be seen to skirt the horizon in the same manner as at the north
pole. The result of such an arrangement would be, that each place upon
the earth would always have one unvarying climate; in which case there
would not exist any of those beneficial changes of season to which we
owe so much.

The changes of season, which we fortunately experience, are due,
however, to the fact that the sun does not appear to move across the sky
each day at one unvarying altitude, but is continually altering the
position of its path; so that at one period of the year it passes across
the sky low down, and remains above the horizon for a short time only,
while at another it moves high up across the heavens, and is above the
horizon for a much longer time. Actually, the sun seems little by little
to creep up the sky during one half of the year, namely, from mid-winter
to mid-summer, and then, just as gradually, to slip down it again during
the other half, namely, from mid-summer to mid-winter. It will therefore
be clear that every region of the earth is much more thoroughly warmed
during one portion of the year than during another, _i.e._ when the
sun's path is high in the heavens than when it is low down.

Once more we find appearances exactly the contrary from the truth. The
earth is in this case the real cause of the deception, just as it was in
the other cases. The sun does not actually creep slowly up the sky, and
then slowly dip down it again, but, owing to the earth's axis being set
aslant, different regions of the earth's surface are presented to the
sun at different times. Thus, in one portion of its orbit, the northerly
regions of the earth are presented to the sun, and in the other portion
the southerly. It follows of course from this, that when it is summer in
the northern hemisphere it is winter in the southern, and _vice versâ_
(see Fig. 13, p. 176).

[Illustration: FIG. 13.--Summer and Winter.]

The fact that, in consequence of this slant of the earth's axis, the sun
is for part of the year on the north side of the equator and part of the
year on the south side, leads to a very peculiar result. The path of the
moon around the earth is nearly on the same plane with the earth's path
around the sun. The moon, therefore, always keeps to the same regions of
the sky as the sun. The slant of the earth's axis thus regularly
displaces the position of both the sun and the moon to the north and
south sides of the equator respectively in the manner we have been
describing. Were the earth, however, a perfect sphere, such change of
position would not produce any effect. We have shown, however, that the
earth is not a perfect sphere, but that it is bulged out all round the
equator. The result is that this bulged-out portion swings slowly under
the pulls of solar and lunar gravitation, in response to the
displacements of the sun and moon to the north and to the south of it.
This slow swing of the equatorial regions results, of course, in a
certain slow change of the direction of the earth's axis, so that the
north pole does not go on pointing continually to the same region of the
sky. The change in the direction of the axis is, however, so extremely
slight, that it shows up only after the lapse of ages. The north pole of
the heavens, that is, the region of the sky towards which the north pole
of the earth's axis points, displaces therefore extremely slowly,
tracing out a wide circle, and arriving back again to the same position
in the sky only after a period of about 25,000 years. At present the
north pole of the heavens is quite close to a bright star in the tail of
the constellation of the Little Bear, which is consequently known as the
Pole Star; but in early Greek times it was at least ten times as far
away from this star as it is now. After some 12,000 years the pole will
point to the constellation of Lyra, and Vega, the most brilliant star in
that constellation, will then be considered as the pole star. This slow
twisting of the earth's axis is technically known as _Precession_, or
the _Precession of the Equinoxes_ (see Plate XIX., p. 292).

The slow displacement of the celestial pole appears to have attracted
the attention of men in very early times, but it was not until the
second century B.C. that precession was established as a fact by the
celebrated Greek astronomer, Hipparchus. For the ancients this strange
cyclical movement had a mystic significance; and they looked towards the
end of the period as the end, so to speak, of a "dispensation," after
which the life of the universe would begin anew:--

"Magnus ab integro sæclorum nascitur ordo.
Jam redit et Virgo, redeunt Saturnia regna;
. . . . . .
Alter erit tum Tiphys, et altera quæ vehat Argo
Delectos heroas; erunt etiam altera bella,
Atque iterum ad Trojam magnus mittetur Achilles."

We have seen that the orbit of the earth is an ellipse, and that the sun
is situated at what is called the _focus_, a point not in the middle of
the ellipse, but rather towards one of its ends. Therefore, during the
course of the year the distance of the earth from the sun varies. The
sun, in consequence of this, is about 3,000,000 miles _nearer_ to us in
our northern _winter_ than it is in our northern summer, a statement
which sounds somewhat paradoxical. This variation in distance, large as
it appears in figures, can, however, not be productive of much
alteration in the amount of solar heat which we receive, for during the
first week in January, when the distance is least, the sun only looks
about _one-eighteenth_ broader than at the commencement of July, when
the distance is greatest. The great disparity in temperature between
winter and summer depends, as we have seen, upon causes of quite another
kind, and varies between such wide limits that the effects of this
slight alteration in the distance of the sun from the earth may be
neglected for practical purposes.

The Tides are caused by the gravitational pull of the sun and moon upon
the water of the earth's surface. Of the two, the moon, being so much
the nearer, exerts the stronger pull, and therefore may be regarded as
the chief cause of the tides. This pull always draws that portion of the
water, which happens to be right underneath the moon at the time, into a
heap; and there is also a _second_ heaping of water at the same moment
_at the contrary side of the earth_, the reasons for which can be shown
mathematically, but cannot be conveniently dealt with here.

As the earth rotates on its axis each portion of its surface passes
beneath the moon, and is swelled up by this pull; the watery portions
being, however, the only ones to yield visibly. A similar swelling up,
as we have seen, takes place at the point exactly away from the moon.
Thus each portion of our globe is borne by the rotation through two
"tide-areas" every day, and this is the reason why there are two tides
during every twenty-four hours.

The crest of the watery swelling is known as high tide. The journey of
the moon around the earth takes about a month, and this brings her past
each place in turn by about fifty minutes later each day, which is the
reason why high tide is usually about twenty-five minutes later each
time.

The moon is, however, not the sole cause of the tides, but the sun, as
we have said, has a part in the matter also. When it is new moon the
gravitational attractions of both sun and moon are clearly acting
together from precisely the same direction, and, therefore, the tide
will be pulled up higher than at other times. At full moon, too, the
same thing happens; for, although the bodies are now acting from
opposite directions, they do not neutralise each other's pulls as one
might imagine, since the sun, in the same manner as the moon, produces a
tide both under it and also at the opposite side of the earth. Thus both
these tides are actually increased in height. The exceptionally high
tides which we experience at new and full moons are known as _Spring
Tides_, in contradistinction to the minimum high tides, which are known
as _Neap Tides_.

The ancients appear to have had some idea of the cause of the tides. It
is said that as early as 1000 B.C. the Chinese noticed that the moon
exerted an influence upon the waters of the sea. The Greeks and Romans,
too, had noticed the same thing; and Cæsar tells us that when he was
embarking his troops for Britain the tide was high _because_ the moon
was full. Pliny went even further than this, in recognising a similar
connection between the waters and the sun.

From casual observation one is inclined to suppose that the high tide
always rises many feet. But that this is not the case is evidenced by
the fact that the tides in the midst of the great oceans are only from
three to four feet high. However, in the seas and straits around our
Isles, for instance, the tides rise very many feet indeed, but this is
merely owing to the extra heaping up which the large volumes of water
undergo in forcing their passage through narrow channels.

As the earth, in rotating, is continually passing through these
tide-areas, one might expect that the friction thus set up would tend to
slow down the rotation itself. Such a slowing down, or "tidal drag," as
it is called, is indeed continually going on; but the effects produced
are so exceedingly minute that it will take many millions of years to
make the rotation appreciably slower, and so to lengthen the day.

Recently it has been proved that the axis of the earth is subject to a
very small displacement, or rather, "wobbling," in the course of a
period of somewhat over a year. As a consequence of this, the pole
shifts its place through a circle of, roughly, a few yards in width
during the time in question. This movement is, perhaps, the combined
result of two causes. One of these is the change of place during the
year of large masses of material upon our earth; such as occurs, for
instance, when ice and snow melt, or when atmospheric and ocean
currents transport from place to place great bodies of air and water.
The other cause is supposed to be the fact that the earth is not
absolutely rigid, and so yields to certain strains upon it. In the
course of investigation of this latter point the interesting conclusion
has been reached by the famous American astronomer, Professor Simon
Newcomb, that our globe as a whole is _a little more rigid than steel_.

We will bring this chapter to a close by alluding briefly to two strange
appearances which are sometimes seen in our night skies. These are known
respectively as the Zodiacal Light and the Gegenschein.

The _Zodiacal Light_ is a faint cone-shaped illumination which is seen
to extend upwards from the western horizon after evening twilight has
ended, and from the eastern horizon before morning twilight has begun.
It appears to rise into the sky from about the position where the sun
would be at that time. The proper season of the year for observing it
during the evening is in the spring, while in autumn it is best seen in
the early morning. In our latitudes its light is not strong enough to
render it visible when the moon is full, but in the tropics it is
reported to be very bright, and easily seen in full moonlight. One
theory regards it as the reflection of light from swarms of meteors
revolving round the sun; another supposes it to be a very rarefied
extension of the corona.

The _Gegenschein_ (German for "counter-glow") is a faint oval patch of
light, seen in the sky exactly opposite to the place of the sun. It is
usually treated of in connection with the zodiacal light, and one theory
regards it similarly as of meteoric origin. Another theory,
however--that of Mr. Evershed--considers it a sort of _tail_ to the
earth (like a comet's tail) composed of hydrogen and helium--the two
_lightest_ gases we know--driven off from our planet in the direction
contrary to the sun.

[13] Every one knows the simple experiment in which a coin lying at the
bottom of an empty basin, and hidden from the eye by its side, becomes
visible when a certain quantity of water has been poured in. This is an
example of refraction. The rays of light coming from the coin ought
_not_ to reach the eye, on account of the basin's side being in the way;
yet by the action of the water they are _refracted_, or bent over its
edge, in such a manner that they do.

CHAPTER XVI

THE MOON

What we call the moon's "phases" are merely the various ways in which we
see the sun shining upon her surface during the course of her monthly
revolutions around the earth (see Fig. 14, p. 184). When she passes in
the neighbourhood of the sun all his light falls upon that side which is
turned away from us, and so the side which is turned towards us is
unillumined, and therefore invisible. When in this position the moon is
spoken of as _new_.

As she continues her motion around the earth, she draws gradually to the
east of the sun's place in the sky. The sunlight then comes somewhat
from the side; and so we see a small portion of the right side of the
lunar disc illuminated. This is the phase known as the _crescent_ moon.

As she moves on in her orbit more and more of her illuminated surface is
brought into view; and so the crescent of light becomes broader and
broader, until we get what is called half-moon, or _first quarter_, when
we see exactly one-half of her surface lit up by the sun's rays. As she
draws still further round yet more of her illuminated surface is brought
into view, until three-quarters of the disc appear lighted up. She is
then said to be _gibbous_.

Eventually she moves round so that she faces the sun completely, and
the whole of her disc appears illuminated. She is then spoken of as
_full_. When in this position it is clear that she is on the contrary
side of the earth to the sun, and therefore rises about the same time
that he is setting. She is now, in fact, at her furthest from the sun.

[Illustration: Direction from which the sun's rays are coming.

Various positions and illumination of the mooon by the sun during her
revolution around the earth.

The corresponding positions as viewed from the earth, showing the
consequent phases.

FIG. 14.--Orbit and Phases of the Moon.]

After this, the motion of the moon in her orbit carries her on back
again in the direction of the sun. She thus goes through her phases as
before, only these of course are _in the reverse order_. The full phase
is seen to give place to the gibbous, and this in turn to the half-moon
and to the crescent; after which her motion carries her into the
neighbourhood of the sun, and she is once more new, and lost to our
sight in the solar glare. Following this she draws away to the east of
the sun again, and the old order of phases repeat themselves as before.

The early Babylonians imagined that the moon had a bright and a dark
side, and that her phases were caused by the bright side coming more and
more into view during her movement around the sky. The Greeks, notably
Aristotle, set to work to examine the question from a mathematical
standpoint, and came to the conclusion that the crescent and other
appearances were such as would necessarily result if the moon were a
dark body of spherical shape illumined merely by the light of the sun.

Although the true explanation of the moon's phases has thus been known
for centuries, it is unfortunately not unusual to see
pictures--advertisement posters, for instance--in which stars appear
_within_ the horns of a crescent moon! Can it be that there are to-day
educated persons who believe that the moon is a thing which _grows_ to a
certain size and then wastes away again; who, in fact, do not know that
the entire body of the moon is there all the while?

When the moon shows a very thin crescent, we are able dimly to see her
still dark portion standing out against the sky. This appearance is
popularly known as the "old moon in the new moon's arms." The dark part
of her surface must, indeed, be to some degree illumined, or we should
not be able to see it at all. Whence then comes the light which
illumines it, since it clearly cannot come from the sun? The riddle is
easily solved, if we consider what kind of view of our earth an observer
situated on this darkened part of the moon would at that moment get. He
would, as a matter of fact, just then see nearly the whole disc of the
earth brightly lit up by sunlight. The lunar landscape all around would,
therefore, be bathed in what to _him_ would be "earthlight," which of
course takes the place there of what _we_ call moonlight. If, then, we
recollect how much greater in size the earth is than the moon, it should
not surprise us that this earthlight will be many times brighter than
moonlight. It is considered, indeed, to be some twenty times brighter.
It is thus not at all astonishing that we can see the dark portion of
the moon illumined merely by sunlight reflected upon it from our earth.

The ancients were greatly exercised in their minds to account for this
"earthlight," or "earthshine," as it is also called. Posidonius (135-51
B.C.) tried to explain it by supposing that the moon was partially
transparent, and that some sunlight consequently filtered through from
the other side. It was not, however, until the fifteenth century that
the correct solution was arrived at.

[Illustration: One side of the moon only is ever presented to the
earth. This side is here indicated by the letters S.F.E. (side facing
earth).

By placing the above positions in a row, we can see at once that the
moon makes one complete rotation on her axis in exactly the same time as
she revolves around the earth.

FIG. 15.--The Rotation of the Moon on her Axis.]

Perhaps the most remarkable thing which one notices about the moon is
that she always turns the same side towards us, and so we never see her
other side. One might be led from this to jump to the conclusion that
she does not rotate upon an axis, as do the other bodies which we see;
but, paradoxical as it may appear, the fact that she turns one face
always towards the earth, is actually a proof that she _does_ rotate
upon an axis. The rotation, however, takes place with such slowness,
that she turns round but once during the time in which she revolves
around the earth (see Fig. 15). In order to understand the matter
clearly, let the reader place an object in the centre of a room and walk
around it once, _keeping his face turned towards it the whole time_,
While he is doing this, it is evident that he will face every one of the
four walls of the room in succession. Now in order to face each of the
four walls of a room in succession one would be obliged _to turn oneself
entirely round_. Therefore, during the act of walking round an object
with his face turned directly towards it, a person at the same time
turns his body once entirely round.

In the long, long past the moon must have turned round much faster than
this. Her rate of rotation has no doubt been slowed down by the action
of some force. It will be recollected how, in the course of the previous
chapter, we found that the tides were tending, though exceedingly
gradually, to slow down the rotation of the earth upon its axis. But, on
account of the earth's much greater mass, the force of gravitation
exercised by it upon the surface of the moon is, of course, much more
powerful than that which the moon exercises upon the surface of the
earth. The tendency to tidal action on the moon itself must, therefore,
be much in excess of anything which we here experience. It is, in
consequence, probable that such a tidal drag, extending over a very long
period of time, has resulted in slowing down the moon's rotation to its
present rate.

The fact that we never see but one side of the moon has given rise from
time to time to fantastic speculations with regard to the other side.
Some, indeed, have wished to imagine that our satellite is shaped like
an egg, the more pointed end being directed away from us. We are here,
of course, faced with a riddle, which is all the more tantalising from
its appearing for ever insoluble to men, chained as they are to the
earth. However, it seems going too far to suppose that any abnormal
conditions necessarily exist at the other side of the moon. As a matter
of fact, indeed, small portions of that side are brought into our view
from time to time in consequence of slight irregularities in the moon's
movement; and these portions differ in no way from those which we
ordinarily see. On the whole, we obtain a view of about 60 per cent. of
the entire lunar surface; that is to say, a good deal more than
one-half.

The actual diameter of the moon is about 2163 miles, which is somewhat
more than one-quarter the diameter of the earth. For a satellite,
therefore, she seems very large compared with her primary, the earth;
when we consider that Jupiter's greatest satellite, although nearly
twice as broad as our moon, has a diameter only one twenty-fifth that of
Jupiter. Furthermore, the moon moves around the earth comparatively
slowly, making only about thirteen revolutions during the entire year.
Seen from space, therefore, she would not give the impression of a
circling body, as other satellites do. Her revolutions are, indeed,
relatively so very slow that she would appear rather like a smaller
planet accompanying the earth in its orbit. In view of all this, some
astronomers are inclined to regard the earth and moon rather as a
"double planet" than as a system of planet and satellite.

When the moon is full she attracts more attention perhaps than in any of
her other phases. The moon, in order to be full, must needs be in that
region of the heavens exactly opposite to the sun. The sun _appears_ to
go once entirely round the sky in the course of a year, and the moon
performs the same journey in the space of about a month. The moon, when
full, having got half-way round this journey, occupies, therefore, that
region of the sky which the sun itself will occupy half a year later.
Thus in winter the full moon will be found roughly to occupy the sun's
summer position in the sky, and in summer the sun's winter position. It
therefore follows that the full moon in winter time is high up in the
heavens, while in summer time it is low down. We thus get the greatest
amount of full moonlight when it is the most needed.

The great French astronomer, Laplace, being struck by the fact that the
"lesser light" did not rule the night to anything like the same extent
that the "greater light" ruled the day, set to work to examine the
conditions under which it might have been made to do so. The result of
his speculations showed that if the moon were removed to such a distance
that she took a year instead of a month to revolve around the earth; and
if she were started off in her orbit at full moon, she would always
continue to remain full--a great advantage for us. Whewell, however,
pointed out that in order to get the moon to move with the requisite
degree of slowness, she would have to revolve so far from the earth that
she would only look one-sixteenth as large as she does at present, which
rather militates against the advantage Laplace had in mind! Finally,
however, it was shown by M. Liouville, in 1845, that the position of a
_perennial full moon_, such as Laplace dreamed of, would be
unstable--that is to say, the body in question could not for long remain
undisturbed in the situation suggested (see Fig. 16, p. 191).

[Illustration: Various positions of Laplace's "Moon" with regard to the
earth and sun during the course of a year.

The same positions of Laplace's "Moon," arranged around the earth, show
that it would make only one revolution in a year.

FIG. 16.--Laplace's "Perennial Full Moon."]

There is a well-known phenomenon called _harvest moon_, concerning the
nature of which there seems to be much popular confusion. An idea in
fact appears to prevail among a good many people that the moon is a
harvest moon when, at rising, it looks bigger and redder than usual.
Such an appearance has, however, nothing at all to say to the matter;
for the moon always _looks_ larger when low down in the sky, and,
furthermore, it usually looks red in the later months of the year, when
there is more mist and fog about than there is in summer. What
astronomers actually term the harvest moon is, indeed, something
entirely different from this. About the month of September the slant at
which the full moon comes up from below the horizon happens to be such
that, during several evenings together, she _rises almost at the same
hour_, instead of some fifty minutes later, as is usually the case. As
the harvest is being gathered in about that time, it has come to be
popularly considered that this is a provision of nature, according to
which the sunlight is, during several evenings, replaced without delay
by more or less full-moonlight, in order that harvesters may continue
their work straight on into the night, and not be obliged to break off
after sunset to wait until the moon rises. The same phenomenon is almost
exactly repeated a month later, but by reason of the pursuits then
carried on it is known as the "hunter's moon."

In this connection should be mentioned that curious phenomenon above
alluded to, and which seems to attract universal notice, namely, that
the moon _looks much larger when near the horizon_--at its rising, for
instance, than when higher up in the sky. This seeming enlargement is,
however, by no means confined to the moon. That the sun also looks much
larger when low down in the sky than when high up, seems to strike even
the most casual watcher of a sunset. The same kind of effect will,
indeed, be noted if close attention be paid to the stars themselves. A
constellation, for instance, appears more spread out when low down in
the sky than when high up. This enlargement of celestial objects when in
the neighbourhood of the horizon is, however, only _apparent_ and not
real. It must be entirely an _illusion_; for the most careful
measurements of the discs of the sun and of the moon fail to show that
the bodies are any larger when near the horizon than when high up in the
sky. In fact, if there be any difference in measurements with regard to
the moon, it will be found to be the other way round; for her disc, when
carefully measured, is actually the slightest degree _greater_ when
_high_ in the sky, than when low down. The reason for this is that, on
account of the rotundity of the earth's surface, she is a trifle nearer
the observer when overhead of him.

This apparent enlargement of celestial objects, when low down in the
sky, is granted on all sides to be an illusion; but although the
question has been discussed with animation time out of mind, none of the
explanations proposed can be said to have received unreserved
acceptance. The one which usually figures in text-books is that we
unconsciously compare the sun and moon, when low down in the sky, with
the terrestrial objects in the same field of view, and are therefore
inclined to exaggerate the size of these orbs. Some persons, on the
other hand, imagine the illusion to have its source in the structure of
the human eye; while others, again, put it down to the atmosphere,
maintaining that the celestial objects in question _loom_ large in the
thickened air near the horizon, in the same way that they do when viewed
through fog or mist.

The writer[14] ventures, however, to think that the illusion has its
origin in our notion of the shape of the celestial vault. One would be
inclined, indeed, to suppose that this vault ought to appear to us as
the half of a hollow sphere; but he maintains that it does not so
appear, as a consequence of the manner in which the eyes of men are set
quite close together in their heads. If one looks, for instance, high up
in the sky, the horizon cannot come within the field of view, and so
there is nothing to make one think that the expanse then gazed upon is
other than quite _flat_--let us say like the ceiling of a room. But, as
the eyes are lowered, a portion of the _encircling_ horizon comes
gradually into the field of view, and the region of the sky then gazed
upon tends in consequence to assume a _hollowed-out_ form. From this it
would seem that our idea of the shape of the celestial vault is, that it
is _flattened down over our heads and hollowed out all around in the
neighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as a
consequence of their very great distance, all the objects in the heavens
necessarily appear to us to move as if they were placed on the
background of the vault; the result being that the mind is obliged to
conceive them as expanded or contracted, in its unconscious attempts to
make them always fill their due proportion of space in the various parts
of this abnormally shaped sky.

From such considerations the writer concludes that the apparent
enlargement in question is merely the natural consequence of the idea we
have of the shape of the celestial vault--an idea gradually built up in
childhood, to become later on what is called "second nature." And in
support of this contention, he would point to the fact that the
enlargement is not by any means confined to the sun and moon, but is
every whit as marked in the case of the constellations. To one who has
not noticed this before, it is really quite a revelation to compare the
appearance of one of the large constellations (Orion, for instance) when
high up in the sky and when low down. The widening apart of the various
stars composing the group, when in the latter position, is very
noticeable indeed.

[Illustration: FIG. 17.--Illustrating the author's explanation of the
apparent enlargement of celestial objects.]

Further, if a person were to stand in the centre of a large dome, he
would be exactly situated as if he were beneath the vaulted heaven, and
one would consequently expect him to suffer the same illusion as to the
shape of the dome. Objects fixed upon its background would therefore
appear to him under the same conditions as objects in the sky, and the
illusions as to their apparent enlargement should hold good here also.

Some years ago a Belgian astronomer, M. Stroobant, in an investigation
of the matter at issue, chanced to make a series of experiments under
the very conditions just detailed. To various portions of the inner
surface of a large dome he attached pairs of electric lights; and on
placing himself at the centre of the building, he noticed that, in every
case, those pairs which were high up appeared closer together than those
which were low down! He does not, however, seem to have sought for the
cause in the vaulted expanse. On the contrary, he attributed the effect
to something connected with our upright stature, to some physiological
reason which regularly makes us estimate objects as larger when in front
than when overhead.

In connection with this matter, it may be noted that it always appears
extremely difficult to estimate with the eye the exact height above the
horizon at which any object (say a star) happens to be. Even skilled
observers find themselves in error in attempting to do so. This seems to
bear out the writer's contention that the form under which the celestial
vault really appears to us is a peculiar one, and tends to give rise to
false judgments.

Before leaving this question, it should also be mentioned that nothing
perhaps is more deceptive than the size which objects in the sky appear
to present. The full moon looks so like a huge plate, that it astonishes
one to find that a threepenny bit held at arm's length will a long way
more than cover its disc.

[Illustration: PLATE VIII. THE MOON

From a photograph taken at the Paris Observatory by M.P. Puiseux.

(Page 197)]

The moon is just too far off to allow us to see the actual detail on
her surface with the naked eye. When thus viewed she merely displays a
patchy appearance,[15] and the imaginary forms which her darker markings
suggest to the fancy are popularly expressed by the term "Man in the
Moon." An examination of her surface with very moderate optical aid is,
however, quite a revelation, and the view we then get is not easily
comparable to what we see with the unaided eye.

Even with an ordinary opera-glass, an observer will be able to note a
good deal of detail upon the lunar disc. If it be his first observation
of the kind, he cannot fail to be struck by the fact to which we have
just made allusion, namely, the great change which the moon appears to
undergo when viewed with magnifying power. "Cain and his Dog," the "Man
in the Moon gathering sticks," or whatever indeed his fancy was wont to
conjure up from the lights and shades upon the shining surface, have now
completely disappeared; and he sees instead a silvery globe marked here
and there with extensive dark areas, and pitted all over with
crater-like formations (see Plate VIII., p. 196). The dark areas retain
even to the present day their ancient name of "seas," for Galileo and
the early telescopic observers believed them to be such, and they are
still catalogued under the mystic appellations given to them in the long
ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of
Dreams."[16] The improved telescopes of later times showed, however,
that they were not really seas (there is no water on the moon), but
merely areas of darker material.

The crater-like formations above alluded to are the "lunar mountains." A
person examining the moon for the first time with telescopic aid, will
perhaps not at once grasp the fact that his view of lunar mountains must
needs be what is called a "bird's-eye" one, namely, a view from above,
like that from a balloon and that he cannot, of course, expect to see
them from the side, as he does the mountains upon the earth. But once he
has realised this novel point of view, he will no doubt marvel at the
formations which lie scattered as it were at his feet. The type of lunar
mountain is indeed in striking contrast to the terrestrial type. On our
earth the range-formation is supreme; on the moon the crater-formation
is the rule, and is so-called from analogy to our volcanoes. A typical
lunar crater may be described as a circular wall, enclosing a central
plain, or "floor," which is often much depressed below the level of the
surface outside. These so-called "craters," or "ring-mountains," as they
are also termed, are often of gigantic proportions. For instance, the
central plain of one of them, known as Ptolemæus,[17] is about 115 miles
across, while that of Plato is about 60. The walls of craters often rise
to great heights; which, in proportion to the small size of the moon,
are very much in excess of our highest terrestrial elevations.
Nevertheless, a person posted at the centre of one of the larger craters
might be surprised to find that he could not see the encompassing
crater-walls, which would in every direction be below his horizon. This
would arise not alone from the great breadth of the crater itself, but
also from the fact that the curving of the moon's surface is very sharp
compared with that of our earth.

[Illustration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINCIPAL
"CRATERS," MOUNTAIN RANGES, AND "SEAS"

In this, as in the other plates of the Moon, the _South_ will be found
at the top of the picture; such being the view given by the ordinary
astronomical telescope, in which all objects are seen _inverted_.

(Page 199)]

We have mentioned Ptolemæus as among the very large craters, or
ring-mountains, on the moon. Its encompassing walls rise to nearly
13,000 feet, and it has the further distinction of being almost in the
centre of the lunar disc. There are, however, several others much wider,
but they are by no means in such a conspicuous position. For instance,
Schickard, close to the south-eastern border, is nearly 130 miles in
diameter, and its wall rises in one point to over 10,000 feet. Grimaldi,
almost exactly at the east point, is nearly as large as Schickard.
Another crater, Clavius, situated near the south point, is about 140
miles across; while its neighbour Bailly--named after a famous French
astronomer of the eighteenth century--is 180, and the largest of those
which we can see (see Plate IX., p. 198).

Many of the lunar craters encroach upon one another; in fact there is
not really room for them all upon the visible hemisphere of the moon.
About 30,000 have been mapped; but this is only a small portion, for
according to the American astronomer, Professor W.H. Pickering, there
are more than 200,000 in all.

Notwithstanding the fact that the crater is the type of mountain
associated in the mind with the moon, it must not be imagined that upon
our satellite there are no mountains at all of the terrestrial type.
There are indeed many isolated peaks, but strangely enough they are
nearly always to be found in the centres of craters. Some of these peaks
are of great altitude, that in the centre of the crater Copernicus being
over 11,000 feet high. A few mountain ranges also exist; the best known
of which are styled, the Lunar Alps and Lunar Apennines (see Plate X.,
p. 200).

Since the _mass_ of the moon is only about one-eightieth that of the
earth, it will be understood that the force of gravity which she
exercises is much less. It is calculated that, at her surface, this is
only about one-sixth of what we experience. A man transported to the
moon would thus be able to jump _six times as high_ as he can here. A
building could therefore be six times as tall as upon our earth, without
causing any more strain upon its foundations. It should not, then, be
any subject for wonder, that the highest peaks in the Lunar Apennines
attain to such heights as 22,000 feet. Such a height, upon a
comparatively small body like the moon, for her _volume_ is only
one-fiftieth that of the earth, is relatively very much in excess of the
29,000 feet of Himalayan structure, Mount Everest, the boast of our
planet, 8000 miles across!

High as are the Lunar Apennines, the highest peaks on the moon are yet
not found among them. There is, for instance, on the extreme southern
edge of the lunar disc, a range known as the Leibnitz Mountains; several
peaks of which rise to a height of nearly 30,000 feet, one peak in
particular being said to attain to 36,000 feet (see Plate IX., p. 198).

[Illustration: PLATE X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON

We have here (see "Map," Plate IX., p. 198) the mountain ranges of the
Apennines, the Caucasus and the Alps; also the craters Plato, Aristotle,
Eudoxus, Cassini, Aristillus, Autolycus, Archimedes and Linné. The
crater Linné is the very bright spot in the dark area at the upper left
hand side of the picture. From a photograph taken at the Paris
Observatory by M.M. Loewy and Puiseux.

(Page 200)]

But the reader will surely ask the question: "How is it possible to
determine the actual height of a lunar mountain, if one cannot go upon
the moon to measure it?" The answer is, that we can calculate its height
from noting the length of the shadow which it casts. Any one will allow
that the length of a shadow cast by the sun depends upon two things:
firstly, upon the height of the object which causes the shadow, and
secondly, upon the elevation of the sun at the moment in the sky. The
most casual observer of nature upon our earth can scarcely have failed
to notice that shadows are shortest at noonday, when the sun is at its
highest in the sky; and that they lengthen out as the sun declines
towards its setting. Here, then, we have the clue. To ascertain,
therefore, the height of a lunar mountain, we have first to consider at
what elevation the sun is at that moment above the horizon of the place
where the mountain in question is situated. Then, having measured the
actual length in miles of the shadow extended before us, all that is
left is to ask ourselves the question: "What height must an object be
whose shadow cast by the sun, when at that elevation in the sky, will
extend to this length?"

There is no trace whatever of water upon the moon. The opinion, indeed,
which seems generally held, is that water has never existed upon its
surface. Erosions, sedimentary deposits, and all those marks which point
to a former occupation by water are notably absent.

Similarly there appears to be no atmosphere on the moon; or, at any
rate, such an excessively rare one, as to be quite inappreciable. Of
this there are several proofs. For instance, in a solar eclipse the
moon's disc always stands out quite clear-cut against that of the sun.
Again during occultations, stars disappear behind the moon with a
suddenness, which could not be the case were there any appreciable
atmosphere. Lastly, we see no traces of twilight upon the lunar surface,
nor any softening at the edges of shadows; both which effects would be
apparent if there were an atmosphere.

The moon's surface is rough and rocky, and displays no marks of the
"weathering" that would necessarily follow, had it possessed anything of
an atmosphere in the past. This makes us rather inclined to doubt that
it ever had one at all. Supposing, however, that it did possess an
atmosphere in the past, it is interesting to inquire what may have
become of it. In the first place it might have gradually disappeared, in
consequence of the gases which composed it uniting chemically with the
materials of which the lunar body is constructed; or, again, its
constituent gases may have escaped into space, in accordance with the
principles of that kinetic theory of which we have already spoken. The
latter solution seems, indeed, the most reasonable of the two, for the
force of gravity at the lunar surface appears too weak to hold down any
known gases. This argument seems also to dispose of the question of
absence of water; for Dr. George Johnstone Stoney, in a careful
investigation of the subject, has shown that the liquid in question,
when in the form of vapour, will escape from a planet if its mass is
less than _one-fourth_ that of our earth. And the mass of the moon is
very much less than this; indeed only the _one-eightieth_, as we have
already stated.

In consequence of this lack of atmosphere, the condition of things upon
the moon will be in marked contrast to what we experience upon the
earth. The atmosphere here performs a double service in shielding us
from the direct rays of the sun, and in bottling the heat as a
glass-house does. On the moon, however, the sun beats down in the
day-time with a merciless force; but its rays are reflected away from
the surface as quickly as they are received, and so the cold of the
lunar night is excessive. It has been calculated that the day
temperature on the moon may, indeed, be as high as our boiling-point,
while the night temperature may be more than twice as low as the
greatest cold known in our arctic regions.

That a certain amount of solar heat is reflected to us from the moon is
shown by the sharp drop in temperature which certain heat-measuring
instruments record when the moon becomes obscured in a lunar eclipse.
The solar heat which is thus reflected to us by the moon is, however, on
the whole extremely small; more light and heat, indeed, reach us
_direct_ from the sun in half a minute than we get by _reflection_ from
the moon during the entire course of the year.

With regard to the origin of the lunar craters there has been much
discussion. Some have considered them to be evidence of violent volcanic
action in the dim past; others, again, as the result of the impact of
meteorites upon the lunar surface, when the moon was still in a plastic
condition; while a third theory holds that they were formed by the
bursting of huge bubbles during the escape into space of gases from the
interior. The question is, indeed, a very difficult one. Though
volcanic action, such as would result in craters of the size of
Ptolemæus, is hard for us to picture, and though the lone peaks which
adorn the centres of many craters have nothing reminiscent of them in
our terrestrial volcanoes, nevertheless the volcanic theory seems to
receive more favour than the others.

In addition to the craters there are two more features which demand
notice, namely, what are known as _rays_ and _rills_. The rays are long,
light-coloured streaks which radiate from several of the large craters,
and extend to a distance of some hundreds of miles. That they are mere
markings on the surface is proved by the fact that they cast no shadows
of any kind. One theory is, that they were originally great cracks which
have been filled with lighter coloured material, welling up from
beneath. The rills, on the other hand, are actually fissures, about a
mile or so in width and about a quarter of a mile in depth.

The rays are seen to the best advantage in connection with the craters
Tycho and Copernicus (see Plate XI., p. 204). In consequence of its
fairly forward position on the lunar disc, and of the remarkable system
of rays which issue from it like spokes from the axle of a wheel, Tycho
commands especial attention. The late Rev. T.W. Webb, a famous observer,
christened it, very happily, the "metropolitan crater of the moon."

[Illustration: PLATE XI. THE MOON

The systems of rays from the craters Tycho, Copernicus and Kepler are
well shown here. From a photograph taken at the Paris Observatory by
M.P. Puiseux.

(Page 204)]

A great deal of attention is, and has been, paid by certain astronomers
to the moon, in the hope of finding out if any changes are actually in
progress at present upon her surface. Sir William Herschel, indeed, once
thought that he saw a lunar volcano in eruption, but this proved to be
merely the effect of the sunlight striking the top of the crater
Aristarchus, while the region around it was still in shadow--sunrise
upon Aristarchus, in fact! No change of any real importance has,
however, been noted, although it is suspected that some minor
alterations have from time to time taken place. For instance, slight
variations of tint have been noticed in certain areas of the lunar
surface. Professor W.H. Pickering puts forward the conjecture that these
may be caused by the growth and decay of some low form of vegetation,
brought into existence by vapours of water, or carbonic acid gas, making
their way out from the interior through cracks near at hand.

Again, during the last hundred years one small crater known as Linné
(Linnæus), situated in the Mare Serenitatis (Sea of Serenity), has
appeared to undergo slight changes, and is even said to have been
invisible for a while (see Plate X., p. 200). It is, however, believed
that the changes in question may be due to the varying angles at which
the sunlight falls upon the crater; for it is an understood fact that
the irregularities of the moon's motion give us views of her surface
which always differ slightly.

The suggestion has more than once been put forward that the surface of
the moon is covered with a thick layer of ice. This is generally
considered improbable, and consequently the idea has received very
little support. It first originated with the late Mr. S.E. Peal, an
English observer of the moon, and has recently been resuscitated by the
German observer, Herr Fauth.

The most unfavourable time for telescopic study of the moon is when she
is full. The sunlight is then falling directly upon her visible
hemisphere, and so the mountains cast no shadows. We thus do not get
that impression of hill and hollow which is so very noticeable in the
other phases.

The first map of the moon was constructed by Galileo. Tobias Mayer
published another in 1775; while during the nineteenth century greatly
improved ones were made by Beer and Mädler, Schmidt, Neison and others.
In 1903, Professor W.H. Pickering brought out a complete photographic
lunar atlas; and a similar publication has recently appeared, the work
of MM. Loewy and Puiseux of the Observatory of Paris.

The so-called "seas" of the moon are, as we have seen, merely dark
areas, and there appears to be no proof that they were ever occupied by
any liquid. They are for the most part found in the _northern_ portion
of the moon; a striking contrast to our seas and oceans, which take up
so much of the _southern_ hemisphere of the earth.

There are many erroneous ideas popularly held with regard to certain
influences which the moon is supposed to exercise upon the earth. For
instance, a change in the weather is widely believed to depend upon a
change in the moon. But the word "change" as here used is meaningless,
for the moon is continually changing her phase during the whole of her
monthly round. Besides, the moon is visible over a great portion of the
earth _at the same moment_, and certainly all the places from which it
can then be seen do not get the same weather! Further, careful
observations, and records extending over the past one hundred years and
more, fail to show any reliable connection between the phases of the
moon and the condition of the weather.

It has been stated, on very good authority, that no telescope ever shows
the surface of the moon as clearly as we could see it with the naked eye
were it only 240 miles distant from us.

Supposing, then, that we were able to approach our satellite, and view
it without optical aid at such comparatively close quarters, it is
interesting to consider what would be the smallest detail which our eye
could take in. The question of the limit of what can be appreciated with
the naked eye is somewhat uncertain, but it appears safe to say that at
a distance of 240 miles the _minutest speck_ visible would have to be
_at least_ some 60 yards across.

Atmosphere and liquid both wanting, the lunar surface must be the seat
of an eternal calm; where no sound breaks the stillness and where
change, as we know it, does not exist. The sun beats down upon the arid
rocks, and inky shadows lie athwart the valleys. There is no mellowing
of the harsh contrasts.

We cannot indeed absolutely affirm that Life has no place at all upon
this airless and waterless globe, since we know not under what strange
conditions it may manifest its presence; and our most powerful
telescopes, besides, do not bring the lunar surface sufficiently near to
us to disprove the existence there of even such large creatures as
disport themselves upon our planet. Still, we find it hard to rid
ourselves of the feeling that we are in the presence of a dead world. On
she swings around the earth month after month, with one face ever
turned towards us, leaving a certain mystery to hang around that hidden
side, the greater part of which men can never hope to see. The rotation
of the moon upon her axis--the lunar day--has become, as we have seen,
equal to her revolution around the earth. An epoch may likewise
eventually be reached in the history of our own planet, when the length
of the terrestrial day has been so slowed down by tidal friction that it
will be equal to the year. Then will the earth revolve around the
central orb, with one side plunged in eternal night and the other in
eternal sunshine. But such a vista need not immediately distress us. It
is millions of years forward in time.

[14] _Journal of the British Astronomical Association_, vol. x.
(1899-1900), Nos. 1 and 3.

[15] Certain of the ancient Greeks thought the markings on the moon to
be merely the reflection of the seas and lands of our earth, as in a
badly polished mirror.

[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum.

[17] The lunar craters have, as a rule, received their names from
celebrated persons, usually men of science. This system of nomenclature
was originated by Riccioli, in 1651.

CHAPTER XVII

THE SUPERIOR PLANETS

Having, in a previous chapter, noted the various aspects which an
inferior planet presents to our view, in consequence of its orbit being
nearer to the sun than the orbit of the earth, it will be well here to
consider in the same way the case of a superior planet, and to mark
carefully the difference.

To begin with, it should be quite evident that we cannot ever have a
transit of a superior planet. The orbit of such a body being entirely
_outside_ that of the earth, the body itself can, of course, never pass
between us and the sun.

A superior planet will be at its greatest distance from us when on the
far side of the sun. It is said then to be in _conjunction_. As it comes
round in its orbit it eventually passes, so to speak, at the _back_ of
us. It is then at its nearest, or in _opposition_, as this is
technically termed, and therefore in the most favourable position for
telescopic observation of its surface. Being, besides, seen by us at
that time in the direction of the heavens exactly opposite to where the
sun is, it will thus at midnight be high up in the south side of the
sky, a further advantage to the observer.

Last of all, a superior planet cannot show crescent shapes like an
interior; for whether it be on the far side of the sun, or behind us,
or again to our right or left, the sunlight must needs appear to fall
more or less full upon its face.

THE PLANETOID EROS

The nearest to us of the superior planets is the tiny body, Eros, which,
as has been already stated, was discovered so late as the year 1898. In
point of view, however, of its small size, it can hardly be considered
as a true planet, and the name "planetoid" seems much more appropriate
to it.

Eros was not discovered, like Uranus, in the course of telescopic
examination of the heavens, nor yet, like Neptune, as the direct result
of difficult calculations, but was revealed by the impress of its light
upon a photographic plate, which had been exposed for some length of
time to the starry sky. Since many of the more recent additions to the
asteroids have been discovered in the same manner, we shall have
somewhat more to say about this special employment of photography when
we come to deal with those bodies later on.

The path of Eros around the sun is so very elliptical, or, to use the
exact technical term, so very "eccentric," that the planetoid does not
keep all the time entirely in the space between our orbit and that of
Mars, which latter happens to be the next body in the order of planetary
succession outwards. In portions of its journey Eros, indeed, actually
goes outside the Martian orbit. The paths of the planetoid and of Mars
are, however, _not upon the same plane_, so the bodies always pass clear
of each other, and there is thus as little chance of their dashing
together as there would be of trains which run across a bridge at an
upper level, colliding with those which pass beneath it at a lower
level.

When Eros is in opposition, it comes within about 13-1/2 million miles
of our earth, and, after the moon, is therefore by a long way our
nearest neighbour in space. It is, however, extremely small, not more,
perhaps, than twenty miles in diameter, and is subject to marked
variations in brightness, which do not appear up to the present to meet
with a satisfactory explanation. But, insignificant as is this little
body, it is of great importance to astronomy; for it happens to furnish
the best method known of calculating the sun's distance from our
earth--a method which Galle, in 1872, and Sir David Gill, in 1877,
suggested that asteroids might be employed for, and which has in
consequence supplanted the old one founded upon transits of Venus. The
sun's distance is now an ascertained fact to within 100,000 miles, or
less than half the distance of the moon.

THE PLANET MARS

We next come to the planet Mars. This body rotates in a period of
slightly more than twenty-four hours. The inclination, or slant, of its
axis is about the same as that of the earth, so that, putting aside its
greater distance from the sun, the variations of season which it
experiences ought to be very much like ours.

The first marking detected upon Mars was the notable one called the
Syrtis Major, also known, on account of its shape, as the Hour-Glass
Sea. This observation was made by the famous Huyghens in 1659; and, from
the movement of the marking in question across the disc, he inferred
that the planet rotated on its axis in a period of about twenty-four
hours.

There appears to be very little atmosphere upon Mars, the result being
that we almost always obtain a clear view of the detail on its surface.
Indeed, it is only to be expected from the kinetic theory that Mars
could not retain much of an atmosphere, as the force of gravity at its
surface is less than one-half of what we experience upon the earth. It
should here be mentioned that recent researches with the spectroscope
seem to show that, whatever atmosphere there may be upon Mars, its
density at the surface of the planet cannot be more than the one-fourth
part of the density of the air at the surface of the earth. Professor
Lowell, indeed, thinks it may be more rarefied than that upon our
highest mountain-tops.

Seen with the naked eye, Mars appears of a red colour. Viewed in the
telescope, its surface is found to be in general of a ruddy hue, varied
here and there with darker patches of a bluish-green colour. These
markings are permanent, and were supposed by the early telescopic
observers to imply a distribution of the planet's surface into land and
water, the ruddy portions being considered as continental areas (perhaps
sandy deserts), and the bluish-green as seas. The similarity to our
earth thus suggested was further heightened by the fact that broad white
caps, situated at the poles, were seen to vary with the planet's
seasons, diminishing greatly in extent during the Martian summer (the
southern cap in 1894 even disappearing altogether), and developing again
in the Martian winter.[18] Readers of Oliver Wendell Holmes will no
doubt recollect that poet's striking lines:--

"The snows that glittered on the disc of Mars
Have melted, and the planet's fiery orb
Rolls in the crimson summer of its year."

A state of things so strongly analogous to what we experience here,
naturally fired the imaginations of men, and caused them to look on Mars
as a world like ours, only upon a much smaller scale. Being smaller, it
was concluded to have cooled quicker, and to be now long past its prime;
and its "inhabitants" were, therefore, pictured as at a later stage of
development than the inhabitants of our earth.

Notwithstanding the strong temptation to assume that the whiteness of
the Martian polar caps is due to fallen snow, such a solution is,
however, by no means so simple as it looks. The deposition of water in
the form of snow, or even of hoar frost, would at least imply that the
atmosphere of Mars should now and then display traces of aqueous vapour,
which it does not appear to do.[19] It has, indeed, been suggested that
the whiteness may not after all be due to this cause, but to carbonic
acid gas (carbon dioxide), which is known to freeze at a _very low_
temperature. The suggestion is plainly based upon the assumption that,
as Mars is so much further from the sun than we are, it would receive
much less heat, and that the little thus received would be quickly
radiated away into space through lack of atmosphere to bottle it in.

We now come to those well-known markings, popularly known as the
"canals" of Mars, which have been the subject of so much discussion
since their discovery thirty years ago.

It was, in fact, in the year 1877, when Mars was in opposition, and thus
at its nearest to us, that the famous Italian astronomer, Schiaparelli,
announced to the world that he had found that the ruddy areas, thought
to be continents, were intersected by a network of straight dark lines.
These lines, he reported, appeared in many cases to be of great length,
so long, indeed, as several thousands of miles, and from about twenty to
sixty miles in width. He christened the lines _channels_, the Italian
word for which, "canali," was unfortunately translated into English as
"canals." The analogy, thus accidentally suggested, gave rise to the
idea that they might be actual waterways.[20]

In the winter of 1881-1882, when Mars was again in opposition,
Schiaparelli further announced that he had found some of these lines
doubled; that is to say, certain of them were accompanied by similar
lines running exactly parallel at no great distance away. There was at
first a good deal of scepticism on the subject of Schiaparelli's
discoveries, but gradually other observers found themselves seeing both
the lines and their doublings. We have in this a good example of a
curious circumstance in astronomical observation, namely, the fact that
when fine detail has once been noted by a competent observer, it is not
long before other observers see the same detail with ease.

An immense amount of close attention has been paid to the planet Mars
during recent years by the American observer, Professor Percival Lowell,
at his famous observatory, 7300 feet above the sea, near the town of
Flagstaff, Arizona, U.S.A. His observations have not, like those of most
astronomers, been confined merely to "oppositions," but he has
systematically kept the planet in view, so far as possible, since the
year 1894.

The instrumental equipment of his observatory is of the very best, and
the "seeing" at Flagstaff is described as excellent. In support of the
latter statement, Mr. Lampland, of the Lowell Observatory, maintains
that the faintest stars shown on charts made at the Lick Observatory
with the 36-inch telescope there, are _perfectly visible_ with the
24-inch telescope at Flagstaff.

Professor Lowell is, indeed, generally at issue with the other observers
of Mars. He finds the canals extremely narrow and sharply defined, and
he attributes the blurred and hazy appearance, which they have presented
to other astronomers, to the unsteady and imperfect atmospheric
conditions in which their observations have been made. He assigns to the
thinnest a width of two or three miles, and from fifteen to twenty to
the larger. Relatively to their width, however, he finds their length
enormous. Many of them are 2000 miles long, while one is even as much
as 3540. Such lengths as these are very great in comparison with the
smallness of the planet. He considers that the canals stand in some
peculiar relation to the polar cap, for they crowd together in its
neighbourhood. In place, too, of ill-defined condensations, he sees
sharp black spots where the canals meet and intersect, and to these he
gives the name of "Oases." He further lays particular stress upon a dark
band of a blue tint, which is always seen closely to surround the edges
of the polar caps all the time that they are disappearing; and this he
takes to be a proof that the white material is something which actually
_melts_. Of all substances which we know, water alone, he affirms, would
act in such a manner.

The question of melting at all may seem strange in a planet which is
situated so far from the sun, and possesses such a rarefied atmosphere.
But Professor Lowell considers that this very thinness of the atmosphere
allows the direct solar rays to fall with great intensity upon the
planet's surface, and that this heating effect is accentuated by the
great length of the Martian summer. In consequence he concludes that,
although the general climate of Mars is decidedly cold, it is above the
freezing point of water.

The observations at Flagstaff appear to do away with the old idea that
the darkish areas are seas, for numerous lines belonging to the
so-called "canal system" are seen to traverse them. Again, there is no
star-like image of the sun reflected from them, as there would be, of
course, from the surface of a great sheet of water. Lastly, they are
observed to vary in tone and colour with the changing Martian seasons,
the blue-green changing into ochre, and later on back again into
blue-green. Professor Lowell regards these areas as great tracts of
vegetation, which are brought into activity as the liquid reaches them
from the melting snows.

[Illustration: PLATE XII. A MAP OF THE PLANET MARS

We see here the Syrtis Major (or "Hour-Glass Sea"), the polar caps,
several "oases," and a large number of "canals," some of which are
double. The South is at the top of the picture, in accordance with the
_inverted_ view given by an astronomical telescope. From a drawing by
Professor Percival Lowell.

(Page 216)]

With respect to the canals, the Lowell observations further inform us
that these are invisible during the Martian winter, but begin to appear
in the spring when the polar cap is disappearing. Professor Lowell,
therefore, inclines to the view that in the middle of the so-called
canals there exist actual waterways which serve the purposes of
irrigation, and that what we see is not the waterways themselves, for
they are too narrow, but the fringe of vegetation which springs up along
the banks as the liquid is borne through them from the melting of the
polar snows. He supports this by his observation that the canals begin
to appear in the neighbourhood of the polar caps, and gradually grow, as
it were, in the direction of the planet's equator.

It is the idea of life on Mars which has given this planet such a
fascination in the eyes of men. A great deal of nonsense has, however,
been written in newspapers upon the subject, and many persons have thus
been led to think that we have obtained some actual evidence of the
existence of living beings upon Mars. It must be clearly understood,
however, that Professor Lowell's advocacy of the existence of life upon
that planet is by no means of this wild order. At the best he merely
indulges in such theories as his remarkable observations naturally call
forth. His views are as follows:--He considers that the planet has
reached a time when "water" has become so scarce that the "inhabitants"
are obliged to employ their utmost skill to make their scanty supply
suffice for purposes of irrigation. The changes of tone and colour upon
the Martian surface, as the irrigation produces its effects, are similar
to what a telescopic observer--say, upon Venus--would notice on our
earth when the harvest ripens over huge tracts of country; that is, of
course, if the earth's atmosphere allowed a clear view of the
terrestrial surface--a very doubtful point indeed. Professor Lowell
thinks that the perfect straightness of the lines, and the geometrical
manner in which they are arranged, are clear evidences of artificiality.
On a globe, too, there is plainly no reason why the liquid which results
from the melting of the polar caps should trend at all in the direction
of the equator. Upon our earth, for instance, the transference of water,
as in rivers, merely follows the slope of the ground, and nothing else.
The Lowell observations show, however, that the Martian liquid is
apparently carried from one pole towards the equator, and then past it
to the other pole, where it once more freezes, only to melt again in due
season, and to reverse the process towards and across the equator as
before. Professor Lowell therefore holds, and it seems a strong point in
favour of his theory, that the liquid must, in some artificial manner,
as by pumping, for instance, be _helped_ in its passage across the
surface of the planet.

A number of attempts have been made to explain the _doubling_ of the
canals merely as effects of refraction or reflection; and it has even
been suggested that it may arise from the telescope not being accurately
focussed.

The actual doubling of the canals once having been doubted, it was an
easy step to the casting of doubt on the reality of the canals
themselves. The idea, indeed, was put forward that the human eye, in
dealing with detail so very close to the limit of visibility, may
unconsciously treat as an actual line several point-like markings which
merely happen to lie in a line. In order to test this theory,
experiments were carried out in 1902 by Mr. E.W. Maunder of Greenwich
Observatory, and Mr. J.E. Evans of the Royal Hospital School at
Greenwich, in which certain schoolboys were set to make drawings of a
white disc with some faint markings upon it. The boys were placed at
various distances from the disc in question; and it was found that the
drawings made by those who were just too far off to see distinctly, bore
out the above theory in a remarkable manner. Recently, however, the
plausibility of the _illusion_ view has been shaken by photographs of
Mars taken during the opposition of 1905 by Mr. Lampland at the Lowell
Observatory, in which a number of the more prominent canals come out as
straight dark lines. Further still, in some photographs made there quite
lately, several canals are said to appear visibly double.

Following up the idea alluded to in Chapter XVI., that the moon may be
covered with a layer of ice, Mr. W.T. Lynn has recently suggested that
this may be the case on Mars; and that, at certain seasons, the water
may break through along definite lines, and even along lines parallel to
these. This, he maintains, would account for the canals becoming
gradually visible across the disc, without the necessity of Professor
Lowell's "pumping" theory.

And now for the views of Professor Lowell himself with regard to the
doubling of the canals. From his observations, he considers that no
pairs of railway lines could apparently be laid down with greater
parallelism. He draws attention to the fact that the doubling does not
take place by any means in every canal; indeed, out of 400 canals seen
at Flagstaff, only fifty-one--or, roughly, one-eighth--have at any time
been seen double. He lays great stress upon this, which he considers
points strongly against the duplication being an optical phenomenon. He
finds that the distance separating pairs of canals is much less in some
doubles than in others, and varies on the whole from 75 to 200 miles.
According to him, the double canals appear to be confined to within 40
degrees of the equator: or, to quote his own words, they are "an
equatorial feature of the planet, confined to the tropic and temperate
belts." Finally, he points out that they seem to _avoid_ the blue-green
areas. But, strangely enough, Professor Lowell does not so far attempt
to fit in the doubling with his body of theory. He makes the obvious
remark that they may be "channels and return channels," and with that he
leaves us.

The conclusions of Professor Lowell have recently been subjected to
strenuous criticism by Professor W.H. Pickering and Dr. Alfred Russel
Wallace. It was Professor Pickering who discovered the "oases," and who
originated the idea that we did not see the so-called "canals"
themselves, but only the growth of vegetation along their borders. He
holds that the oases are craterlets, and that the canals are cracks
which radiate from them, as do the rifts and streaks from craters upon
the moon. He goes on to suggest that vapours of water, or of carbonic
acid gas, escaping from the interior, find their way out through these
cracks, and promote the growth of a low form of vegetation on either
side of them. In support of this view he draws attention to the
existence of long "steam-cracks," bordered by vegetation, in the deserts
of the highly volcanic island of Hawaii. We have already seen, in an
earlier chapter, how he has applied this idea to the explanation of
certain changes which are suspected to be taking place upon the moon.

In dealing with the Lowell canal system, Professor Pickering points out
that under such a slight atmospheric pressure as exists on Mars, the
evaporation of the polar caps--supposing them to be formed of
snow--would take place with such extraordinary rapidity that the
resulting water could never be made to travel along open channels, but
that a system of gigantic tubes or water-mains would have to be
employed!

As will be gathered from his theories regarding vegetation, Professor
Pickering does not deny the existence of a form of life upon Mars. But
he will not hear of civilisation, or of anything even approaching it. He
thinks, however, that as Mars is intermediate physically between the
moon and earth, the form of life which it supports may be higher than
that on the moon and lower than that on the earth.

In a small book published in the latter part of 1907, and entitled _Is
Mars Habitable?_ Dr. Alfred Russel Wallace sets himself, among other
things, to combat the idea of a comparatively high temperature, such as
Professor Lowell has allotted to Mars. He shows the immense service
which the water-vapour in our atmosphere exercises, through keeping the
solar heat from escaping from the earth's surface. He then draws
attention to the fact that there is no spectroscopic evidence of
water-vapour on Mars[21]; and points out that its absence is only to be
expected, as Dr. George Johnstone Stoney has shown that it will escape
from a body whose mass is less than one-quarter the mass of the earth.
The mass of Mars is, in fact, much less than this, _i.e._ only
one-ninth. Dr. Wallace considers, therefore, that the temperature of
Mars ought to be extremely low, unless the constitution of its
atmosphere is very different from ours. With regard to the latter
statement, it should be mentioned that the Swedish physicist, Arrhenius,
has recently shown that the carbonic acid gas in our atmosphere has an
important influence upon climate. The amount of it in our air is, as we
have seen, extremely small; but Arrhenius shows that, if it were
doubled, the temperature would be more uniform and much higher. We thus
see how futile it is, with our present knowledge, to dogmatise on the
existence or non-existence of life in other celestial orbs.

As to the canals Dr. Wallace puts forward a theory of his own. He
contends that after Mars had cooled to a state of solidity, a great
swarm of meteorites and small asteroids fell in upon it, with the result
that a thin molten layer was formed all over the planet. As this layer
cooled, the imprisoned gases escaped, producing vents or craterlets; and
as it attempted to contract further upon the solid interior, it split in
fissures radiating from points of weakness, such, for instance, as the
craterlets. And he goes on to suggest that the two tiny Martian
satellites, with which we shall deal next, are the last survivors of his
hypothetical swarm. Finally, with regard to the habitability of Mars,
Dr. Wallace not only denies it, but asserts that the planet is
"absolutely uninhabitable."

For a long time it was supposed that Mars did not possess any
satellites. In 1877, however, during that famous opposition in which
Schiaparelli first saw the canals, two tiny satellites were discovered
at the Washington Observatory by an American astronomer, Professor Asaph
Hall. These satellites are so minute, and so near to the planet, that
they can only be seen with very large telescopes; and even then the
bright disc of the planet must be shielded off. They have been
christened Phobos and Deimos (Fear and Dread); these being the names of
the two subordinate deities who, according to Homer, attended upon Mars,
the god of war.

It is impossible to measure the exact sizes of these satellites, as they
are too small to show any discs, but an estimate has been formed from
their brightness. The diameter of Phobos was at first thought to be six
miles, and that of Deimos, seven. As later estimates, however,
considerably exceed this, it will, perhaps, be not far from the truth to
state that they are each roughly about the size of the planetoid Eros.
Phobos revolves around Mars in about 7-1/2 hours, at a distance of about
only 4000 miles from the planet's surface, and Deimos in about 30 hours,
at a distance of about 12,000 miles. As Mars rotates on its axis in
about 24 hours, it will be seen that Phobos makes more than three
revolutions while the planet is rotating once--a very interesting
condition of things.

A strange foreshadowing of the discovery of the satellites of Mars will
be familiar to readers of _Gulliver's Travels_. According to Dean
Swift's hero, the astronomers on the Flying Island of Laputa had found
two tiny satellites to Mars, one of which revolved around the planet in
ten hours. The correctness of this guess is extraordinarily close,
though at best it is, of course, nothing more than a pure coincidence.

It need not be at all surprising that much uncertainty should exist with
regard to the actual condition of the surface of Mars. The circumstances
in which we are able to see that planet at the best are, indeed, hardly
sufficient to warrant us in propounding any hard and fast theories. One
of the most experienced of living observers, the American astronomer,
Professor E.E. Barnard, considers that the view we get of Mars with the
best telescope may be fairly compared with our naked eye view of the
moon. Since we have seen that a view with quite a small telescope
entirely alters our original idea of the lunar surface, a slight
magnification revealing features of whose existence we had not
previously the slightest conception, it does not seem too much to say
that a further improvement in optical power might entirely subvert the
present notions with regard to the Martian canals. Therefore, until we
get a still nearer view of these strange markings, it seems somewhat
futile to theorise. The lines which we see are perhaps, indeed, a
foreshortened and all too dim view of some type of formation entirely
novel to us, and possibly peculiar to Mars. Differences of gravity and
other conditions, such as obtain upon different planets, may perhaps
produce very diverse results. The earth, the moon, and Mars differ
greatly from one another in size, gravitation, and other such
characteristics. Mountain-ranges so far appear typical of our globe, and
ring-mountains typical of the moon. May not the so-called "canals" be
merely some special formation peculiar to Mars, though quite a natural
result of its particular conditions and of its past history?

THE ASTEROIDS (OR MINOR PLANETS)

We now come to that belt of small planets which are known by the name of
asteroids. In the general survey of the solar system given in Chapter
II., we saw how it was long ago noticed that the distances of the
planetary orbits from the sun would have presented a marked appearance
of orderly sequence, were it not for a gap between the orbits of Mars
and Jupiter where no large planet was known to circulate. The suspicion
thus aroused that some planet might, after all, be moving in this
seemingly empty space, gave rise to the gradual discovery of a great
number of small bodies; the largest of which, Ceres, is less than 500
miles in diameter. Up to the present day some 600 of these bodies have
been discovered; the four leading ones, in order of size, being named
Ceres, Pallas, Juno, and Vesta. All the asteroids are invisible to the
naked eye, with the exception of Vesta, which, though by no means the
largest, happens to be the brightest. It is, however, only just visible
to the eye under favourable conditions. No trace of an atmosphere has
been noted upon any of the asteroids, but such a state of things is only
to be expected from the kinetic theory.

For a good many years the discoveries of asteroids were made by means of
the telescope. When, in the course of searching the heavens, an object
was noticed which did not appear upon any of the recognised star charts,
it was kept under observation for several nights to see whether it
changed its place in the sky. Since asteroids move around the sun in
orbits, just as planets do, they, of course, quickly reveal themselves
by their change of position against the starry background.

The year 1891 started a new era in the discovery of asteroids. It
occurred to the Heidelberg observer, Dr. Max Wolf, one of the most
famous of the hunters of these tiny planets, that photography might be
employed in the quest with success. This photographic method, to which
allusion has already been made in dealing with Eros, is an extremely
simple one. If a photograph of a portion of the heavens be taken through
an "equatorial"--that is, a telescope, moving by machinery, so as to
keep the stars, at which it is pointed, always exactly in the field of
view during their apparent movement across the sky--the images of these
stars will naturally come out in the photograph as sharply defined
points. If, however, there happens to be an asteroid, or other planetary
body, in the same field of view, its image will come out as a short
white streak; because the body has a comparatively rapid motion of its
own, and will, during the period of exposure, have moved sufficiently
against the background of the stars to leave a short trail, instead of a
dot, upon the photographic plate. By this method Wolf himself has
succeeded in discovering more than a hundred asteroids (see Plate XIII.,
p. 226). It was, indeed, a little streak of this kind, appearing upon a
photograph taken by the astronomer Witt, at Berlin, in 1898, which first
informed the world of the existence of Eros.

[Illustration: PLATE XIII. MINOR PLANET TRAILS

Two trails of minor planets (asteroids) imprinted _at the same time_
upon one photographic plate. In the white streak on the left-hand side
of the picture we witness the _discovery_ of a new minor planet. The
streak on the right was made by a body already known--the minor planet
"Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on
the 4th of November, 1901, with the aid of a 16-inch telescope. The time
of exposure was two hours.

(Page 227)]

It has been calculated that the total mass of the asteroids must be
much less than one-quarter that of the earth. They circulate as a rule
within a space of some 30,000,000 miles in breadth, lying about midway
between the paths of Mars and Jupiter. Two or three, however, of the
most recently discovered of these small bodies have been found to pass
quite close to Jupiter. The orbits of the asteroids are by no means in
the one plane, that of Pallas being the most inclined to the plane of
the earth's orbit. It is actually three times as much inclined as that
of Eros.

Two notable theories have been put forward to account for the origin of
the asteroids. The first is that of the celebrated German astronomer,
Olbers, who was the discoverer of Pallas and Vesta. He suggested that
they were the fragments of an exploded planet. This theory was for a
time generally accepted, but has now been abandoned in consequence of
certain definite objections. The most important of these objections is
that, in accordance with the theory of gravitation, the orbits of such
fragments would all have to pass through the place where the explosion
originally occurred. But the wide area over which the asteroids are
spread points rather against the notion that they all set out originally
from one particular spot. Another objection is that it does not appear
possible that, within a planet already formed, forces could originate
sufficiently powerful to tear the body asunder.

The second theory is that for some reason a planet here failed in the
making. Possibly the powerful gravitational action of the huge body of
Jupiter hard by, disturbed this region so much that the matter
distributed through it was never able to collect itself into a single
mass.

[18] Sir William Herschel was the first to note these polar changes.

[19] Quite recently, however, Professor Lowell has announced that his
observer, Mr. E.C. Slipher, finds with the spectroscope faint traces of
water vapour in the Martian atmosphere.

[20] In a somewhat similar manner the term "crater," as applied to the
ring-mountain formation on the moon, has evidently given a bias in
favour of the volcanic theory as an explanation of that peculiar
structure.

[21] Mr. Slipher's results (see note 2, page 213) were not then known.

CHAPTER XVIII

THE SUPERIOR PLANETS--_continued_

The planets, so far, have been divided into inferior and superior. Such
a division, however, refers merely to the situation of their orbits with
regard to that of our earth. There is, indeed, another manner in which
they are often classed, namely, according to size. On this principle
they are divided into two groups; one group called the _Terrestrial
Planets_, or those which have characteristics like our earth, and the
other called the _Major Planets_, because they are all of very great
size. The terrestrial planets are Mercury, Venus, the earth, and Mars.
The major planets are the remainder, namely, Jupiter, Saturn, Uranus,
and Neptune. As the earth's orbit is the boundary which separates the
inferior from the superior planets, so does the asteroidal belt divide
the terrestrial from the major planets. We found the division into
inferior and superior useful for emphasising the marked difference in
aspect which those two classes present as seen from our earth; the
inferior planets showing phases like the moon when viewed in the
telescope, whereas the superior planets do not. But the division into
terrestrial and major planets is the more far-reaching classification of
the two, for it includes the whole number of planets, whereas the other
arrangement necessarily excludes the earth. The members of each of
these classes have many definite characteristics in common. The
terrestrial planets are all of them relatively small in size,
comparatively near together, and have few or no satellites. They are,
moreover, rather dense in structure. The major planets, on the other
hand, are huge bodies, circulating at great distances from each other,
and are, as a rule, provided with a number of satellites. With respect
to structure, they may be fairly described as being loosely put
together. Further, the markings on the surfaces of the terrestrial
planets are permanent, whereas those on the major planets are
continually shifting.

THE PLANET JUPITER

Jupiter is the greatest of the major planets. It has been justly called
the "Giant" planet, for both in volume and in mass it exceeds all the
other planets put together. When seen through the telescope it exhibits
a surface plentifully covered with markings, the most remarkable being a
series of broad parallel belts. The chief belt lies in the central parts
of the planet, and is at present about 10,000 miles wide. It is bounded
on either side by a reddish brown belt of about the same width. Bright
spots also appear upon the surface of the planet, last for a while, and
then disappear. The most notable of the latter is one known as the
"Great Red Spot." This is situated a little beneath the southern red
belt, and appeared for the first time about thirty years ago. It has
undergone a good many changes in colour and brightness, and is still
faintly visible. This spot is the most permanent marking which has yet
been seen upon Jupiter. In general, the markings change so often that
the surface which we see is evidently not solid, but of a fleeting
nature akin to cloud (see Plate XIV., p. 230).

[Illustration: PLATE XIV. THE PLANET JUPITER

The Giant Planet as seen at 11.30 p.m., on the 11th of January, 1908,
with a 12-1/2-inch reflecting telescope. The extensive oval marking in
the upper portion of the disc is the "Great Red Spot." The South is at
the top of the picture, the view being the _inverted_ one given by an
astronomical telescope. From a drawing by the Rev. Theodore E.R.
Phillips, M.A., F.R.A.S., Director of the Jupiter Section of the British
Astronomical Association.

(Page 231)]

Observations of Jupiter's markings show that on an average the planet
rotates on its axis in a period of about 9 hours 54 minutes. The mention
here of _an average_ with reference to the rotation will, no doubt,
recall to the reader's mind the similar case of the sun, the different
portions of which rotate with different velocities. The parts of Jupiter
which move quickest take 9 hours 50 minutes to go round, while those
which move slowest take 9 hours 57 minutes. The middle portions rotate
the fastest, a phenomenon which the reader will recollect was also the
case with regard to the sun.

Jupiter is a very loosely packed body. Its density is on an average only
about 1-1/2 times that of water, or about one-fourth the density of the
earth; but its bulk is so great that the gravitation at that surface
which we see is about 2-1/2 times what it is on the surface of the
earth. In accordance, therefore, with the kinetic theory, we may expect
the planet to retain an extensive layer of gases around it; and this is
confirmed by the spectroscope, which gives evidence of the presence of a
dense atmosphere.

All things considered, it may be safely inferred that the interior of
Jupiter is very hot, and that what we call its surface is not the actual
body of the planet, but a voluminous layer of clouds and vapours driven
upwards from the heated mass underneath. The planet was indeed formerly
thought to be self-luminous; but this can hardly be the case, for those
portions of the surface which happen to lie at any moment in the
shadows cast by the satellites appear to be quite black. Again, when a
satellite passes into the great shadow cast by the planet it becomes
entirely invisible, which would not be the case did the planet emit any
perceptible light of its own.

In its revolutions around the sun, Jupiter is attended, so far as we
know, by seven[22] satellites. Four of these were among the first
celestial objects which Galileo discovered with his "optick tube," and
he named them the "Medicean Stars" in honour of his patron, Cosmo de
Medici. Being comparatively large bodies they might indeed just be seen
with the naked eye, were it not for the overpowering glare of the
planet.

It was only in quite recent times, namely, in 1892, that a fifth
satellite was added to the system of Jupiter. This body, discovered by
Professor E.E. Barnard, is very small. It circulates nearer to the
planet than the innermost of Galileo's moons; and, on account of the
glare, is a most difficult object to obtain a glimpse of, even in the
best of telescopes. In December 1904 and January 1905 respectively, two
more moons were added to the system, these being found by _photography_,
by the American astronomer, Professor C.D. Perrine. Both the bodies in
question revolve at a greater distance from the planet than the
outermost of the older known satellites.

Galileo's moons, though the largest bodies of Jupiter's satellite
system, are, as we have already pointed out, very small indeed when
compared with the planet itself. The diameters of two of them, Europa
and Io, are, however, about the same as that of our moon, while those of
the other two, Callisto and Ganymede, are more than half as large again.
The recently discovered satellites are, on the other hand,
insignificant; that found by Barnard, for example, being only about 100
miles in diameter.

Of the four original satellites Io is the nearest to Jupiter, and, seen
from the planet, it would show a disc somewhat larger than that of our
moon. The others would appear somewhat smaller. However, on account of
the great distance of the sun, the entire light reflected to Jupiter by
all the satellites should be very much less than what we get from our
moon.

Barnard's satellite circles around Jupiter at a distance less than our
moon is from us, and in a period of about 12 hours. Galileo's four
satellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 days
respectively, at distances lying roughly between a quarter of a million
and one million miles. Perrine's two satellites are at a distance of
about seven million miles, and take about nine months to complete their
revolutions.

The larger satellites, when viewed in the telescope, exhibit certain
defined markings; but the bodies are so far away from us, that only
those details which are of great extent can be seen. The satellite Io,
according to Professor Barnard, shows a darkish disc, with a broad white
belt across its middle regions. Mr. Douglass, one of the observers at
the Lowell Observatory, has noted upon Ganymede a number of markings
somewhat resembling those seen on Mars, and he concludes, from their
movement, that this satellite rotates on its axis in about seven days.
Professor Barnard, on the other hand, does not corroborate this, though
he claims to have discovered bright polar caps on both Ganymede and
Callisto.

In an earlier chapter we dealt at length with eclipses, occultations,
and transits, and endeavoured to make clear the distinction between
them. The system of Jupiter's satellites furnishes excellent examples of
all these phenomena. The planet casts a very extensive shadow, and the
satellites are constantly undergoing obscuration by passing through it.
Such occurrences are plainly comparable to our lunar eclipses. Again,
the satellites may, at one time, be occulted by the huge disc of the
planet, and at another time seen in transit over its face. A fourth
phenomenon is what is known as an _eclipse of the planet by a
satellite_, which is the exact equivalent of what we style on the earth
an eclipse of the sun. In this last case the shadow, cast by the
satellite, appears as a round black spot in movement across the planet's
surface.

In the passages of these attendant bodies behind the planet, into its
shadow, or across its face, respectively, it occasionally happens that
Galileo's four satellites all disappear from view, and the planet is
then seen for a while in the unusual condition of being apparently
without its customary attendants. An instance of this phenomenon took
place on the 3rd of October 1907. On that occasion, the satellites known
as I. and III. (_i.e._ Io and Ganymede) were eclipsed, that is to say,
obscured by passing into the planet's shadow; Satellite IV. (Callisto)
was occulted by the planet's disc; while Satellite II. (Europa), being
at the same moment in transit across the planet's face, was invisible
against that brilliant background. A number of instances of this kind of
occurrence are on record. Galileo, for example, noted one on the 15th of
March 1611, while Herschel observed another on the 23rd of May 1802.

It was indirectly to Jupiter's satellites that the world was first
indebted for its knowledge of the velocity of light. When the periods of
revolution of the satellites were originally determined, Jupiter
happened, at the time, to be at his nearest to us. From the periods thus
found tables were made for the prediction of the moments at which the
eclipses and other phenomena of the satellites should take place. As
Jupiter, in the course of his orbit, drew further away from the earth,
it was noticed that the disappearances of the satellites into the shadow
of the planet occurred regularly later than the time predicted. In the
year 1675, Roemer, a Danish astronomer, inferred from this, not that the
predictions were faulty, but that light did not travel instantaneously.
It appeared, in fact, to take longer to reach us, the greater the
distance it had to traverse. Thus, when the planet was far from the
earth, the last ray given out by the satellite, before its passage into
the shadow, took a longer time to cross the intervening space, than when
the planet was near. Modern experiments in physics have quite confirmed
this, and have proved for us that light does not travel across space in
the twinkling of an eye, as might hastily be supposed, but actually
moves, as has been already stated, at the rate of about 186,000 miles
per second.

THE PLANET SATURN

Seen in the telescope the planet Saturn is a wonderful and very
beautiful object. It is distinguished from all the other planets, in
fact from all known celestial bodies, through being girt around its
equator by what looks like a broad, flat ring of exceeding thinness.
This, however, upon closer examination, is found to be actually composed
of three concentric rings. The outermost of these is nearly of the same
brightness as the body of the planet itself. The ring which comes
immediately within it is also bright, and is separated from the outer
one all the way round by a relatively narrow space, known as "Cassini's
division," because it was discovered by the celebrated French
astronomer, J.D. Cassini, in the year 1675. Inside the second ring, and
merging insensibly into it, is a third one, known as the "crape ring,"
because it is darker in hue than the others and partly transparent, the
body of Saturn being visible through it. The inner boundary of this
third and last ring does not adjoin the planet, but is everywhere
separated from it by a definite space. This ring was discovered
_independently_[23] in 1850 by Bond in America and Dawes in England.

[Illustration: PLATE XV. THE PLANET SATURN

From a drawing made by Professor Barnard with the Great Lick Telescope.
The black band fringing the outer ring, where it crosses the disc, is
portion of the _shadow which the rings cast upon the planet_. The black
wedge-shaped mark, where the rings disappear behind the disc at the
left-hand side, is portion of the _shadow which the planet casts upon
the rings_.

(Page 237)]

As distinguished from the crape ring, the bright rings must have a
considerable closeness of texture; for the shadow of the planet may be
seen projected upon them, and their shadows in turn projected upon the
surface of the planet (see Plate XV., p. 236).

According to Professor Barnard, the entire breadth of the ring system,
that is to say, from one side to the other of the outer ring, is 172,310
miles, or somewhat more than double the planet's diameter.

In the varying views which we get of Saturn, the system of the rings is
presented to us at very different angles. Sometimes we are enabled to
gaze upon its broad expanse; at other times, however, its thin edge is
turned exactly towards us, an occurrence which takes place after
intervals of about fifteen years. When this happened in 1892 the rings
are said to have disappeared entirely from view in the great Lick
telescope. We thus get an idea of their small degree of thickness, which
would appear to be only about 50 miles. The last time the system of
rings was exactly edgewise to the earth was on the 3rd of October 1907.

The question of the composition of these rings has given rise to a good
deal of speculation. It was formerly supposed that they were either
solid or liquid, but in 1857 it was proved by Clerk Maxwell that a
structure of this kind would not be able to stand. He showed, however,
that they could be fully explained by supposing them to consist of an
immense number of separate solid particles, or, as one might otherwise
put it, extremely small satellites, circling in dense swarms around the
middle portions of the planet. It is therefore believed that we have
here the materials ready for the formation of a satellite or satellites;
but that the powerful gravitative action, arising through the planet's
being so near at hand, is too great ever to allow these materials to
aggregate themselves into a solid mass. There is, as a matter of fact, a
minimum distance from the body of any planet within which it can be
shown that a satellite will be unable to form on account of
gravitational stress. This is known as "Roche's limit," from the name of
a French astronomer who specially investigated the question.

There thus appears to be a certain degree of analogy between Saturn's
rings and the asteroids. Empty spaces, too, exist in the asteroidal
zone, the relative position of one of which bears a striking resemblance
to that of "Cassini's division." It is suggested, indeed, that this
division had its origin in gravitational disturbances produced by the
attraction of the larger satellites, just as the empty spaces in the
asteroidal zone are supposed to be the result of perturbations caused by
the Giant Planet hard by.

It has long been understood that the system of the rings must be
rotating around Saturn, for if they were not in motion his intense
gravitational attraction would quickly tear them in pieces. This was at
length proved to be the fact by the late Professor Keeler, Director of
the Lick Observatory, who from spectroscopic observations found that
those portions of the rings situated near to the planet rotated faster
than those farther from it. This directly supports the view that the
rings are composed of satellites; for, as we have already seen, the
nearer a satellite is to its primary the faster it will revolve. On the
other hand, were the rings solid, their outer portions would move the
fastest; as we have seen takes place in the body of the earth, for
example. The mass of the ring system, however, must be exceedingly
small, for it does not appear to produce any disturbances in the
movements of Saturn's satellites. From the kinetic theory, therefore,
one would not expect to find any atmosphere on the rings, and the
absence of it is duly shown by spectroscopic observations.

The diameter of Saturn, roughly speaking, is about one-fifth less than
that of Jupiter. The planet is very flattened at the poles, this
flattening being quite noticeable in a good telescope. For instance, the
diameter across the equator is about 76,470 miles, while from pole to
pole it is much less, namely, 69,770.

The surface of Saturn bears a strong resemblance to that of Jupiter. Its
markings, though not so well defined, are of the same belt-like
description; and from observation of them it appears that the planet
rotates _on an average_ in a little over ten hours. The rotation is in
fact of the same peculiar kind as that of the sun and Jupiter; but the
difference of speed at which the various portions of Saturn go round are
even more marked than in the case of the Giant Planet. The density of
Saturn is less than that of Jupiter; so that it must be largely in a
condition of vapour, and in all probability at a still earlier stage of
planetary evolution.

Up to the present we know of as many as ten satellites circling around
Saturn, which is more than any other planet of the solar system can lay
claim to. Two of these, however, are very recent discoveries; one,
Phoebe, having been found by photography in August 1898, and the other,
Themis, in 1904, also by the same means. For both of these we are
indebted to Professor W.H. Pickering. Themis is said to be _the faintest
object in the solar system_. It cannot be _seen_, even with the largest
telescope in existence; a fact which should hardly fail to impress upon
one the great advantage the photographic plate possesses in these
researches over the human eye.

The most important of the whole Saturnian family of satellites are the
two known as Titan and Japetus. These were discovered respectively by
Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size
as our moon; while the diameter of Titan, the largest of the satellites,
is about half as much again. Titan takes about sixteen days to revolve
around Saturn, while Japetus takes more than two months and a half. The
former is about three-quarters of a million miles distant from the
planet, and the latter about two and a quarter millions. To Sir William
Herschel we are indebted for the discovery of two more satellites, one
of which he found on the evening that he used his celebrated 40-foot
telescope for the first time. The ninth satellite, Phoebe, one of the
two discovered by Professor Pickering, is perhaps the most remarkable
body in the solar system, for all the other known members of that system
perform their revolutions in one fixed direction, whereas this satellite
revolves in the _contrary_ direction.

In consequence of the great distance of Saturn, the sun, as seen from
the planet, would appear so small that it would scarcely show any disc.
The planet, indeed, only receives from the sun about one-ninetieth of
the heat and light which the earth receives. Owing to this diminished
intensity of illumination, the combined light reflected to Saturn by the
whole of its satellites must be very small.

With the sole exception of Jupiter, not one of the planets circulating
nearer to the sun could be seen from Saturn, as they would be entirely
lost in the solar glare. For an observer upon Saturn, Jupiter would,
therefore, fill much the same position as Venus does for us, regularly
displaying phases and being alternately a morning and an evening star.

It is rather interesting to consider the appearances which would be
produced in our skies were the earth embellished with a system of rings
similar to those of Saturn. In consequence of the curving of the
terrestrial surface, they would not be seen at all from within the
Arctic or Antarctic circles, as they would be always below the horizon.
From the equator they would be continually seen edgewise, and so would
appear merely as line of light stretching right across the heaven and
passing through the zenith. But the dwellers in the remaining regions
would find them very objectionable, for they would cut off the light of
the sun during lengthy periods of time.

Saturn was a sore puzzle to the early telescopic observers. They did not
for a long time grasp the fact that it was surrounded by a ring--so slow
is the human mind to seek for explanations out of the ordinary course of
things. The protrusions of the ring on either side of the planet, at
first looked to Galileo like two minor globes placed on opposite sides
of it, and slightly overlapping the disc. He therefore informed Kepler
that "Saturn consists of three stars in contact with one another." Yet
he was genuinely puzzled by the fact that the two attendant bodies (as
he thought them) always retained the same position with regard to the
planet's disc, and did not appear to revolve around it, nor to be in any
wise shifted as a consequence of the movements of our earth.

About a year and a half elapsed before he again examined Saturn; and, if
he was previously puzzled, he was now thoroughly amazed. It happened
just then to be one of those periods when the ring is edgewise towards
the earth, and of course he only saw a round disc like that of Jupiter.
What, indeed, had become of the attendant orbs? Was some demon mocking
him? Had Saturn devoured his own children? He was, however, fated to be
still more puzzled, for soon the minor orbs reappeared, and, becoming
larger and larger as time went on, they ended by losing their globular
appearance and became like two pairs of arms clasping the planet from
each side! (see Plate XVI., p. 242).

Galileo went to his grave with the riddle still unsolved, and it
remained for the famous Dutch astronomer, Huyghens, to clear up the
matter. It was, however, some little time before he hit upon the real
explanation. Having noticed that there were dark spaces between the
strange appendages and the body of the planet, he imagined Saturn to be
a globe fitted with handles at each side; "ansæ" these came to be
called, from the Latin _ansa_, which means a handle. At length, in the
year 1656, he solved the problem, and this he did by means of that
123-foot tubeless telescope, of which mention has already been made. The
ring happened then to be at its edgewise period, and a careful study of
the behaviour of the ansæ when disappearing and reappearing soon
revealed to Huyghens the true explanation.

[Illustration: PLATE XVI. EARLY REPRESENTATIONS OF SATURN

From an illustration in the _Systema Saturnium_ of Christian Huyghens.

(Page 242)]

THE PLANETS URANUS AND NEPTUNE

We have already explained (in Chapter II.) the circumstances in which
both Uranus and Neptune were discovered. It should, however, be added
that after the discovery of Uranus, that planet was found to have been
already noted upon several occasions by different observers, but always
without the least suspicion that it was other than a mere faint star.
Again, with reference to the discovery of Neptune, it may here be
mentioned that the apparent amount by which that planet had pulled
Uranus out of its place upon the starry background was exceedingly
small--so small, indeed, that no eye could have detected it without the
aid of a telescope!

Of the two predictions of the place of Neptune in the sky, that of Le
Verrier was the nearer. Indeed, the position calculated by Adams was
more than twice as far out. But Adams was by a long way the first in the
field with his results, and only for unfortunate delays the prize would
certainly have fallen to him. For instance, there was no star-map at
Cambridge, and Professor Challis, the director of the observatory there,
was in consequence obliged to make a laborious examination of the stars
in the suspected region. On the other hand, all that Galle had to do was
to compare that part of the sky where Le Verrier told him to look with
the Berlin star-chart which he had by him. This he did on September 23,
1846, with the result that he quickly noted an eighth magnitude star
which did not figure in that chart. By the next night this star had
altered its position in the sky, thus disclosing the fact that it was
really a planet.

Six days later Professor Challis succeeded in finding the planet, but of
course he was now too late. On reviewing his labours he ascertained that
he had actually noted down its place early in August, and had he only
been able to sift his observations as he made them, the discovery would
have been made then.

Later on it was found that Neptune had only just missed being discovered
about fifty years earlier. In certain observations made during 1795, the
famous French astronomer, Lalande, found that a star, which he had
mapped in a certain position on the 8th of May of that year, was in a
different position two days later. The idea of a planet does not appear
to have entered his mind, and he merely treated the first observation as
an error!

The reader will, no doubt, recollect how the discovery of the asteroids
was due in effect to an apparent break in the seemingly regular sequence
of the planetary orbits outwards from the sun. This curious sequence of
relative distances is usually known as "Bode's Law," because it was
first brought into general notice by an astronomer of that name. It had,
however, previously been investigated mathematically by Titius in 1772.
Long before this, indeed, the unnecessarily wide space between the
orbits of Mars and Jupiter had attracted the attention of the great
Kepler to such a degree, that he predicted that a planet would some day
be found to fill the void. Notwithstanding the service which the
so-called Law of Bode has indirectly rendered to astronomy, it has
strangely enough been found after all not to rest upon any scientific
foundation. It will not account for the distance from the sun of the
orbit of Neptune, and the very sequence seems on the whole to be in the
nature of a mere coincidence.

Neptune is invisible to the naked eye; Uranus is just at the limit of
visibility. Both planets are, however, so far from us that we can get
but the poorest knowledge of their condition and surroundings. Uranus,
up to the present, is known to be attended by four satellites, and
Neptune by one. The planets themselves are about equal in size; their
diameters, roughly speaking, being about one-half that of Saturn. Some
markings have, indeed, been seen upon the disc of Uranus, but they are
very indistinct and fleeting. From observation of them, it is assumed
that the planet rotates on its axis in a period of some ten to twelve
hours. No definite markings have as yet been seen upon Neptune, which
body is described by several observers as resembling a faint planetary
nebula.

With regard to their physical condition, the most that can be said about
these two planets is that they are probably in much the same vaporous
state as Jupiter and Saturn. On account of their great distance from the
sun they can receive but little solar heat and light. Seen from
Neptune, in fact, the sun would appear only about the size of Venus at
her best, though of a brightness sufficiently intense to illumine the
Neptunian landscape with about seven hundred times our full moonlight.

[22] Mr. P. Melotte, of Greenwich Observatory, while examining a
photograph taken there on February 28, 1908, discovered upon it a very
faint object which it is firmly believed will prove to be an _eighth_
satellite of Jupiter. This object was afterwards found on plates exposed
as far back as January 27. It has since been photographed several times
at Greenwich, and also at Heidelberg (by Dr. Max Wolf) and at the Lick
Observatory. Its movement is probably _retrograde_, like that of Phoebe
(p. 240).

[23] In the history of astronomy two salient points stand out.

The first of these is the number of "independent" discoveries which have
taken place; such, for instance, as in the cases of Le Verrier and Adams
with regard to Neptune, and of Lockyer and Janssen in the matter of the
spectroscopic method of observing solar prominences.

The other is the great amount of "anticipation." Copernicus, as we have
seen, was anticipated by the Greeks; Kepler was not actually the first
who thought of elliptic orbits; others before Newton had imagined an
attractive force.

Both these points furnish much food for thought!

CHAPTER XIX

COMETS

The reader has, no doubt, been struck by the marked uniformity which
exists among those members of the solar system with which we have dealt
up to the present. The sun, the planets, and their satellites are all
what we call solid bodies. The planets move around the sun, and the
satellites around the planets, in orbits which, though strictly
speaking, ellipses, are yet not in any instance of a very oval form. Two
results naturally follow from these considerations. Firstly, the bodies
in question hide the light coming to us from those further off, when
they pass in front of them. Secondly, the planets never get so far from
the sun that we lose sight of them altogether.

With the objects known as Comets it is, however, quite the contrary.
These objects do not conform to our notions of solidity. They are so
transparent that they can pass across the smallest star without dimming
its light in the slightest degree. Again, they are only visible to us
during a portion of their orbits. A comet may be briefly described as an
illuminated filmy-looking object, made up usually of three portions--a
head, a nucleus, or brighter central portion within this head, and a
tail. The heads of comets vary greatly in size; some, indeed, appear
quite small, like stars, while others look even as large as the moon.
Occasionally the nucleus is wanting, and sometimes the tail also.

[Illustration: FIG. 18.--Showing how the Tail of a Comet is directed
away from the Sun.]

These mysterious visitors to our skies come up into view out of the
immensities beyond, move towards the sun at a rapidly increasing speed,
and, having gone around it, dash away again into the depths of space. As
a comet approaches the sun, its body appears to grow smaller and
smaller, while, at the same time, it gradually throws out behind it an
appendage like a tail. As the comet moves round the central orb this
tail is always directed _away_ from the sun; and when it departs again
into space the tail goes in advance. As the comet's distance from the
sun increases, the tail gradually shrinks away and the head once more
grows in size (see Fig. 18). In consequence of these changes, and of the
fact that we lose sight of comets comparatively quickly, one is much
inclined to wonder what further changes may take place after the bodies
have passed beyond our ken.

The orbits of comets are, as we have seen, very elliptic. In some
instances this ellipticity is so great as to take the bodies out into
space to nearly six times the distance of Neptune from the sun. For a
long time, indeed, it was considered that comets were of two kinds,
namely, those which actually _belonged_ to the solar system, and those
which were merely _visitors_ to it for the first and only time--rushing
in from the depths of space, rapidly circuiting the sun, and finally
dashing away into space again, never to return. On the contrary,
nowadays, astronomers are generally inclined to regard comets as
permanent members of the solar system.

The difficulty, however, of deciding absolutely whether the orbits of
comets are really always _closed_ curves, that is to say, curves which
must sooner or later bring the bodies back again towards the sun, is,
indeed, very great. Comets, in the first place, are always so diffuse,
that it is impossible to determine their exact position, or, rather, the
exact position of that important point within them, known as the centre
of gravity. Secondly, that stretch of its orbit along which we can
follow a comet, is such a very small portion of the whole path, that the
slightest errors of observation which we make will result in
considerably altering our estimate of the actual shape of the orbit.

Comets have been described as so transparent that they can pass across
the sky without dimming the lustre of the smallest stars, which the
thinnest fog or mist would do. This is, indeed, true of every portion
of a comet except the nucleus, which is, as its name implies, the
densest part. And yet, in contrast to this ghostlike character, is the
strange fact that when comets are of a certain brightness they may
actually be seen in full daylight.

As might be gathered from their extreme tenuity, comets are so
exceedingly small in mass that they do not appear to exert any
gravitational attraction upon the other bodies of our system. It is,
indeed, a known fact that in the year 1886 a comet passed right amidst
the satellites of Jupiter without disturbing them in the slightest
degree. The attraction of the planet, on the other hand, so altered the
comet's orbit, as to cause it to revolve around the sun in a period of
seven years, instead of twenty-seven, as had previously been the case.
Also, in 1779, the comet known as Lexell's passed quite close to
Jupiter, and its orbit was so changed by that planet's attraction that
it has never been seen since. The density of comets must, as a rule, be
very much less than the one-thousandth part of that of the air at the
surface of our globe; for, if the density of the comet were even so
small as this, its mass would _not_ be inappreciable.

If comets are really undoubted members of the solar system, the
circumstances in which they were evolved must have been different from
those which produced the planets and satellites. The axial rotations of
both the latter, and also their revolutions, take place in one certain
direction;[24] their orbits, too, are ellipses which do not differ much
from circles, and which, furthermore, are situated fairly in the one
plane. Comets, on the other hand, do not necessarily travel round the
sun in the same fixed direction as the planets. Their orbits, besides,
are exceedingly elliptic; and, far from keeping to one plane, or even
near it, they approach the sun from all directions.

Broadly speaking, comets may be divided into two distinct classes, or
"families." In the first class, the same orbit appears to be shared in
common by a series of comets which travel along it, one following the
other. The comets which appeared in the years 1668, 1843, 1880, 1882,
and 1887 are instances of a number of different bodies pursuing the same
path around the sun. The members of a comet family of this kind are
observed to have similar characteristics. The idea is that such comets
are merely portions of one much larger cometary body, which became
broken up by the gravitational action of other bodies in the system, or
through violent encounter with the sun's surroundings.

The second class is composed of comets which are supposed to have been
seized by the gravitative action of certain planets, and thus forced to
revolve in short ellipses around the sun, well within the limits of the
solar system. These comets are, in consequence, spoken of as "captures."
They move around the sun in the same direction as the planets do.
Jupiter has a fairly large comet family of this kind attached to him. As
a result of his overpowering gravitation, it is imagined that during the
ages he must have attracted a large number of these bodies on his own
account, and, perhaps, have robbed other planets of their captures. His
family at present numbers about thirty. Of the other planets, so far as
we know, Saturn possesses a comet family of two, Uranus three, and
Neptune six. There are, indeed, a few comets which appear as if under
the influence of some force situated outside the known bounds of the
solar system, a circumstance which goes to strengthen the idea that
other planets may revolve beyond the orbit of Neptune. The terrestrial
planets, on the other hand, cannot have comet families; because the
enormous gravitative action of the sun in their vicinity entirely
overpowers the attractive force which they exert upon those comets which
pass close to them. Besides this, a comet, when in the inner regions of
the solar system, moves with such rapidity, that the gravitational pull
of the planets there situated is not powerful enough to deflect it to
any extent. It must not be presumed, however, that a comet once captured
should always remain a prisoner. Further disturbing causes might
unsettle its newly acquired orbit, and send it out again into the
celestial spaces.

With regard to the matter of which comets are composed, the spectroscope
shows the presence in them of hydrocarbon compounds (a notable
characteristic of these bodies), and at times, also, of sodium and iron.
Some of the light which we get from comets is, however, merely reflected
sunlight.

The fact that the tails of comets are always directed away from the sun,
has given rise to the idea that this is caused by some repelling action
emanating from the sun itself, which is continually driving off the
smallest particles. Two leading theories have been formulated to account
for the tails themselves upon the above assumption. One of these, first
suggested by Olbers in 1812, and now associated with the name of the
Russian astronomer, the late Professor Brédikhine, who carefully worked
it out, presumes an electrical action emanating from the sun; the other,
that of Arrhenius, supposes a pressure exerted by the solar light in its
radiation outwards into space. It is possible, indeed, that repelling
forces of both these kinds may be at work together. Minute particles are
probably being continually produced by friction and collisions among the
more solid parts in the heads of comets. Supposing that such particles
are driven off altogether, one may therefore assume that the so-called
captured comets are disintegrating at a comparatively rapid rate. Kepler
long ago maintained that "comets die," and this actually appears to be
the case. The ordinary periodic ones, such, for instance, as Encke's
Comet, are very faint, and becoming fainter at each return. Certain of
these comets have, indeed, failed altogether to reappear. It is notable
that the members of Jupiter's comet family are not very conspicuous
objects. They have small tails, and even in some cases have none at all.
The family, too, does not contain many members, and yet one cannot but
suppose that Jupiter, on account of his great mass, has had many
opportunities for making captures adown the ages.

Of the two theories to which allusion has above been made, that of
Brédikhine has been worked out so carefully, and with such a show of
plausibility, that it here calls for a detailed description. It appears
besides to explain the phenomena of comets' tails so much more
satisfactorily than that of Arrhenius, that astronomers are inclined to
accept it the more readily of the two. According to Brédikhine's theory
the electrical repulsive force, which he assumes for the purposes of his
argument, will drive the minutest particles of the comet in a direction
away from the sun much more readily than the gravitative action of that
body will pull them towards it. This may be compared to the ease with
which fine dust may be blown upwards, although the earth's gravitation
is acting upon it all the time.

The researches of Brédikhine, which began seriously with his
investigation of Coggia's Comet of 1874, led him to classify the tails
of comets in _three types_. Presuming that the repulsive force emanating
from the sun did not vary, he came to the conclusion that the different
forms assumed by cometary tails must be ascribed to the special action
of this force upon the various elements which happen to be present in
the comet. The tails which he classes as of the first type, are those
which are long and straight and point directly away from the sun.
Examples of such tails are found in the comets of 1811, 1843, and 1861.
Tails of this kind, he thinks, are in all probability formed of
_hydrogen_. His second type comprises those which are pointed away from
the sun, but at the same time are considerably curved, as was seen in
the comets of Donati and Coggia. These tails are formed of _hydrocarbon
gas_. The third type of tail is short, brush-like, and strongly bent,
and is formed of the _vapour of iron_, mixed with that of sodium and
other elements. It should, however, be noted that comets have
occasionally been seen which possess several tails of these various
types.

We will now touch upon a few of the best known comets of modern times.

The comet of 1680 was the first whose orbit was calculated according to
the laws of gravitation. This was accomplished by Newton, and he found
that the comet in question completed its journey round the sun in a
period of about 600 years.

In 1682 there appeared a great comet, which has become famous under the
name of Halley's Comet, in consequence of the profound investigations
made into its motion by the great astronomer, Edmund Halley. He fixed
its period of revolution around the sun at about seventy-five years, and
predicted that it would reappear in the early part of 1759. He did not,
however, live to see this fulfilled, but the comet duly returned--_the
first body of the kind to verify such a prediction_--and was detected on
Christmas Day, 1758, by George Palitzch, an amateur observer living near
Dresden. Halley also investigated the past history of the comet, and
traced it back to the year 1456. The orbit of Halley's comet passes out
slightly beyond the orbit of Neptune. At its last visit in 1835, this
comet passed comparatively close to us, namely, within five million
miles of the earth. According to the calculations of Messrs P.H. Cowell
and A.C.D. Crommelin of Greenwich Observatory, its next return will be
in the spring of 1910; the nearest approach to the earth taking place
about May 12.

On the 26th of March, 1811, a great comet appeared, which remained
visible for nearly a year and a half. It was a magnificent object; the
tail being about 100 millions of miles in length, and the head about
127,000 miles in diameter. A detailed study which he gave to this comet
prompted Olbers to put forward that theory of electrical repulsion
which, as we have seen, has since been so carefully worked out by
Brédikhine. Olbers had noticed that the particles expelled from the head
appeared to travel to the end of the tail in about eleven minutes, thus
showing a velocity per second very similar to that of light.

The discovery in 1819 of the comet known as Encke's, because its orbit
was determined by an astronomer of that name, drew attention for the
first time to Jupiter's comet family, and, indeed, to short-period
comets in general. This comet revolves around the sun in the shortest
known period of any of these bodies, namely, 3-1/3 years. Encke
predicted that it would return in 1822. This duly occurred, the comet
passing at its nearest to the sun within three hours of the time
indicated; being thus the second instance of the fulfilment of a
prediction of the kind. A certain degree of irregularity which Encke's
Comet displays in the dates of its returns to the sun, has been supposed
to indicate that it passes in the course of its orbit through some
retarding medium, but no definite conclusions have so far been arrived
at in this matter.

A comet, which appeared in 1826, goes by the name of Biela's Comet,
because of its discovery by an Austrian military officer, Wilhelm von
Biela. This comet was found to have a period of between six and seven
years. Certain calculations made by Olbers showed that, at its return in
1832, it would pass _through the earth's orbit_. The announcement of
this gave rise to a panic; for people did not wait to inquire whether
the earth would be anywhere near that part of its orbit when the comet
passed. The panic, however, subsided when the French astronomer, Arago,
showed that at the moment in question the earth would be some 50
millions of miles away from the point indicated!

[Illustration: PLATE XVII. DONATI'S COMET

From a drawing made on October 9th, 1858, by G.P. Bond, of Harvard
College Observatory, U.S.A. A good illustration of Brédikhine's theory:
note the straight tails of his _first_ type, and the curved tail of his
_second_.

(Page 257)]

In 1846, shortly after one of its returns, Biela's Comet divided into
two portions. At its next appearance (1852) these portions had separated
to a distance of about 1-1/2 millions of miles from each other. This
comet, or rather its constituents, have never since been seen.

Perhaps the most remarkable comet of recent times was that of 1858,
known as Donati's, it having been discovered at Florence by the Italian
astronomer, G.B. Donati. This comet, a magnificent object, was visible
for more than three months with the naked eye. Its tail was then 54
millions of miles in length. It was found to revolve around the sun in a
period of over 2000 years, and to go out in its journey to about 5-1/2
times the distance of Neptune. Its motion is retrograde, that is to say,
in the contrary direction to the usual movement in the solar system. A
number of beautiful drawings of Donati's Comet were made by the American
astronomer, G.P. Bond. One of the best of these is reproduced on Plate
XVII., p. 256.

In 1861 there appeared a great comet. On the 30th of June of that year
the earth and moon actually passed through its tail; but no effects were
noticed, other than a peculiar luminosity in the sky.

In the year 1881 there appeared another large comet, known as Tebbutt's
Comet, from the name of its discoverer. This was the _first comet of
which a satisfactory photograph was obtained_. The photograph in
question was taken by the late M. Janssen.

The comet of 1882 was of vast size and brilliance. It approached so
close to the sun that it passed through some 100,000 miles of the solar
corona. Though its orbit was not found to have been altered by this
experience, its nucleus displayed signs of breaking up. Some very fine
photographs of this comet were obtained at the Cape of Good Hope by Mr.
(now Sir David) Gill.

The comet of 1889 was followed with the telescope nearly up to the orbit
of Saturn, which seems to be the greatest distance at which a comet has
ever been seen.

The _first discovery of a comet by photographic means_[25] was made by
Professor Barnard in 1892; and, since then, photography has been
employed with marked success in the detection of small periodic comets.

The best comet seen in the Northern hemisphere since that of 1882,
appears to have been Daniel's Comet of 1907 (see Plate XVIII., p. 258).
This comet was discovered on June 9, 1907, by Mr. Z. Daniel, at
Princeton Observatory, New Jersey, U.S.A. It became visible to the naked
eye about mid-July of that year, and reached its greatest brilliancy
about the end of August. It did not, however, attract much popular
attention, as its position in the sky allowed it to be seen only just
before dawn.

[24] With the exception, of course, of such an anomaly as the retrograde
motion of the ninth satellite of Saturn.

[25] If we except the case of the comet which was photographed near the
solar corona in the eclipse of 1882.

[Illustration: PLATE XVIII. DANIEL'S COMET OF 1907

From a photograph taken, on August 11th, 1907, by Dr. Max Wolf, at the
Astrophysical Observatory, Heidelberg. The instrument used was a 28-inch
reflecting telescope, and the time of exposure was fifteen minutes. As
the telescope was guided to follow the moving comet, the stars have
imprinted themselves upon the photographic plate as short trails. This
is clearly the opposite to what is depicted on Plate XIII.

(Page 258)]

CHAPTER XX

REMARKABLE COMETS

If eclipses were a cause of terror in past ages, comets appear to have
been doubly so. Their much longer continuance in the sight of men had no
doubt something to say to this, and also the fact that they arrived
without warning; it not being then possible to give even a rough
prediction of their return, as in the case of eclipses. As both these
phenomena were occasional, and out of the ordinary course of things,
they drew exceptional attention as unusual events always do; for it must
be allowed that quite as wonderful things exist, but they pass unnoticed
merely because men have grown accustomed to them.

For some reason the ancients elected to class comets along with meteors,
the aurora borealis, and other phenomena of the atmosphere, rather than
with the planets and the bodies of the spaces beyond. The sudden
appearance of these objects led them to be regarded as signs sent by the
gods to announce remarkable events, chief among these being the deaths
of monarchs. Shakespeare has reminded us of this in those celebrated
lines in _Julius Cæsar_:--

"When beggars die there are no comets seen,
The heavens themselves blaze forth the death of princes."

Numbed by fear, the men of old blindly accepted these presages of fate;
and did not too closely question whether the threatened danger was to
their own nation or to some other, to their ruler or to his enemy. Now
and then, as in the case of the Roman Emperor Vespasian, there was a
cynical attempt to apply some reasoning to the portent. That emperor, in
alluding to the comet of A.D. 79, is reported to have said: "This hairy
star does not concern me; it menaces rather the King of the Parthians,
for he is hairy and I am bald." Vespasian, all the same, died shortly
afterwards!

Pliny, in his natural history, gives several instances of the terrible
significance which the ancients attached to comets. "A comet," he says,
"is ordinarily a very fearful star; it announces no small effusion of
blood. We have seen an example of this during the civil commotion of
Octavius."

A very brilliant comet appeared in 371 B.C., and about the same time an
earthquake caused Helicè and Bura, two towns in Achaia, to be swallowed
up by the sea. The following remark made by Seneca concerning it shows
that the ancients did not consider comets merely as precursors, but even
as actual _causes_ of fatal events: "This comet, so anxiously observed
by every one, _because of the great catastrophe which it produced as
soon as it appeared_, the submersion of Bura and Helicè."

Comets are by no means rare visitors to our skies, and very few years
have elapsed in historical times without such objects making their
appearance. In the Dark and Middle Ages, when Europe was split up into
many small kingdoms and principalities, it was, of course, hardly
possible for a comet to appear without the death of some ruler occurring
near the time. Critical situations, too, were continually arising in
those disturbed days. The end of Louis le Debonnaire was hastened, as
the reader will, no doubt, recollect, by the great eclipse of 840; but
it was firmly believed that a comet which had appeared a year or two
previously presaged his death. The comet of 1556 is reported to have
_influenced_ the abdication of the Emperor Charles V.; but curiously
enough, this event had already taken place before the comet made its
appearance! Such beliefs, no doubt, had a very real effect upon rulers
of a superstitious nature, or in a weak state of health. For instance,
Gian Galeazzo Visconti, Duke of Milan, was sick when the comet of 1402
appeared. After seeing it, he is said to have exclaimed: "I render
thanks to God for having decreed that my death should be announced to
men by this celestial sign." His malady then became worse, and he died
shortly afterwards.

It is indeed not improbable that such superstitious fears in monarchs
were fanned by those who would profit by their deaths, and yet did not
wish to stain their own hands with blood.

Evil though its effects may have been, this morbid interest which past
ages took in comets has proved of the greatest service to our science.
Had it not been believed that the appearance of these objects was
attended with far-reaching effects, it is very doubtful whether the old
chroniclers would have given themselves the trouble of alluding to them
at all; and thus the modern investigators of cometary orbits would have
lacked a great deal of important material.

We will now mention a few of the most notable comets which historians
have recorded.

A comet which appeared in 344 B.C. was thought to betoken the success
of the expedition undertaken in that year by Timoleon of Corinth against
Sicily. "The gods by an extraordinary prodigy announced his success and
future greatness: a burning torch appeared in the heavens throughout the
night and preceded the fleet of Timoleon until it arrived off the coast
of Sicily."

The comet of 43 B.C. was generally believed to be the soul of Cæsar on
its way to heaven.

Josephus tells us that in A.D. 69 several prodigies, and amongst them a
comet in the shape of a sword, announced the destruction of Jerusalem.
This comet is said to have remained over the city for the space of a
year!

A comet which appeared in A.D. 336 was considered to have announced the
death of the Emperor Constantine.

But perhaps the most celebrated comet of early times was the one which
appeared in A.D. 1000. That year was, in more than one way, big with
portent, for there had long been a firm belief that the Christian era
could not possibly run into four figures. Men, indeed, steadfastly
believed that when the thousand years had ended, the millennium would
immediately begin. Therefore they did not reap neither did they sow,
they toiled not, neither did they spin, and the appearance of the comet
strengthened their convictions. The fateful year, however, passed by
without anything remarkable taking place; but the neglect of husbandry
brought great famine and pestilence over Europe in the years which
followed.

In April 1066, that year fraught with such immense consequences for
England, a comet appeared. No one doubted but that it was a presage of
the success of the Conquest, and perhaps, indeed, it had its due weight
in determining the minds and actions of the men who took part in the
expedition. _Nova stella, novus rex_ ("a new star, a new sovereign") was
a favourite proverb of the time. The chroniclers, with one accord, have
delighted to relate that the Normans, "guided by a comet," invaded
England. A representation of this object appears in the Bayeux Tapestry
(see Fig. 19, p. 263).[26]

[Illustration: FIG. 19.--The comet of 1066, as represented in the Bayeux
Tapestry.

(From the _World of Comets_.)]

We have mentioned Halley's Comet of 1682, and how it revisits the
neighbourhood of the earth at intervals of seventy-six years. The comet
of 1066 has for many years been supposed to be Halley's Comet on one of
its visits. The identity of these two, however, was only quite recently
placed beyond all doubt by the investigations of Messrs Cowell and
Crommelin. This comet appeared also in 1456, when John Huniades was
defending Belgrade against the Turks led by Mahomet II., the conqueror
of Constantinople, and is said to have paralysed both armies with fear.

The Middle Ages have left us descriptions of comets, which show only too
well how the imagination will run riot under the stimulus of terror. For
instance, the historian, Nicetas, thus describes the comet of the year
1182: "After the Romans were driven from Constantinople a prognostic was
seen of the excesses and crimes to which Andronicus was to abandon
himself. A comet appeared in the heavens similar to a writhing serpent;
sometimes it extended itself, sometimes it drew itself in; sometimes, to
the great terror of the spectators, it opened a huge mouth; it seemed
that, as if thirsting for human blood, it was upon the point of
satiating itself." And, again, the celebrated Ambrose Paré, the father
of surgery, has left us the following account of the comet of 1528,
which appeared in his own time: "This comet," said he, "was so horrible,
so frightful, and it produced such great terror in the vulgar, that some
died of fear, and others fell sick. It appeared to be of excessive
length, and was of the colour of blood. At the summit of it was seen the
figure of a bent arm, holding in its hand a great sword, as if about to
strike. At the end of the point there were three stars. On both sides of
the rays of this comet were seen a great number of axes, knives,
blood-coloured swords, among which were a great number of hideous human
faces, with beards and bristling hair." Paré, it is true, was no
astronomer; yet this shows the effect of the phenomenon, even upon a man
of great learning, as undoubtedly he was. It should here be mentioned
that nothing very remarkable happened at or near the year 1528.

Concerning the comet of 1680, the extraordinary story got about that, at
Rome, a hen had laid an egg on which appeared a representation of the
comet!

But the superstitions with regard to comets were now nearing their end.
The last blow was given by Halley, who definitely proved that they
obeyed the laws of gravitation, and circulated around the sun as planets
do; and further announced that the comet of 1682 had a period of
seventy-six years, which would cause it to reappear in the year 1759. We
have seen how this prediction was duly verified. We have seen, too, how
this comet appeared again in 1835, and how it is due to return in the
early part of 1910.

[26] With regard to the words "Isti mirant stella" in the figure, Mr.
W.T. Lynn suggests that they may not, after all, be the grammatically
bad Latin which they appear, but that the legend is really "Isti
mirantur stellam," the missing letters being supposed to be hidden by
the building and the comet.

CHAPTER XXI

METEORS OR SHOOTING STARS

Any one who happens to gaze at the sky for a short time on a clear night
is pretty certain to be rewarded with a view of what is popularly known
as a "shooting star." Such an object, however, is not a star at all, but
has received its appellation from an analogy; for the phenomenon gives
to the inexperienced in these matters an impression as if one of the
many points of light, which glitter in the vaulted heaven, had suddenly
become loosened from its place, and was falling towards the earth. In
its passage across the sky the moving object leaves behind a trail of
light which usually lasts for a few moments. Shooting stars, or meteors,
as they are technically termed, are for the most part very small bodies,
perhaps no larger than peas or pebbles, which, dashing towards our earth
from space beyond, are heated to a white heat, and reduced to powder by
the friction resulting from their rapid passage into our atmosphere.
This they enter at various degrees of speed, in some cases so great as
45 miles a second. The speed, of course, will depend greatly upon
whether the earth and the meteors are rushing towards each other, or
whether the latter are merely overtaking the earth. In the first of
these cases the meteors will naturally collide with the atmosphere with
great force; in the other case they will plainly come into it with much
less rapidity. As has been already stated, it is from observations of
such bodies that we are enabled to estimate, though very imperfectly,
the height at which the air around our globe practically ceases, and
this height is imagined to be somewhere about 100 miles. Fortunate,
indeed, is it for us that there is a goodly layer of atmosphere over our
heads, for, were this not so, these visitors from space would strike
upon the surface of our earth night and day, and render existence still
more unendurable than many persons choose to consider it. To what a
bombardment must the moon be continually subject, destitute as she is of
such an atmospheric shield!

It is only in the moment of their dissolution that we really learn
anything about meteors, for these bodies are much too small to be seen
before they enter our atmosphere. The débris arising from their
destruction is wafted over the earth, and, settling down eventually upon
its surface, goes to augment the accumulation of that humble domestic
commodity which men call dust. This continual addition of material
tends, of course, to increase the mass of the earth, though the effect
thus produced will be on an exceedingly small scale.

The total number of meteors moving about in space must be practically
countless. The number which actually dash into the earth's atmosphere
during each year is, indeed, very great. Professor Simon Newcomb, the
well-known American astronomer, has estimated that, of the latter, those
large enough to be seen with the naked eye cannot be in all less than
146,000,000,000 per annum. Ten times more numerous still are thought to
be those insignificant ones which are seen to pass like mere sparks of
light across the field of an observer's telescope.

Until comparatively recent times, perhaps up to about a hundred years
ago, it was thought that meteors were purely terrestrial phenomena which
had their origin in the upper regions of the air. It, however, began to
be noticed that at certain periods of the year these moving objects
appeared to come from definite areas of the sky. Considerations,
therefore, respecting their observed velocities, directions, and
altitudes, gave rise to the theory that they are swarms of small bodies
travelling around the sun in elongated elliptical orbits, all along the
length of which they are scattered, and that the earth, in its annual
revolution, rushing through the midst of such swarms at the same epoch
each year, naturally entangles many of them in its atmospheric net.

The dates at which the earth is expected to pass through the principal
meteor-swarms are now pretty well known. These swarms are distinguished
from one another by the direction of the sky from which the meteors seem
to arrive. Many of the swarms are so wide that the earth takes days, and
even weeks, to pass through them. In some of these swarms, or streams,
as they are also called, the meteors are distributed with fair evenness
along the entire length of their orbits, so that the earth is greeted
with a somewhat similar shower at each yearly encounter. In others, the
chief portions are bunched together, so that, in certain years, the
display is exceptional (see Fig. 20, p. 269). That part of the heavens
from which a shower of meteors is seen to emanate is called the
"radiant," or radiant point, because the foreshortened view we get of
the streaks of light makes it appear as if they radiated outwards from
this point. In observations of these bodies the attention of astronomers
is directed to registering the path and speed of each meteor, and to
ascertaining the position of the radiant. It is from data such as these
that computations concerning the swarms and their orbits are made.

[Illustration: FIG. 20.--Passage of the Earth through the thickest
portion of a Meteor Swarm. The Earth and the Meteors are here
represented as approaching each other from opposite directions.]

For the present state of knowledge concerning meteors, astronomy is
largely indebted to the researches of Mr. W.F. Denning, of Bristol, and
of the late Professor A.S. Herschel.

During the course of each year the earth encounters a goodly number of
meteor-swarms. Three of these, giving rise to fine displays, are very
well known--the "Perseids," or August Meteors, and the "Leonids" and
"Bielids," which appear in November.

Of the above three the _Leonid_ display is by far the most important,
and the high degree of attention paid to it has laid the foundation of
meteoric astronomy in much the same way that the study of the
fascinating corona has given such an impetus to our knowledge of the
sun. The history of this shower of meteors may be traced back as far as
A.D. 902, which was known as the "Year of the Stars." It is related that
in that year, on the night of October 12th--the shower now comes about a
month later--whilst the Moorish King, Ibrahim Ben Ahmed, lay dying
before Cosenza, in Calabria, "a multitude of falling stars scattered
themselves across the sky like rain," and the beholders shuddered at
what they considered a dread celestial portent. We have, however, little
knowledge of the subsequent history of the Leonids until 1698, since
which time the maximum shower has appeared with considerable regularity
at intervals of about thirty-three years. But it was not until 1799 that
they sprang into especial notice. On the 11th November in that year a
splendid display was witnessed at Cumana, in South America, by the
celebrated travellers, Humboldt and Bonpland. Finer still, and
surpassing all displays of the kind ever seen, was that of November 12,
1833, when the meteors fell thick as snowflakes, 240,000 being estimated
to have appeared during seven hours. Some of them were even so bright as
to be seen in full daylight. The radiant from which the meteors seem to
diverge was ascertained to be situated in the head of the constellation
of the Lion, or "Sickle of Leo," as it is popularly termed, whence
their name--Leonids. It was from a discussion of the observations then
made that the American astronomer, Olmsted, concluded that these meteors
sprang upon us from interplanetary space, and were not, as had been
hitherto thought, born of our atmosphere. Later on, in 1837, Olbers
formulated the theory that the bodies in question travelled around the
sun in an elliptical orbit, and at the same time he established the
periodicity of the maximum shower.

The periodic time of recurrence of this maximum, namely, about
thirty-three years, led to eager expectancy as 1866 drew near. Hopes
were then fulfilled, and another splendid display took place, of which
Sir Robert Ball, who observed it, has given a graphic description in his
_Story of the Heavens_. The display was repeated upon a smaller scale in
the two following years. The Leonids were henceforth deemed to hold an
anomalous position among meteor swarms. According to theory the earth
cut through their orbit at about the same date each year, and so a
certain number were then seen to issue from the radiant. But, in
addition, after intervals of thirty-three years, as has been seen, an
exceptional display always took place; and this state of things was not
limited to one year alone, but was repeated at each meeting for about
three years running. The further assumption was, therefore, made that
the swarm was much denser in one portion of the orbit than
elsewhere,[27] and that this congested part was drawn out to such an
extent that the earth could pass through the crossing place during
several annual meetings, and still find it going by like a long
procession (see Fig. 20, p. 269).

In accordance with this ascertained period of thirty-three years, the
recurrence of the great Leonid shower was timed to take place on the
15th of November 1899. But there was disappointment then, and the
displays which occurred during the few years following were not of much
importance. A good deal of comment was made at the time, and theories
were accordingly put forward to account for the failure of the great
shower. The most probable explanation seems to be, that the attraction
of one of the larger planets--Jupiter perhaps--has diverted the orbit
somewhat from its old position, and the earth does not in consequence
cut through the swarm in the same manner as it used to do.

The other November display alluded to takes place between the 23rd and
27th of that month. It is called the _Andromedid_ Shower, because the
meteors appear to issue from the direction of the constellation of
Andromeda, which at that period of the year is well overhead during the
early hours of the night. These meteors are also known by the name of
_Bielids_, from a connection which the orbit assigned to them appears to
have with that of the well-known comet of Biela.

M. Egenitis, Director of the Observatory of Athens, accords to the
Bielids a high antiquity. He traces the shower back to the days of the
Emperor Justinian. Theophanes, the Chronicler of that epoch, writing of
the famous revolt of Nika in the year A.D. 532, says:--"During the same
year a great fall of stars came from the evening till the dawn." M.
Egenitis notes another early reference to these meteors in A.D. 752,
during the reign of the Eastern Emperor, Constantine Copronymous.
Writing of that year, Nicephorus, a Patriarch of Constantinople, has as
follows:--"All the stars appeared to be detached from the sky, and to
fall upon the earth."

The Bielids, however, do not seem to have attracted particular notice
until the nineteenth century. Attention first began to be riveted upon
them on account of their suspected connection with Biela's comet. It
appeared that the same orbit was shared both by that comet and the
Bielid swarm. It will be remembered that the comet in question was not
seen after its appearance in 1852. Since that date, however, the Bielid
shower has shown an increased activity; which was further noticed to be
especially great in those years in which the comet, had it still
existed, would be due to pass near the earth.

The third of these great showers to which allusion has above been made,
namely, the _Perseids_, strikes the earth about the 10th of August; for
which reason it is known on the Continent under the name of the "tears
of St. Lawrence," the day in question being sacred to that Saint. This
shower is traceable back many centuries, even as far as the year A.D.
811. The name given to these meteors, "Perseids," arises from the fact
that their radiant point is situated in the constellation of Perseus.
This shower is, however, not by any means limited to the particular
night of August 10th, for meteors belonging to the swarm may be observed
to fall in more or less varying quantities from about July 8th to August
22nd. The Perseid meteors sometimes fall at the rate of about sixty per
hour. They are noted for their great rapidity of motion, and their
trails besides often persist for a minute or two before being
disseminated. Unlike the other well-known showers, the radiants of which
are stationary, that of the Perseids shifts each night a little in an
easterly direction.

The orbit of the Perseids cuts that of the earth almost perpendicularly.
The bodies are generally supposed to be the result of the disintegration
of an ancient comet which travelled in the same orbit. Tuttle's Comet,
which passed close to the earth in 1862, also belongs to this orbit; and
its period of revolution is calculated to be 131 years. The Perseids
appear to be disseminated all along this great orbit, for we meet them
in considerable quantities each year. The bodies in question are in
general particularly small. The swarm has, however, like most others, a
somewhat denser portion, and through this the earth passed in 1848. The
_aphelion_, or point where the far end of the orbit turns back again
towards the sun, is situated right away beyond the path of Neptune, at a
distance of forty-eight times that of the earth from the sun. The comet
of 1532 also belongs to the Perseid orbit. It revisited the
neighbourhood of the earth in 1661, and should have returned in 1789.
But we have no record of it in that year; for which omission the then
politically disturbed state of Europe may account. If not already
disintegrated, this comet is due to return in 1919.

This supposed connection between comets and meteor-swarms must be also
extended to the case of the Leonids. These meteors appear to travel
along the same track as Tempel's Comet of 1866.

It is considered that the attractions of the various bodies of the
solar system upon a meteor swarm must eventually result in breaking up
the "bunched" portion, so that in time the individual meteors should
become distributed along the whole length of the orbit. Upon this
assumption the Perseid swarm, in which the meteors are fairly well
scattered along its path, should be of greater age than the Leonid. As
to the Leonid swarm itself, Le Verrier held that it was first brought
into the solar system in A.D. 126, having been captured from outer space
by the gravitative action of the planet Uranus.

The acknowledged theory of meteor swarms has naturally given rise to an
idea, that the sunlight shining upon such a large collection of
particles ought to render a swarm visible before its collision with the
earth's atmosphere. Several attempts have therefore been made to search
for approaching swarms by photography, but, so far, it appears without
success. It has also been proposed, by Mr. W.H.S. Monck, that the stars
in those regions from which swarms are due, should be carefully watched,
to see if their light exhibits such temporary diminutions as would be
likely to arise from the momentary interposition of a cloud of moving
particles.

Between ten and fifteen years ago it happened that several well-known
observers, employed in telescopic examination of the sun and moon,
reported that from time to time they had seen small dark bodies,
sometimes singly, sometimes in numbers, in passage across the discs of
the luminaries. It was concluded that these were meteors moving in space
beyond the atmosphere of the earth. The bodies were called "dark
meteors," to emphasise the fact that they were seen in their natural
condition, and not in that momentary one in which they had hitherto been
always seen; _i.e._ when heated to white heat, and rapidly vaporised, in
the course of their passage through the upper regions of our air. This
"discovery" gave promise of such assistance to meteor theories, that
calculations were made from the directions in which they had been seen
to travel, and the speeds at which they had moved, in the hope that some
information concerning their orbits might be revealed. But after a while
some doubt began to be thrown upon their being really meteors, and
eventually an Australian observer solved the mystery. He found that they
were merely tiny particles of dust, or of the black coating on the inner
part of the tube of the telescope, becoming detached from the sides of
the eye-piece and falling across the field of view. He was led to this
conclusion by having noted that a gentle tapping of his instrument
produced the "dark" bodies in great numbers! Thus the opportunity of
observing meteors beyond our atmosphere had once more failed.

_Meteorites_, also known as ærolites and fireballs, are usually placed
in quite a separate category from meteors. They greatly exceed the
latter in size, are comparatively rare, and do not appear in any way
connected with the various showers of meteors. The friction of their
passage through the atmosphere causes them to shine with a great light;
and if not shattered to pieces by internal explosions, they reach the
ground to bury themselves deep in it with a great rushing and noise.
When found by uncivilised peoples, or savages, they are, on account of
their celestial origin, usually regarded as objects of wonder and of
worship, and thus have arisen many mythological legends and deifications
of blackened stones. On the other hand, when they get into the
possession of the civilised, they are subjected to careful examinations
and tests in chemical laboratories. The bodies are, as a rule, composed
of stone, in conjunction with iron, nickel, and such elements as exist
in abundance upon our earth; though occasionally specimens are found
which are practically pure metal. In the museums of the great capitals
of both Continents are to be seen some fine collections of meteorites.
Several countries--Greenland and Mexico, for instance--contain in the
soil much meteoric iron, often in masses so large as to baffle all
attempts at removal. Blocks of this kind have been known to furnish the
natives in their vicinity for many years with sources of workable iron.

The largest meteorite in the world is one known as the Anighito
meteorite. It was brought to the United States by the explorer Peary,
who found it at Cape York in Greenland. He estimates its weight at from
90 to 100 tons. One found in Mexico, called the Bacubirito, comes next,
with an estimated weight of 27-1/2 tons. The third in size is the
Willamette meteorite, found at Willamette in Oregon in 1902. It measures
10 × 6-1/2 × 4-1/2 feet, and weighs about 15-1/2 tons.

[27] The "gem" of the meteor ring, as it has been termed.

CHAPTER XXII

THE STARS

In the foregoing chapters we have dealt at length with those celestial
bodies whose nearness to us brings them into our especial notice. The
entire room, however, taken up by these bodies, is as a mere point in
the immensities of star-filled space. The sun, too, is but an ordinary
star; perhaps quite an insignificant one[28] in comparison with the
majority of those which stud that background of sky against which the
planets are seen to perform their wandering courses.

Dropping our earth and the solar system behind, let us go afield and
explore the depths of space.

We have seen how, in very early times, men portioned out the great mass
of the so-called "fixed stars" into divisions known as constellations.
The various arrangements, into which the brilliant points of light fell
as a result of perspective, were noticed and roughly compared with such
forms as were familiar to men upon the earth. Imagination quickly saw in
them the semblances of heroes and of mighty fabled beasts; and, around
these monstrous shapes, legends were woven, which told how the great
deeds done in the misty dawn of historical time had been enshrined by
the gods in the sky as an example and a memorial for men. Though the
centuries have long outlived such fantasies, yet the constellation
figures and their ancient names have been retained to this day, pretty
well unaltered for want of any better arrangement. The Great and Little
Bears, Cassiopeia, Perseus, and Andromeda, Orion and the rest, glitter
in our night skies just as they did centuries and centuries ago.

Many persons seem to despair of gaining any real knowledge of astronomy,
merely because they are not versed in recognising the constellations.
For instance, they will say:--"What is the use of my reading anything
about the subject? Why, I believe I couldn't even point out the Great
Bear, were I asked to do so!" But if such persons will only consider for
a moment that what we call the Great Bear has no existence in fact, they
need not be at all disheartened. Could we but view this familiar
constellation from a different position in space, we should perhaps be
quite unable to recognise it. Mountain masses, for instance, when seen
from new directions, are often unrecognisable.

It took, as we have seen, a very long time for men to acknowledge the
immense distances of the stars from our earth. Their seeming
unchangeableness of position was, as we have seen, largely responsible
for the idea that the earth was immovable in space. It is a wonder that
the Copernican system ever gained the day in the face of this apparent
fixity of the stars. As time went on, it became indeed necessary to
accord to these objects an almost inconceivable distance, in order to
account for the fact that they remained apparently quite undisplaced,
notwithstanding the journey of millions of miles which the earth was now
acknowledged to make each year around the sun. In the face of the
gradual and immense improvement in telescopes, this apparent immobility
of the stars was, however, not destined to last. The first ascertained
displacement of a star, namely that of 61 Cygni, noted by Bessel in the
year 1838, definitely proved to men the truth of the Copernican system.
Since then some forty more stars have been found to show similar tiny
displacements. We are, therefore, in possession of the fact, that the
actual distances of a few out of the great host can be calculated.

To mention some of these. The nearest star to the earth, so far as we
yet know, is Alpha Centauri, which is distant from us about 25 billions
of miles. The light from this star, travelling at the stupendous rate of
about 186,000 miles per second, takes about 4-1/4 years to reach our
earth, or, to speak astronomically, Alpha Centauri is about 4-1/4 "light
years" distant from us. Sirius--the brightest star in the whole sky--is
at twice this distance, _i.e._ about 8-1/2 light years. Vega is about 30
light years distant from us, Capella about 32, and Arcturus about 100.

The displacements, consequent on the earth's movement, have, however,
plainly nothing to say to any real movements on the part of the stars
themselves. The old idea was that the stars were absolutely fixed; hence
arose the term "fixed stars"--a term which, though inaccurate, has not
yet been entirely banished from the astronomical vocabulary. But careful
observations extending over a number of years have shown slight changes
of position among these bodies; and such alterations cannot be ascribed
to the revolution of the earth in its orbit, for they appear to take
place in every direction. These evidences of movement are known as
"proper motions," that is to say, actual motions in space proper to the
stars themselves. Stars which are comparatively near to us show, as a
rule, greater proper motions than those which are farther off. It must
not, however, be concluded that these proper motions are of any very
noticeable amounts. They are, as a matter of fact, merely upon the same
apparently minute scale as other changes in the heavens; and would
largely remain unnoticed were it not for the great precision of modern
astronomical instruments.

One of the swiftest moving of the stars is a star of the sixth magnitude
in the constellation of the Great Bear; which is known as "1830
Groombridge," because this was the number assigned to it in a catalogue
of stars made by an astronomer of that name. It is popularly known as
the "Runaway Star," a name given to it by Professor Newcomb. Its speed
is estimated to be at least 138 miles per second. It may be actually
moving at a much greater rate, for it is possible that we see its path
somewhat foreshortened.

A still greater proper motion--the greatest, in fact, known--is that of
an eighth magnitude star in the southern hemisphere, in the
constellation of Pictor. Nothing, indeed, better shows the enormous
distance of the stars from us, and the consequent inability of even such
rapid movements to alter the appearance of the sky during the course of
ages, than the fact that it would take more than two centuries for the
star in question to change its position in the sky by a space equal to
the apparent diameter of the moon; a statement which is equivalent to
saying that, were it possible to see this star with the naked eye, which
it is not, at least twenty-five years would have to elapse before one
would notice that it had changed its place at all!

Both the stars just mentioned are very faint. That in Pictor is, as has
been said, not visible to the naked eye. It appears besides to be a very
small body, for Sir David Gill finds a parallax which makes it only as
far off from us as Sirius. The Groombridge star, too, is just about the
limit of ordinary visibility. It is, indeed, a curious fact that the
fainter stars seem, on the average, to be moving more rapidly than the
brighter.

Investigations into proper motions lead us to think that every one of
the stars must be moving in space in some particular direction. To take
a few of the best known. Sirius and Vega are both approaching our system
at a rate of about 10 miles per second, Arcturus at about 5 miles per
second, while Capella is receding from us at about 15 miles per second.
Of the twin brethren, Castor and Pollux, Castor is moving away from us
at about 4-1/2 miles per second, while Pollux is coming towards us at
about 33 miles per second.

Much of our knowledge of proper motions has been obtained indirectly by
means of the spectroscope, on the Doppler principle already treated of,
by which we are enabled to ascertain whether a source from which light
is coming is approaching or receding.

The sun being, after all, a mere star, it will appear only natural for
it also to have a proper motion of its own. This is indeed the case; and
it is rushing along in space at a rate of between ten and twelve miles
per second, carrying with it its whole family of planets and satellites,
of comets and meteors. The direction in which it is advancing is towards
a point in the constellation of Lyra, not far from its chief star Vega.
This is shown by the fact that the stars about the region in question
appear to be opening out slightly, while those in the contrary portion
of the sky appear similarly to be closing together.

Sir William Herschel was the first to discover this motion of the sun
through space; though in the idea that such a movement might take place
he seems to have been anticipated by Mayer in 1760, by Michell in 1767,
and by Lalande in 1776.

A suggestion has been made that our solar system, in its motion through
the celestial spaces, may occasionally pass through regions where
abnormal magnetic conditions prevail, in consequence of which
disturbances may manifest themselves throughout the system at the same
instant. Thus the sun may be getting the credit of _producing_ what it
merely reacts to in common with the rest of its family. But this
suggestion, plausible though it may seem, will not explain why the
magnetic disturbances experienced upon our earth show a certain
dependence upon such purely local facts, as the period of the sun's
rotation, for instance.

One would very much like to know whether the movement of the sun is
along a straight line, or in an enormous orbit around some centre. The
idea has been put forward that it may be moving around the centre of
gravity of the whole visible stellar universe. Mädler, indeed,
propounded the notion that Alcyone--the chief star in the group known as
the Pleiades--occupied this centre, and that everything revolved around
it. He went even further to proclaim that here was the Place of the
Almighty, the Mansion of the Eternal! But Mädler's ideas upon this point
have long been shelved.

To return to the general question of the proper motion of stars.

In several instances these motions appear to take place in groups, as if
certain stars were in some way associated together. For example, a large
number of the stars composing the Pleiades appear to be moving through
space in the same direction. Also, of the seven stars composing the
Plough, all but two--the star at the end of its "handle," and that one
of the "pointers," as they are called, which is the nearer to the pole
star--have a common proper motion, _i.e._ are moving in the same
direction and nearly at the same rate.

Further still, the well-known Dutch astronomer, Professor Kapteyn, of
Groningen, has lately reached the astonishing conclusion that a great
part of the visible universe is occupied by two vast streams of stars
travelling in opposite directions. In both these great streams, the
individual bodies are found, besides, to be alike in design, alike in
chemical constitution, and alike in the stage of their development.

A fable related by the Persian astronomer, Al Sufi (tenth century, A.D.)
shows well the changes in the face of the sky which proper motions are
bound to produce after great lapses of time. According to this fable the
stars Sirius and Procyon were the sisters of the star Canopus. Canopus
married Rigel (another star,) but, having murdered her, he fled towards
the South Pole, fearing the anger of his sisters. The fable goes on to
relate, among other things, that Sirius followed him across the Milky
Way. Mr. J. E. Gore, in commenting on the story, thinks that it may be
based upon a tradition of Sirius having been seen by the men of the
Stone Age on the opposite side of the Milky Way to that on which it now
is.

Sirius is in that portion of the heavens _from_ which the sun is
advancing. Its proper motion is such that it is gaining upon the earth
at the rate of about ten miles per second, and so it must overtake the
sun after the lapse of great ages. Vega, on the other hand, is coming
towards us from that part of the sky _towards_ which the sun is
travelling. It should be about half a million years before the sun and
Vega pass by one another. Those who have specially investigated this
question say that, as regards the probability of a near approach, it is
much more likely that Vega will be then so far to one side of the sun,
that her brightness will not be much greater than it is at this moment.

Considerations like these call up the chances of stellar collisions.
Such possibilities need not, however, give rise to alarm; for the stars,
as a rule, are at such great distances from each other, that the
probability of relatively near approaches is slight.

We thus see that the constellations do not in effect exist, and that
there is in truth no real background to the sky. We find further that
the stars are strewn through space at immense distances from each other,
and are moving in various directions hither and thither. The sun, which
is merely one of them, is moving also in a certain direction, carrying
the solar system along with it. It seems, therefore, but natural to
suppose that many a star may be surrounded by some planetary system in a
way similar to ours, which accompanies it through space in the course of
its celestial journeyings.

[28] Vega, for instance, shines one hundred times more brightly than the
sun would do, were it to be removed to the distance at which that star
is from us.

CHAPTER XXIII

THE STARS--_continued_

The stars appear to us to be scattered about the sky without any orderly
arrangement. Further, they are of varying degrees of brightness; some
being extremely brilliant, whilst others can but barely be seen. The
brightness of a star may arise from either of two causes. On the one
hand, the body may be really very bright in itself; on the other hand,
it may be situated comparatively near to us. Sometimes, indeed, both
these circumstances may come into play together.

Since variation in brightness is the most noticeable characteristic of
the stars, men have agreed to class them in divisions called
"magnitudes." This term, it must be distinctly understood, is employed
in such classification without any reference whatever to actual size,
being merely taken to designate roughly the amount of light which we
receive from a star. The twenty brightest stars in the sky are usually
classed in the first magnitude. In descending the scale, each magnitude
will be noticed to contain, broadly speaking, three times as many stars
as the one immediately above it. Thus the second magnitude contains 65,
the third 190, the fourth 425, the fifth 1100, and the sixth 3200. The
last of these magnitudes is about the limit of the stars which we are
able to see with the naked eye. Adding, therefore, the above numbers
together, we find that, without the aid of the telescope, we cannot see
more than about 5000 stars in the entire sky--northern and southern
hemispheres included. Quite a small telescope will, however, allow us to
see down to the ninth magnitude, so that the total number of stars
visible to us with such very moderate instrumental means will be well
over 100,000.

It must not, however, be supposed that the stars included within each
magnitude are all of exactly the same brightness. In fact, it would be
difficult to say if there exist in the whole sky two stars which send us
precisely the same amount of light. In arranging the magnitudes, all
that was done was to make certain broad divisions, and to class within
them such stars as were much on a par with regard to brightness. It may
here be noted that a standard star of the first magnitude gives us about
one hundred times as much light as a star of the sixth magnitude, and
about one million times as much as one of the sixteenth magnitude--which
is near the limit of what we can see with the very best telescope.

Though the first twenty stars in the sky are popularly considered as
being of the first magnitude, yet several of them are much brighter than
an average first magnitude star would be. For instance, Sirius--the
brightest star in the whole sky--is equal to about eleven first
magnitude stars, like, say, Aldebaran. In consequence of such
differences, astronomers are agreed in classifying the brightest of them
as _brighter_ than the standard first magnitude star. On this principle
Sirius would be about two and a half magnitudes _above_ the first. This
notation is usefully employed in making comparisons between the amount
of light which we receive from the sun, and that which we get from an
individual star. Thus the sun will be about twenty-seven and a half
magnitudes _above_ the first magnitude. The range, therefore, between
the light which we receive from the sun (considered merely as a very
bright star) and the first magnitude stars is very much greater than
that between the latter and the faintest star which can be seen with the
telescope, or even registered upon the photographic plate.

To classify stars merely by their magnitudes, without some definite note
of their relative position in the sky, would be indeed of little avail.
We must have some simple method of locating them in the memory, and the
constellations of the ancients here happily come to our aid. A system
combining magnitudes with constellations was introduced by Bayer in
1603, and is still adhered to. According to this the stars in each
constellation, beginning with the brightest star, are designated by the
letters of the Greek alphabet taken in their usual order. For example,
in the constellation of Canis Major, or the Greater Dog, the brightest
star is the well-known Sirius, called by the ancients the "Dog Star";
and this star, in accordance with Bayer's method, has received the Greek
letter [a] (alpha), and is consequently known as Alpha Canis
Majoris.[29] As soon as the Greek letters are used up in this way the
Roman alphabet is brought into requisition, after which recourse is had
to ordinary numbers.

Notwithstanding this convenient arrangement, some of the brightest
stars are nearly always referred to by certain proper names given to
them in old times. For instance, it is more usual to speak of Sirius,
Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of
[a] Canis Majoris, [a] Boötis, [a] Lyræ, [a] Aurigæ, [a] Canis Minoris,
[a] Tauri, [a] Leonis, &c. &c.

In order that future generations might be able to ascertain what changes
were taking place in the face of the sky, astronomers have from time to
time drawn up catalogues of stars. These lists have included stars of a
certain degree of brightness, their positions in the sky being noted
with the utmost accuracy possible at the period. The earliest known
catalogue of this kind was made, as we have seen, by the celebrated
Greek astronomer, Hipparchus, about the year 125 B.C. It contained 1080
stars. It was revised and brought up to date by Ptolemy in A.D. 150.
Another celebrated list was that drawn up by the Persian astronomer, Al
Sufi, about the year A.D. 964. In it 1022 stars were noted down. A
catalogue of 1005 stars was made in 1580 by the famous Danish
astronomer, Tycho Brahe. Among modern catalogues that of Argelander
(1799-1875) contained as many as 324,198 stars. It was extended by
Schönfeld so as to include a portion of the Southern Hemisphere, in
which way 133,659 more stars were added.

In recent years a project was placed on foot of making a photographic
survey of the sky, the work to be portioned out among various nations. A
great part of this work has already been brought to a conclusion. About
15,000,000 stars will appear upon the plates; but, so far, it has been
proposed to catalogue only about a million and a quarter of the
brightest of them. This idea of surveying the face of the sky by
photography sprang indirectly from the fine photographs which Sir David
Gill took, when at the Cape of Good Hope, of the Comet of 1882. The
immense number of star-images which had appeared upon his plates
suggested the idea that photography could be very usefully employed to
register the relative positions of the stars.

The arrangement of seven stars known as the "Plough" is perhaps the most
familiar configuration in the sky (see Plate XIX., p. 292). In the
United States it is called the "Dipper," on account of its likeness to
the outline of a saucepan, or ladle. "Charles' Wain" was the old English
name for it, and readers of Cæsar will recollect it under
_Septentriones_, or the "Seven Stars," a term which that writer uses as
a synonym for the North. Though identified in most persons' minds with
_Ursa Major_, or the Great Bear, the Plough is actually only a small
portion of that famous constellation. Six out of the seven stars which
go to make up the well-known figure are of the second magnitude, while
the remaining one, which is the middle star of the group, is of the
third.

The Greek letters, as borne by the individual stars of the Plough, are a
plain transgression of Bayer's method as above described, for they have
certainly not been allotted here in accordance with the proper order of
brightness. For instance, the third magnitude star, just alluded to as
being in the middle of the group, has been marked with the Greek letter
[d] (Delta); and so is made to take rank _before_ the stars composing
what is called the "handle" of the Plough, which are all of the second
magnitude. Sir William Herschel long ago drew attention to the irregular
manner in which Bayer's system had been applied. It is, indeed, a great
pity that this notation was not originally worked out with greater care
and correctness; for, were it only reliable, it would afford great
assistance to astronomers in judging of what changes in relative
brightness have taken place among the stars.

Though we may speak of using the constellations as a method of finding
our way about the sky, it is, however, to certain marked groupings in
them of the brighter stars that we look for our sign-posts.

Most of the constellations contain a group or so of noticeable stars,
whose accidental arrangement dimly recalls the outline of some familiar
geometrical figure and thus arrests the attention.[30] For instance, in
an almost exact line with the two front stars of the Plough, or
"pointers" as they are called,[31] and at a distance about five times as
far away as the interval between them, there will be found a third star
of the second magnitude. This is known as Polaris, or the Pole Star, for
it very nearly occupies that point of the heaven towards which the north
pole of the earth's axis is _at present_ directed (see Plate XIX., p.
292). Thus during the apparently daily rotation of the heavens, this
star looks always practically stationary. It will, no doubt, be
remembered how Shakespeare has put into the mouth of Julius Cæsar these
memorable words:--

"But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament."

[Illustration: PLATE XIX. THE SKY AROUND THE NORTH POLE

We see here the Plough, the Pole Star, Ursa Minor, Auriga, Cassiopeia's
Chair, and Lyra. Also the Circle of Precession, along which the Pole
makes a complete revolution in a period of 25,868 years, and the
Temporary Star discovered by Tycho Brahe in the year 1572.

(Page 291)]

On account of the curvature of the earth's surface, the height at which
the Pole Star is seen above the horizon at any place depends regularly
upon the latitude; that is to say, the distance of the place in question
from the equator. For instance, at the north pole of the earth, where
the latitude is greatest, namely, 90°, the Pole Star will appear
directly overhead; whereas in England, where the latitude is about 50°,
it will be seen a little more than half way up the northern sky. At the
equator, where the latitude is _nil_, the Pole Star will be on the
horizon due north.

In consequence of its unique position, the Pole Star is of very great
service in the study of the constellations. It is a kind of centre
around which to hang our celestial ideas--a starting point, so to speak,
in our voyages about the sky.

According to the constellation figures, the Pole Star is in _Ursa
Minor_, or the Little Bear, and is situated at the end of the tail of
that imaginary figure (see Plate XIX., p. 292). The chief stars of this
constellation form a group not unlike the Plough, except that the
"handle" is turned in the contrary direction. The Americans, in
consequence, speak of it as the "Little Dipper."

Before leaving this region of the sky, it will be well to draw attention
to the second magnitude star [z] in the Great Bear (Zeta Ursæ Majoris),
which is the middle star in the "handle" of the Plough. This star is
usually known as Mizar, a name given to it by the Arabians. A person
with good eyesight can see quite near to it a fifth magnitude star,
known under the name of Alcor. We have here a very good example of that
deception in the estimation of objects in the sky, which has been
alluded to in an earlier chapter. Alcor is indeed distant from Mizar by
about one-third the apparent diameter of the moon, yet no one would
think so!

On the other side of Polaris from the Plough, and at about an equal
apparent distance, will be found a figure in the form of an irregular
"W", made up of second and third magnitude stars. This is the well-known
"Cassiopeia's Chair"--portion of the constellation of _Cassiopeia_ (see
Plate XIX., p. 292).

On either side of the Pole Star, about midway between the Plough and
Cassiopeia's Chair, but a little further off from it than these, are the
constellations of _Auriga_ and _Lyra_ (see Plate XIX., p. 292). The
former constellation will be easily recognised, because its chief
features are a brilliant yellowish first magnitude star, with one of the
second magnitude not far from it. The first magnitude star is Capella,
the other is [b] Aurigæ. Lyra contains only one first magnitude
star--Vega, pale blue in colour. This star has a certain interest for us
from the fact that, as a consequence of that slow shift of direction of
the earth's axis known as Precession, it will be very near the north
pole of the heavens in some 12,000 years, and so will then be considered
the pole star (see Plate XIX., p. 292). The constellation of Lyra
itself, it must also be borne in mind, occupies that region of the
heavens towards which the solar system is travelling.

The handle of the Plough points roughly towards the constellation of
_Boötes_, in which is the brilliant first magnitude star Arcturus. This
star is of an orange tint.

Between Boötes and Lyra lie the constellations of _Corona Borealis_ (or
the Northern Crown) and _Hercules_. The chief feature of Corona
Borealis, which is a small constellation, is a semicircle of six small
stars, the brightest of which is of the second magnitude. The
constellation of Hercules is very extensive, but contains no star
brighter than the third magnitude.

Near to Lyra, on the side away from Hercules, are the constellations of
_Cygnus_ and _Aquila_. Of the two, the former is the nearer to the Pole
Star, and will be recognised by an arrangement of stars widely set in
the form of a cross, or perhaps indeed more like the framework of a
boy's kite. The position of Aquila will be found through the fact that
three of its brightest stars are almost in a line and close together.
The middle of these is Altair, a yellowish star of the first magnitude.

At a little distance from Ursa Major, on the side away from the Pole
Star, is the constellation of _Leo_, or the Lion. Its chief feature is a
series of seven stars, supposed to form the head of that animal. The
arrangement of these stars is, however, much more like a sickle,
wherefore this portion of the constellation is usually known as the
"Sickle of Leo." At the end of the handle of the sickle is a white first
magnitude star--Regulus.

The reader will, no doubt, recollect that it is from a point in the
Sickle of Leo that the Leonid meteors appear to radiate.

The star second in brightness in the constellation of Leo is known as
Denebola. This star, now below the second magnitude, seems to have been
very much brighter in the past. It is noted, indeed, as a brilliant
first magnitude star by Al Sufi, that famous Persian astronomer who
lived, as we have seen, in the tenth century. Ptolemy also notes it as
of the first magnitude.

In the neighbourhood of Auriga, and further than it from the Pole Star,
are several remarkable constellations--Taurus, Orion, Gemini, Canis
Minor, and Canis Major (see Plate XX., p. 296).

The first of these, _Taurus_ (or the Bull), contains two conspicuous
star groups--the Pleiades and the Hyades. The Pleiades are six or seven
small stars quite close together, the majority of which are of the
fourth magnitude. This group is sometimes occulted by the moon. The way
in which the stars composing it are arranged is somewhat similar to that
in the Plough, though of course on a scale ever so much smaller. The
impression which the group itself gives to the casual glance is thus
admirably pictured in Tennyson's _Locksley Hall_:--

"Many a night I saw the Pleiads, rising through the mellow shade,
Glitter like a swarm of fire-flies tangled in a silver braid."

[Illustration: PLATE XX. ORION AND HIS NEIGHBOURS

We see here that magnificent region of the sky which contains the
brightest star of all--Sirius. Note also especially the Milky Way, the
Pleiades, the Hyades, and the "Belt" and "Sword" of Orion.

(Page 296)]

The group of the Hyades occupies the "head" of the Bull, and is much
more spread out than that of the Pleiades. It is composed besides of
brighter stars, the brightest being one of the first magnitude,
Aldebaran. This star is of a red colour, and is sometimes known as the
"Eye of the Bull."

The constellation of _Orion_ is easily recognised as an irregular
quadrilateral formed of four bright stars, two of which, Betelgeux
(reddish) and Rigel (brilliant white), are of the first magnitude. In
the middle of the quadrilateral is a row of three second magnitude
stars, known as the "Belt" of Orion. Jutting off from this is another
row of stars called the "Sword" of Orion.

The constellation of _Gemini_, or the Twins, contains two bright
stars--Castor and Pollux--close to each other. Pollux, though marked
with the Greek letter [b], is the brighter of the two, and nearly of the
standard first magnitude.

Just further from the Pole than Gemini, is the constellation of _Canis
Minor_, or the Lesser Dog. Its chief star is a white first magnitude
one--Procyon.

Still further again from the Pole than Canis Minor is the constellation
of _Canis Major_, or the Greater Dog. It contains the brightest star in
the whole sky, the first magnitude star Sirius, bluish-white in colour,
also known as the "Dog Star." This star is almost in line with the stars
forming the Belt of Orion, and is not far from that constellation.

Taken in the following order, the stars Capella, [b] Aurigæ, Castor,
Pollux, Procyon, and Sirius, when they are all above the horizon at the
same time, form a beautiful curve stretching across the heaven.

The groups of stars visible in the southern skies have by no means the
same fascination for us as those in the northern. The ancients were in
general unacquainted with the regions beyond the equator, and so their
scheme of constellations did not include the sky around the South Pole
of the heavens. In modern times, however, this part of the celestial
expanse was also portioned out into constellations for the purpose of
easy reference; but these groupings plainly lack that simplicity of
conception and legendary interest which are so characteristic of the
older ones.

The brightest star in the southern skies is found in the constellation
of _Argo_, and is known as Canopus. In brightness it comes next to
Sirius, and so is second in that respect in the entire heaven. It does
not, however, rise above the English horizon.

Of the other southern constellations, two call for especial notice, and
these adjoin each other. One is _Centaurus_ (or the Centaur), which
contains the two first magnitude stars, [a] and [b] Centauri. The first
of these, Alpha Centauri, comes next in brightness to Canopus, and is
notable as being the nearest of all the stars to our earth. The other
constellation is called _Crux_, and contains five stars set in the form
of a rough cross, known as the "Southern Cross." The brightest of these,
[a] Crucis, is of the first magnitude.

Owing to the Precession of the Equinoxes, which, as we have seen,
gradually shifts the position of the Pole among the stars, certain
constellations used to be visible in ancient times in more northerly
latitudes than at present. For instance, some five thousand years ago
the Southern Cross rose above the English horizon, and was just visible
in the latitude of London. It has, however, long ago even ceased to be
seen in the South of Europe. The constellation of Crux happens to be
situated in that remarkable region of the southern skies, in which are
found the stars Canopus and Alpha Centauri, and also the most brilliant
portion of the Milky Way. It is believed to be to this grand celestial
region that allusion is made in the Book of Job (ix. 9), under the title
of the "Chambers of the South." The "Cross" must have been still a
notable feature in the sky of Palestine in the days when that ancient
poem was written.

There is no star near enough to the southern pole of the heavens to earn
the distinction of South Polar Star.

The Galaxy, or _Milky Way_ (see Plate XX., p. 296), is a broad band of
diffused light which is seen to stretch right around the sky. The
telescope, however, shows it to be actually composed of a great host of
very faint stars--too faint, indeed, to be separately distinguished with
the naked eye. Along a goodly stretch of its length it is cleft in two;
while near the south pole of the heavens it is entirely cut across by a
dark streak.

In this rapid survey of the face of the sky, we have not been able to do
more than touch in the broadest manner upon some of the most noticeable
star groups and a few of the most remarkable stars. To go any further is
not a part of our purpose; our object being to deal with celestial
bodies as they actually are, and not in those groupings under which they
display themselves to us as a mere result of perspective.

[29] Attention must here be drawn to the fact that the name of the
constellation is always put in the genitive case.

[30] The early peoples, as we have seen, appear to have been attracted
by those groupings of the stars which reminded them in a way of the
figures of men and animals. We moderns, on the other hand, seek almost
instinctively for geometrical arrangements. This is, perhaps,
symptomatic of the evolution of the race. In the growth of the
individual we find, for example, something analogous. A child, who has
been given pencil and paper, is almost certain to produce grotesque
drawings of men and animals; whereas the idle and half-conscious
scribblings which a man may make upon his blotting-paper are usually of
a geometrical character.

[31] Because the line joining them _points_ in the direction of the Pole
Star.

CHAPTER XXIV

SYSTEMS OF STARS

Many stars are seen comparatively close together. This may plainly arise
from two reasons. Firstly, the stars may happen to be almost in the same
line of sight; that is to say, seen in nearly the same direction; and
though one star may be ever so much nearer to us than the other, the
result will give all the appearance of a related pair. A seeming
arrangement of two stars in this way is known as a "double," or double
star; or, indeed, to be very precise, an "optical double." Secondly, in
a pair of stars, both bodies may be about the same distance from us, and
actually connected as a system like, for instance, the moon and the
earth. A pairing of stars in this way, though often casually alluded to
as a double star, is properly termed a "binary," or binary system.

But collocations of stars are by no means limited to two. We find,
indeed, all over the sky such arrangements in which there are three or
more stars; and these are technically known as "triple" or "multiple"
stars respectively. Further, groups are found in which a great number of
stars are closely massed together, such a massing together of stars
being known as a "cluster."

The Pole Star (Polaris) is a double star, one of the components being of
a little below the second magnitude, and the other a little below the
ninth. They are so close together that they appear as one star to the
naked eye, but they may be seen separate with a moderately sized
telescope. The brighter star is yellowish, and the faint one white. This
brighter star is found _by means of the spectroscope_ to be actually
composed of three stars so very close together that they cannot be seen
separately even with a telescope. It is thus a triple star, and the
three bodies of which it is composed are in circulation about each
other. Two of them are darker than the third.

The method of detecting binary stars by means of the spectroscope is an
application of Doppler's principle. It will, no doubt, be remembered
that, according to the principle in question, we are enabled, from
certain shiftings of the lines in the spectrum of a luminous body, to
ascertain whether that body is approaching us or receding from us. Now
there are certain stars which always appear single even in the largest
telescopes, but when the spectroscope is directed to them a spectrum
_with two sets of lines_ is seen. Such stars must, therefore, be double.
Further, if the shiftings of the lines, in a spectrum like this, tell us
that the component stars are making small movements to and from us which
go on continuously, we are therefore justified in concluding that these
are the orbital revolutions of a binary system greatly compressed by
distance. Such connected pairs of stars, since they cannot be seen
separately by means of any telescope, no matter how large, are known as
"spectroscopic binaries."

In observations of spectroscopic binaries we do not always get a double
spectrum. Indeed, if one of the components be below a certain
magnitude, its spectrum will not appear at all; and so we are left in
the strange uncertainty as to whether this component is merely faint or
actually dark. It is, however, from the shiftings of the lines in the
spectrum of the other component that we see that an orbital movement is
going on, and are thus enabled to conclude that two bodies are here
connected into a system, although one of these bodies resolutely refuses
directly to reveal itself even to the all-conquering spectroscope.

Mizar, that star in the handle of the Plough to which we have already
drawn attention, will be found with a small telescope to be a fine
double, one of the components being white and the other greenish.
Actually, however, as the American astronomer, Professor F.R. Moulton,
points out, these stars are so far from each other that if we could be
transferred to one of them we should see the other merely as an ordinary
bright star. The spectroscope shows that the brighter of these stars is
again a binary system of two huge suns, the components revolving around
each other in a period of about twenty days. This discovery made by
Professor E.C. Pickering, the _first_ of the kind by means of the
spectroscope, was announced in 1889 from the Harvard Observatory in the
United States.

A star close to Vega, known as [e] (Epsilon) Lyræ (see Plate XIX., p.
292), is a double, the components of which may be seen separately with
the naked eye by persons with very keen eyesight. If this star, however,
be viewed with the telescope, the two companions will be seen far apart;
and it will be noticed that each of them is again a double.

By means of the spectroscope Capella is shown to be really composed of
two stars (one about twice as bright as the other) situated very close
together and forming a binary system. Sirius is also a binary system;
but it is what is called a "visual" one, for its component stars may be
_seen_ separately in very large telescopes. Its double, or rather
binary, nature, was discovered in 1862 by the celebrated optician Alvan
G. Clark, while in the act of testing the 18-inch refracting telescope,
then just constructed by his firm, and now at the Dearborn Observatory,
Illinois, U.S.A. The companion is only of the tenth magnitude, and
revolves around Sirius in a period of about fifty years, at a mean
distance equal to about that of Uranus from the sun. Seen from Sirius,
it would shine only something like our full moon. It must be
self-luminous and not a mere planet; for Mr. Gore has shown that if it
shone only by the light reflected from Sirius, it would be quite
invisible even in the Great Yerkes Telescope.

Procyon is also a binary, its companion having been discovered by
Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of
revolution in this system is about forty years. Observations by Mr. T.
Lewis of Greenwich seem, however, to point to the companion being a
small nebula rather than a star.

The star [ê] (Eta) Cassiopeiæ (see Plate XIX., p. 292), is easily seen
as a fine double in telescopes of moderate size. It is a binary system,
the component bodies revolving around their common centre of gravity in
a period of about two hundred years. This system is comparatively near
to us, _i.e._ about nine light years, or a little further off than
Sirius.

In a small telescope the star Castor will be found double, the
components, one of which is brighter than the other, forming a binary
system. The fainter of these was found by Belopolsky, with the
spectroscope, to be composed of a system of two stars, one bright and
the other either dark or not so bright, revolving around each other in a
period of about three days. The brighter component of Castor is also a
spectroscopic binary, with a period of about nine days; so that the
whole of what we see with the naked eye as Castor, is in reality a
remarkable system of four stars in mutual orbital movement.

Alpha Centauri--the nearest star to the earth--is a visual binary, the
component bodies revolving around each other in a period of about
eighty-one years. The extent of this system is about the same as that of
Sirius. Viewed from each other, the bodies would shine only like our sun
as seen from Neptune.

Among the numerous binary stars the orbits of some fifty have been
satisfactorily determined. Many double stars, for which this has not yet
been done, are, however, believed to be, without doubt, binary. In some
cases a parallax has been found; so that we are enabled to estimate in
miles the actual extent of such systems, and the masses of the bodies in
terms of the sun's mass.

Most of the spectroscopic binaries appear to be upon a smaller scale
than the telescopic ones. Some are, indeed, comparatively speaking,
quite small. For instance, the component stars forming [b] Aurigæ are
about eight million miles apart, while in [z] Geminorum, the distance
between the bodies is only a little more than a million miles.

Spectroscopic binaries are probably very numerous. Professor W.W.
Campbell, Director of the Lick Observatory, estimates, for instance,
that, out of about every half-a-dozen stars, one is a spectroscopic
binary.

It is only in the case of binary systems that we can discover the masses
of stars at all. These are ascertained from their movements with regard
to each other under the influence of their mutual gravitative
attractions. In the case of simple stars we have clearly nothing of the
kind to judge by; though, if we can obtain a parallax, we may hazard a
guess from their brightness.

Binary stars were incidentally discovered by Sir William Herschel. In
his researches to get a stellar parallax he had selected a number of
double stars for test purposes, on the assumption that, if one of such a
pair were much nearer than the other, it might show a displacement with
regard to its neighbour as a direct consequence of the earth's orbital
movement around the sun. He, however, failed entirely to obtain any
parallaxes, the triumph in this being, as we have seen, reserved for
Bessel. But in some of the double stars which he had selected, he found
certain alterations in the relative positions of the bodies, which
plainly were not a consequence of the earth's motion, but showed rather
that there was an actual circling movement of the bodies themselves
under their mutual attractions. It is to be noted that the existence of
such connected pairs had been foretold as probable by the Rev. John
Michell, who lived a short time before Herschel.

The researches into binary systems--both those which can be seen with
the eye and those which can be observed by means of the spectroscope,
ought to impress upon us very forcibly the wide sway of the law of
gravitation.

Of star clusters about 100 are known, and such systems often contain
several thousand stars. They usually cover an area of sky somewhat
smaller than the moon appears to fill. In most clusters the stars are
very faint, and, as a rule, are between the twelfth and sixteenth
magnitudes. It is difficult to say whether these are actually small
bodies, or whether their faintness is due merely to their great distance
from us, since they are much too far off to show any appreciable
parallactic displacement. Mr. Gore, however, thinks there is good
evidence to show that the stars in clusters are really close, and that
the clusters themselves fill a comparatively small space.

One of the finest examples of a cluster is the great globular one, in
the constellation of Hercules, discovered by Halley in 1714. It contains
over 5000 stars, and upon a clear, dark night is visible to the naked
eye as a patch of light. In the telescope, however, it is a wonderful
object. There are also fine clusters in the constellations of Auriga,
Pegasus, and Canes Venatici. In the southern heavens there are some
magnificent examples of globular clusters. This hemisphere seems,
indeed, to be richer in such objects than the northern. For instance,
there is a great one in the constellation of the Centaur, containing
some 6000 stars (see Plate XXI., p. 306).

[Illustration: PLATE XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN
CONSTELLATION OF CENTAURUS

From a photograph taken at the Cape Observatory, on May 24th, 1903. Time
of exposure, 1 hour.

(Page 306)]

Certain remarkable groups of stars, of a nature similar to clusters,
though not containing such faint or densely packed stars as those we
have just alluded to, call for a mention in this connection. The best
example of such star groups are the Pleiades and the Hyades (see Plate
XX., p. 296), Coma Berenices, and Præsepe (or the Beehive), the
last-named being in the constellation of Cancer.

Stars which alter in their brightness are called _Variable Stars_, or
"variables." The first star whose variability attracted attention is
that known as Omicron Ceti, namely, the star marked with the Greek
letter [o] (Omicron) in the constellation of Cetus, or the Whale, a
constellation situated not far from Taurus. This star, the variability
of which was discovered by Fabricius in 1596, is also known as Mira, or
the "Wonderful," on account of the extraordinary manner in which its
light varies from time to time. The star known by the name of Algol,[32]
popularly called the "Demon Star"--whose astronomical designation is [b]
(Beta) Persei, or the star second in brightness in the constellation of
Perseus--was discovered by Goodricke, in the year 1783, to be a variable
star. In the following year [b] Lyræ, the star in Lyra next in order of
brightness after Vega, was also found by the same observer to be a
variable. It may be of interest to the reader to know that Goodricke was
deaf and dumb, and that he died in 1786 at the early age of twenty-one
years!

It was not, however, until the close of the nineteenth century that much
attention was paid to variable stars. Now several hundreds of these are
known, thanks chiefly to the observations of, amongst others, Professor
S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr.
A.W. Roberts of South Africa. This branch of astronomy has not, indeed,
attracted as much popular attention as it deserves, no doubt because the
nature of the work required does not call for the glamour of an
observatory or a large telescope.

The chief discoveries with regard to variable stars have been made by
the naked eye, or with a small binocular. The amount of variation is
estimated by a comparison with other stars. As in many other branches of
astronomy, photography is now employed in this quest with marked
success; and lately many variable stars have been found to exist in
clusters and nebulæ.

It was at one time considered that a variable star was in all
probability a body, a portion of whose surface had been relatively
darkened in some manner akin to that in which sun spots mar the face of
the sun; and that when its axial rotation brought the less illuminated
portions in turn towards us, we witnessed a consequent diminution in the
star's general brightness. Herschel, indeed, inclined to this
explanation, for his belief was that all the stars bore spots like those
of the sun. It appears preferably thought nowadays that disturbances
take place periodically in the atmosphere or surroundings of certain
stars, perhaps through the escape of imprisoned gases, and that this may
be a fruitful cause of changes of brilliancy. The theory in question
will, however, apparently account for only one class of variable star,
namely, that of which Mira Ceti is the best-known example. The scale on
which it varies in brightness is very great, for it changes from the
second to the ninth magnitude. For the other leading type of variable
star, Algol, of which mention has already been made, is the best
instance. The shortness of the period in which the changes of brightness
in such stars go their round, is the chief characteristic of this latter
class. The period of Algol is a little under three days. This star when
at its brightest is of about the second magnitude, and when least bright
is reduced to below the third magnitude; from which it follows that its
light, when at the minimum, is only about one-third of what it is when
at the maximum. It seems definitely proved by means of the spectroscope
that variables of this kind are merely binary stars, too close to be
separated by the telescope, which, as a consequence of their orbits
chancing to be edgewise towards us, eclipse each other in turn time
after time. If, for instance, both components of such a pair are bright,
then when one of them is right behind the other, we will not, of course,
get the same amount of light as when they are side by side. If, on the
other hand, one of the components happens to be dark or less luminous
and the other bright, the manner in which the light of the bright star
will be diminished when the darker star crosses its face should easily
be understood. It is to the second of these types that Algol is supposed
to belong. The Algol system appears to be composed of a body about as
broad as our sun, which regularly eclipses a brighter body which has a
diameter about half as great again.

Since the companion of Algol is often spoken of as a _dark_ body, it
were well here to point out that we have no evidence at all that it is
entirely devoid of light. We have already found, in dealing with
spectroscopic binaries, that when one of the component stars is below a
certain magnitude[33] its spectrum will not be seen; so one is left in
the glorious uncertainty as to whether the body in question is
absolutely dark, or darkish, or faint, or indeed only just out of range
of the spectroscope.

It is thought probable by good authorities that the companion of Algol
is not quite dark, but has some inherent light of its own. It is, of
course, much too near Algol to be seen with the largest telescope. There
is in fact a distance of only from two to three millions of miles
between the bodies, from which Mr. Gore infers that they would probably
remain unseparated even in the largest telescope which could ever be
constructed by man.

The number of known variables of the Algol type is, so far, small; not
much indeed over thirty. In some of them the components are believed to
revolve touching each other, or nearly so. An extreme example of this is
found in the remarkable star V. Puppis, an Algol variable of the
southern hemisphere. Both its components are bright, and the period of
light variation is about one and a half days. Dr. A. W. Roberts finds
that the bodies are revolving around each other in actual contact.

_Temporary stars_ are stars which have suddenly blazed out in regions of
the sky where no star was previously seen, and have faded away more or
less gradually.

It was the appearance of such a star, in the year 134 B.C., which
prompted Hipparchus to make his celebrated catalogue, with the object of
leaving a record by which future observers could note celestial changes.
In 1572 another star of this kind flashed out in the constellation of
Cassiopeia (see Plate XIX., p. 292), and was detected by Tycho Brahe. It
became as bright as the planet Venus, and eventually was visible in the
day-time. Two years later, however, it disappeared, and has never since
been seen. In 1604 Kepler recorded a similar star in the constellation
of Ophiuchus which grew to be as bright as Jupiter. It also lasted for
about two years, and then faded away, leaving no trace behind. It is
rarely, however, that temporary stars attain to such a brilliance; and
so possibly in former times a number of them may have appeared, but not
have risen to a sufficient magnitude to attract attention. Even now,
unless such a star becomes clearly visible to the naked eye, it runs a
good chance of not being detected. A curious point, worth noting, with
regard to temporary stars is that the majority of them have appeared in
the Milky Way.

These sudden visitations have in our day received the name of _Novæ_;
that is to say, "New" Stars. Two, in recent years, attracted a good deal
of attention. The first of these, known as Nova Aurigæ, or the New Star
in the constellation of Auriga, was discovered by Dr. T.D. Anderson at
Edinburgh in January 1892. At its greatest brightness it attained to
about the fourth magnitude. By April it had sunk to the twelfth, but
during August it recovered to the ninth magnitude. After this last
flare-up it gradually faded away.

The startling suddenness with which temporary stars usually spring into
being is the groundwork upon which theories to account for their origin
have been erected. That numbers of dark stars, extinguished suns, so to
speak, may exist in space, there is a strong suspicion; and it is just
possible that we have an instance of one dark stellar body in the
companion of Algol. That such dark stars might be in rapid motion is
reasonable to assume from the already known movements of bright stars.
Two dark bodies might, indeed, collide together, or a collision might
take place between a dark star and a star too faint to be seen even with
the most powerful telescope. The conflagration produced by the impact
would thus appear where nothing had been seen previously. Again, a
similar effect might be produced by a dark body, or a star too faint to
be seen, being heated to incandescence by plunging in its course through
a nebulous mass of matter, of which there are many examples lying about
in space.

The last explanation, which is strongly reminiscent of what takes place
in shooting stars, appears more probable than the collision theory. The
flare-up of new stars continues, indeed, only for a comparatively short
time; whereas a collision between two bodies would, on the other hand,
produce an enormous nebula which might take even millions of years to
cool down. We have, indeed, no record of any such sudden appearance of a
lasting nebula.

The other temporary star, known as Nova Persei, or the new star in the
constellation of Perseus, was discovered early in the morning of
February 22, 1901, also by Dr. Anderson. A day later it had grown to be
brighter than Capella. Photographs which had been taken, some three days
previous to its discovery, of the very region of the sky in which it had
burst forth, were carefully examined, and it was not found in these. At
the end of two days after its discovery Nova Persei had lost one-third
of its light. During the ensuing six months it passed through a series
of remarkable fluctuations, varying in brightness between the third and
fifth magnitudes. In the month of August it was seen to be surrounded by
luminous matter in the form of a nebula, which appeared to be gradually
spreading to some distance around. Taking into consideration the great
way off at which all this was taking place, it looked as if the new star
had ejected matter which was travelling outward with a velocity
equivalent to that of light. The remarkable theory was, however, put
forward by Professor Kapteyn and the late Dr. W.E. Wilson that there
might be after all no actual transmission of matter; but that perhaps
the real explanation was the gradual _illumination_ of hitherto
invisible nebulous matter, as a consequence of the flare-up which had
taken place about six months before. It was, therefore, imagined that
some dark body moving through space at a very rapid rate had plunged
through a mass of invisible nebulous matter, and had consequently become
heated to incandescence in its passage, very much like what happens to a
meteor when moving through our atmosphere. The illumination thus set up
temporarily in one point, being transmitted through the nebulous wastes
around with the ordinary velocity of light, had gradually rendered this
surrounding matter visible. On the assumptions required to fit in with
such a theory, it was shown that Nova Persei must be at a distance from
which light would take about three hundred years in coming to us. The
actual outburst of illumination, which gave rise to this temporary star,
would therefore have taken place about the beginning of the reign of
James I.

Some recent investigations with regard to Nova Persei have, however,
greatly narrowed down the above estimate of its distance from us. For
instance, Bergstrand proposes a distance of about ninety-nine light
years; while the conclusions of Mr. F.W. Very would bring it still
nearer, _i.e._ about sixty-five light years.

The last celestial objects with which we have here to deal are the
_Nebulæ_. These are masses of diffused shining matter scattered here and
there through the depths of space. Nebulæ are of several kinds, and have
been classified under the various headings of Spiral, Planetary, Ring,
and Irregular.

A typical _spiral_ nebula is composed of a disc-shaped central portion,
with long curved arms projecting from opposite sides of it, which give
an impression of rapid rotatory movement.

The discovery of spiral nebulæ was made by Lord Rosse with his great
6-foot reflector. Two good examples of these objects will be found in
Ursa Major, while there is another fine one in Canes Venatici (see Plate
XXII., p. 314), a constellation which lies between Ursa Major and
Boötes. But the finest spiral of all, perhaps the most remarkable nebula
known to us, is the Great Nebula in the constellation of Andromeda, (see
Plate XXIII., p. 316)--a constellation just further from the pole than
Cassiopeia. When the moon is absent and the night clear this nebula can
be easily seen with the naked eye as a small patch of hazy light. It is
referred to by Al Sufi.

[Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES
VENATICI

From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S.

(Page 314)]

Spiral nebulæ are white in colour, whereas the other kinds of nebula
have a greenish tinge. They are also by far the most numerous; and the
late Professor Keeler, who considered this the normal type of nebula,
estimated that there were at least 120,000 of such spirals within the
reach of the Crossley reflector of the Lick Observatory. Professor
Perrine has indeed lately raised this estimate to half a million, and
thinks that with more sensitive photographic plates and longer exposures
the number of spirals would exceed a million. The majority of these
objects are very small, and appear to be distributed over the sky in a
fairly uniform manner.

_Planetary_ nebulæ are small faint roundish objects which, when seen in
the telescope, recall the appearance of a planet, hence their name. One
of these nebulæ, known astronomically as G.C. 4373, has recently been
found to be rushing through space towards the earth at a rate of between
thirty and forty miles per second. It seems strange, indeed, that any
gaseous mass should move at such a speed!

What are known as _ring_ nebulæ were until recently believed to form a
special class. These objects have the appearance of mere rings of
nebulous matter. Much doubt has, however, been thrown upon their being
rings at all; and the best authorities regard them merely as spiral
nebulæ, of which we happen to get a foreshortened view. Very few
examples are known, the most famous being one in the constellation of
Lyra, usually known as the Annular Nebula in Lyra. This object is so
remote from us as to be entirely invisible to the naked eye. It contains
a star of the fifteenth magnitude near to its centre. From photographs
taken with the Crossley reflector, Professor Schaeberle finds in this
nebula evidences of spiral structure. It may here be mentioned that the
Great Nebula in Andromeda, which has now turned out to be a spiral, had
in earlier photographs the appearance of a ring.

There also exist nebulæ of _irregular_ form, the most notable being the
Great Nebula in the constellation of Orion (see Plate XXIV., p. 318). It
is situated in the centre of the "Sword" of Orion (see Plate XX., p.
296). In large telescopes it appears as a magnificent object, and in
actual dimensions it must be much on the same scale as the Andromeda
Nebula. The spectroscope tells us that it is a mass of glowing gas.

The Trifid Nebula, situated in the constellation of Sagittarius, is an
object of very strange shape. Three dark clefts radiate from its centre,
giving it an appearance as if it had been torn into shreds.

The Dumb-bell Nebula, a celebrated object, so called from its likeness
to a dumb-bell, turns out, from recent photographs taken by Professor
Schaeberle, which bring additional detail into view, to be after all a
great spiral.

There is a nest, or rather a cluster of nebulæ in the constellation of
Coma Berenices; over a hundred of these objects being here gathered into
a space of sky about the size of our full moon.

[Illustration: PLATE XXIII. THE GREAT NEBULA IN THE CONSTELLATION OF
ANDROMEDA

From a photograph taken at the Yerkes Observatory.

(Page 314)]

The spectroscope informs us that spiral nebulæ are composed of
partially-cooled matter. Their colour, as we have seen, is white. Nebulæ
of a greenish tint are, on the other hand, found to be entirely in a
gaseous condition. Just as the solar corona contains an unknown element,
which for the time being has been called "Coronium," so do the gaseous
nebulæ give evidence of the presence of another unknown element. To this
Sir William Huggins has given the provisional name of "Nebulium."

The _Magellanic Clouds_ are two patches of nebulous-looking light, more
or less circular in form, which are situated in the southern hemisphere
of the sky. They bear a certain resemblance to portions of the Milky
Way, but are, however, not connected with it. They have received their
name from the celebrated navigator, Magellan, who seems to have been one
of the first persons to draw attention to them. "Nubeculæ" is another
name by which they are known, the larger cloud being styled _nubecula
major_ and the smaller one _nubecula minor_. They contain within them
stars, clusters, and gaseous nebulæ. No parallax has yet been found for
any object which forms part of the nubeculæ, so it is very difficult to
estimate at what distance from us they may lie. They are, however,
considered to be well within our stellar universe.

Having thus brought to a conclusion our all too brief review of the
stars and the nebulæ--of the leading objects in fine which the celestial
spaces have revealed to man--we will close this chapter with a recent
summation by Sir David Gill of the relations which appear to obtain
between these various bodies. "Huggins's spectroscope," he says, "has
shown that many nebulæ are not stars at all; that many well-condensed
nebulæ, as well as vast patches of nebulous light in the sky, are but
inchoate masses of luminous gas. Evidence upon evidence has accumulated
to show that such nebulæ consist of the matter out of which stars
(_i.e._ suns) have been and are being evolved. The different types of
star spectra form such a complete and gradual sequence (from simple
spectra resembling those of nebulæ onwards through types of gradually
increasing complexity) as to suggest that we have before us, written in
the cryptograms of these spectra, the complete story of the evolution of
suns from the inchoate nebula onwards to the most active sun (like our
own), and then downward to the almost heatless and invisible ball. The
period during which human life has existed upon our globe is probably
too short--even if our first parents had begun the work--to afford
observational proof of such a cycle of change in any particular star;
but the fact of such evolution, with the evidence before us, can hardly
be doubted."[34]

[32] The name Al gûl, meaning the Demon, was what the old Arabian
astronomers called it, which looks very much as if they had already
noticed its rapid fluctuations in brightness.

[33] Mr. Gore thinks that the companion of Algol may be a star of the
sixth magnitude.

[34] Presidential Address to the British Association for the Advancement
of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S.,
&c. &c.

[Illustration: PLATE XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF
ORION

From a photograph taken at the Yerkes Observatory.

(Page 316)]

CHAPTER XXV

THE STELLAR UNIVERSE

The stars appear fairly evenly distributed all around us, except in one
portion of the sky where they seem very crowded, and so give one an
impression of being very distant. This portion, known as the Milky Way,
stretches, as we have already said, in the form of a broad band right
round the entire heavens. In those regions of the sky most distant from
the Milky Way the stars appear to be thinly sown, but become more and
more closely massed together as the Milky Way is approached.

This apparent distribution of the stars in space has given rise to a
theory which was much favoured by Sir William Herschel, and which is
usually credited to him, although it was really suggested by one Thomas
Wright of Durham in 1750; that is to say, some thirty years or more
before Herschel propounded it. According to this, which is known as the
"Disc" or "Grindstone" Theory, the stars are considered as arranged in
space somewhat in the form of a thick disc, or grindstone, close to the
_central_ parts of which our solar system is situated.[35] Thus we
should see a greater number of stars when we looked out through the
_length_ of such a disc in any direction, than when we looked out
through its _breadth_. This theory was, for a time, supposed to account
quite reasonably for the Milky Way, and for the gradual increase in the
number of stars in its vicinity.

It is quite impossible to verify directly such a theory, for we know the
actual distance of only about forty-three stars. We are unable,
therefore, definitely to assure ourselves whether, as the grindstone
theory presupposes, the stellar universe actually reaches out very much
further from us in the direction of the Milky Way than in the other
parts of the sky. The theory is clearly founded upon the supposition
that the stars are more or less equal in size, and are scattered through
space at fairly regular distances from each other.

Brightness, therefore, had been taken as implying nearness to us, and
faintness great distance. But we know to-day that this is not the case,
and that the stars around us are, on the other hand, of various degrees
of brightness and of all orders of size. Some of the faint stars--for
instance, the galloping star in Pictor--are indeed nearer to us than
many of the brighter ones. Sirius, on the other hand, is twice as far
off from us as [a] Centauri, and yet it is very much brighter; while
Canopus, which in brightness is second only to Sirius out of the whole
sky, is too far off for its distance to be ascertained! It must be
remembered that no parallax had yet been found for any star in the days
of Herschel, and so his estimations of stellar distances were
necessarily of a very circumstantial kind. He did not, however, continue
always to build upon such uncertain ground; but, after some further
examination of the Milky Way, he gave up his idea that the stars were
equally disposed in space, and eventually abandoned the grindstone
theory.

Since we have no means of satisfactorily testing the matter, through
finding out the various distances from us at which the stars are really
placed, one might just as well go to the other extreme, and assume that
the thickening of stars in the region of the Milky Way is not an effect
of perspective at all, but that the stars in that part of the sky are
actually more crowded together than elsewhere--a thing which astronomers
now believe to be the case. Looked at in this way, the shape of the
stellar universe might be that of a globe-shaped aggregation of stars,
in which the individuals are set at fairly regular distances from each
other; the whole being closely encircled by a belt of densely packed
stars. It must, however, be allowed that the gradual increase in the
number of stars towards the Milky Way appears a strong argument in
favour of the grindstone theory; yet the belt theory, as above detailed,
seems to meet with more acceptance.

There is, in fact, one marked circumstance which is remarkably difficult
of explanation by means of the grindstone theory. This is the existence
of vacant spaces--holes, so to speak, in the groundwork of the Milky
Way. For instance, there is a cleft running for a good distance along
its length, and there is also a starless gap in its southern portion. It
seems rather improbable that such a great number of stars could have
arranged themselves so conveniently, as to give us a clear view right
out into empty space through such a system in its greatest thickness;
as if, in fact, holes had been bored, and clefts made, from the boundary
of the disc clean up to where our solar system lies. Sir John Herschel
long ago drew attention to this point very forcibly. It is plain that
such vacant spaces can, on the other hand, be more simply explained as
mere holes in a belt; and the best authorities maintain that the
appearance of the Milky Way confirms a view of this kind.

Whichever theory be indeed the correct one, it appears at any rate that
the stars do not stretch out in every direction to an infinite distance;
but that _the stellar system is of limited extent_, and has in fact a
boundary.

In the first place, Science has no grounds for supposing that light is
in any way absorbed or destroyed merely by its passage through the
"ether," that imponderable medium which is believed to transmit the
luminous radiations through space. This of course is tantamount to
saying that all the direct light from all the stars should reach us,
excepting that little which is absorbed in its passage through our own
atmosphere. If stars, and stars, and stars existed in every direction
outwards without end, it can be proved mathematically that in such
circumstances there could not remain the tiniest space in the sky
without a star to fill it, and that therefore the heavens would always
blaze with light, and the night would be as bright as the noonday.[36]
How very far indeed this is from being the case, may be gathered from an
estimate which has been made of the general amount of light which we
receive from the stars. According to this estimate the sky is considered
as more or less dark, the combined illumination sent to us by all the
stars being only about the one-hundreth part of what we get from the
full moon.[37]

Secondly, it has been suggested that although light may not suffer any
extinction or diminution from the ether itself, still a great deal of
illumination may be prevented from reaching us through myriads of
extinguished suns, or dark meteoric matter lying about in space. The
idea of such extinguished suns, dark stars in fact, seems however to be
merely founded upon the sole instance of the invisible companion of
Algol; but, as we have seen, there is no proof whatever that it is a
dark body. Again, some astronomers have thought that the dark holes in
the Milky Way, "Coal Sacks," as they are called, are due to masses of
cool, or partially cooled matter, which cuts off the light of the stars
beyond. The most remarkable of these holes is one in the neighbourhood
of the Southern Cross, known as the "Coal Sack in Crux." But Mr. Gore
thinks that the cause of the holes is to be sought for rather in what
Sir William Herschel termed "clustering power," _i.e._ a tendency on the
part of stars to accumulate in certain places, thus leaving others
vacant; and the fact that globular and other clusters are to be found
very near to such holes certainly seems corroborative of this theory. In
summing up the whole question, Professor Newcomb maintains that there
does not appear any evidence of the light from the Milky Way stars,
which are apparently the furthest bodies we see, being intercepted by
dark bodies or dark matter. As far as our telescopes can penetrate, he
holds that we see the stars _just as they are_.

Also, if there did exist an infinite number of stars, one would expect
to find evidence in some direction of an overpoweringly great
force,--the centre of gravity of all these bodies.

It is noticed, too, that although the stars increase in number with
decrease in magnitude, so that as we descend in the scale we find three
times as many stars in each magnitude as in the one immediately above
it, yet this progression does not go on after a while. There is, in
fact, a rapid falling off in numbers below the twelfth magnitude; which
looks as if, at a certain distance from us, the stellar universe were
beginning to _thin out_.

Again, it is estimated, by Mr. Gore and others, that only about 100
millions of stars are to be seen in the whole of the sky with the best
optical aids. This shows well the limited extent of the stellar system,
for the number is not really great. For instance, there are from fifteen
to sixteen times as many persons alive upon the earth at this moment!

Last of all, there appears to be strong photographic evidence that our
sidereal system is limited in extent. Two photographs taken by the late
Dr. Isaac Roberts of a region rich in stellar objects in the
constellation of Cygnus, clearly show what has been so eloquently called
the "darkness behind the stars." One of these photographs was taken in
1895, and the other in 1898. On both occasions the state of the
atmosphere was practically the same, and the sensitiveness of the films
was of the same degree. The exposure in the first case was only one
hour; in the second it was about two hours and a half. And yet both
photographs show _exactly the same stars, even down to the faintest_.
From this one would gather that the region in question, which is one of
the most thickly star-strewn in the Milky Way, is _penetrable right
through_ with the means at our command. Dr. Roberts himself in
commenting upon the matter drew attention to the fact, that many
astronomers seemed to have tacitly adopted the assumption that the stars
extend indefinitely through space.

From considerations such as these the foremost astronomical authorities
of our time consider themselves justified in believing that the
collection of stars around us is _finite_; and that although our best
telescopes may not yet be powerful enough to penetrate to the final
stars, still the rapid decrease in numbers as space is sounded with
increasing telescopic power, points strongly to the conclusion that the
boundaries of the stellar system may not lie very far beyond the
uttermost to which we can at present see.

Is it possible then to make an estimate of the extent of this stellar
system?

Whatever estimates we may attempt to form cannot however be regarded as
at all exact, for we know the actual distances of such a very few only
of the nearest of the stars. But our knowledge of the distances even of
these few, permits us to assume that the stars close around us may be
situated, on an average, at about eight light-years from each other; and
that this holds good of the stellar spaces, with the exception of the
encircling girdle of the Milky Way, where the stars seem actually to be
more closely packed together. This girdle further appears to contain the
greater number of the stars. Arguing along these lines, Professor
Newcomb reaches the conclusion that the farthest stellar bodies which we
see are situated at about between 3000 and 4000 light-years from us.

Starting our inquiry from another direction, we can try to form an
estimate by considering the question of proper motions.

It will be noticed that such motions do not depend entirely upon the
actual speed of the stars themselves, but that some of the apparent
movement arises indirectly from the speed of our own sun. The part in a
proper motion which can be ascribed to the movement of our solar system
through space is clearly a displacement in the nature of a parallax--Sir
William Herschel called it "_Systematic_ Parallax"; so that knowing the
distance which we move over in a certain lapse of time, we are able to
hazard a guess at the distances of a good many of the stars. An inquiry
upon such lines must needs be very rough, and is plainly based upon the
assumption that the stars whose distances we attempt to estimate are
moving at an average speed much like that of our own sun, and that they
are not "runaway stars" of the 1830 Groombridge order. Be that as it
may, the results arrived at by Professor Newcomb from this method of
reasoning are curiously enough very much on a par with those founded on
the few parallaxes which we are really certain about; with the exception
that they point to somewhat closer intervals between the individual
stars, and so tend to narrow down our previous estimate of the extent of
the stellar system.

Thus far we get, and no farther. Our solar system appears to lie
somewhere near the centre of a great collection of stars, separated each
one from the other, on an average, by some 40 billions of miles; the
whole being arranged in the form of a mighty globular cluster. Light
from the nearest of these stars takes some four years to come to us. It
takes about 1000 times as long to reach us from the confines of the
system. This globe of stars is wrapt around closely by a stellar girdle,
the individual stars in which are set together more densely than those
in the globe itself. The entire arrangement appears to be constructed
upon a very regular plan. Here and there, as Professor Newcomb points
out, the aspect of the heavens differs in small detail; but generally it
may be laid down that the opposite portions of the sky, whether in the
Milky Way itself, or in those regions distant from it, show a marked
degree of symmetry. The proper motions of stars in corresponding
portions of the sky reveal the same kind of harmony, a harmony which may
even be extended to the various colours of the stars. The stellar
system, which we see disposed all around us, appears in fine to bear all
the marks of an _organised whole_.

The older astronomers, to take Sir William Herschel as an example,
supposed some of the nebulæ to be distant "universes." Sir William was
led to this conclusion by the idea he had formed that, when his
telescopes failed to show the separate stars of which he imagined these
objects to be composed, he must put down the failure to their stupendous
distance from us. For instance, he thought the Orion Nebula, which is
now known to be made up of glowing gas, to be an external stellar
system. Later on, however, he changed his mind upon this point, and came
to the conclusion that "shining fluid" would better account both for
this nebula, and for others which his telescopes had failed to separate
into component stars.

The old ideas with regard to external systems and distant universes have
been shelved as a consequence of recent research. All known clusters and
nebulæ are now firmly believed to lie _within_ our stellar system.

This view of the universe of stars as a sort of island in the
immensities, does not, however, give us the least idea about the actual
extent of space itself. Whether what is called space is really infinite,
that is to say, stretches out unendingly in every direction, or whether
it has eventually a boundary somewhere, are alike questions which the
human mind seems utterly unable to picture to itself.

[35] The Ptolemaic idea dies hard!

[36] Even the Milky Way itself is far from being a blaze of light, which
shows that the stars composing it do not extend outwards indefinitely.

[37] Mr. Gore has recently made some remarkable deductions, with regard
to the amount of light which we get from the stars. He considers that
most of this light comes from stars below the sixth magnitude; and
consequently, if all the stars visible to the naked eye were to be
blotted out, the glow of the night sky would remain practically the same
as it is at present. Going to the other end of the scale, he thinks also
that the combined light which we get from all the stars below the
seventeenth magnitude is so very small, that it may be neglected in such
an estimation. He finds, indeed, that if there are stars so low as the
twentieth magnitude, one hundred millions of them would only be equal in
brightness to a single first-magnitude star like Vega. On the other
hand, it is possible that the light of the sky at night is not entirely
due to starlight, but that some of it may be caused by phosphorescent
glow.

CHAPTER XXVI

THE STELLAR UNIVERSE--_continued_

It is very interesting to consider the proper motions of stars with
reference to such an isolated stellar system as has been pictured in the
previous chapter. These proper motions are so minute as a rule, that we
are quite unable to determine whether the stars which show them are
moving along in straight lines, or in orbits of immense extent. It
would, in fact, take thousands of years of careful observation to
determine whether the paths in question showed any degree of curving. In
the case of the more distant stars, the accurate observations which have
been conducted during the last hundred years have not so far revealed
any proper motions with regard to them; but one cannot escape the
conclusion that these stars move as the others do.

If space outside our stellar system is infinite in extent, and if all
the stars within that system are moving unchecked in every conceivable
direction, the result must happen that after immense ages these stars
will have drawn apart to such a distance from each other, that the
system will have entirely disintegrated, and will cease to exist as a
connected whole. Eventually, indeed, as Professor Newcomb points out,
the stars will have separated so far from each other that each will be
left by itself in the midst of a black and starless sky. If, however, a
certain proportion of stars have a speed sufficiently slow, they will
tend under mutual attraction to be brought to rest by collisions, or
forced to move in orbits around each other. But those stars which move
at excessive speeds, such, for instance, as 1830 Groombridge, or the
star in the southern constellation of Pictor, seem utterly incapable of
being held back in their courses by even the entire gravitative force of
our stellar system acting as a whole. These stars must, therefore, move
eventually right through the system and pass out again into the empty
spaces beyond. Add to this; certain investigations, made into the speed
of 1830 Groombridge, furnish a remarkable result. It is calculated,
indeed, that had this star been _falling through infinite space for
ever_, pulled towards us by the combined gravitative force of our entire
system of stars, it could not have gathered up anything like the speed
with which it is at present moving. No force, therefore, which we can
conjure out of our visible universe, seems powerful enough either to
have impressed upon this runaway star the motion which it now has, or to
stay it in its wild course. What an astounding condition of things!

Speculations like this call up a suspicion that there may yet exist
other universes, other centres of force, notwithstanding the apparent
solitude of our stellar system in space. It will be recollected that the
idea of this isolation is founded upon such facts as, that the heavens
do not blaze with light, and that the stars gradually appear to thin out
as we penetrate the system with increasing telescopic power. But
perchance there is something which hinders us from seeing out into space
beyond our cluster of stars; which prevents light, in fact, from
reaching us from other possible systems scattered through the depths
beyond. It has, indeed, been suggested by Mr. Gore[38] that the
light-transmitting ether may be after all merely a kind of "atmosphere"
of the stars; and that it may, therefore, thin off and cease a little
beyond the confines of our stellar system, just as the air thins off and
practically ceases at a comparatively short distance from the earth. A
clashing together of solid bodies outside our atmosphere could plainly
send us no sound, for there is no air extending the whole way to bear to
our ears the vibrations thus set up; so light emitted from any body
lying beyond our system of stars, would not be able to come to us if the
ether, whose function it is to convey the rays of light, ceased at or
near the confines of that system.

Perchance we have in this suggestion the key to the mystery of how our
sun and the other stellar bodies maintain their functions of temperature
and illumination. The radiations of heat and light arriving at the
limits of this ether, and unable to pass any further, may be thrown back
again into the system in some altered form of energy.

But these, at best, are mere airy and fascinating speculations. We have,
indeed, no evidence whatever that the luminiferous ether ceases at the
boundary of the stellar system. If, therefore, it extends outwards
infinitely in every direction, and if it has no absorbing or weakening
effect on the vibrations which it transmits, we cannot escape from the
conclusion that practically all the rays of light ever emitted by all
the stars must chase one another eternally through the never-ending
abysses of space.

[38] _Planetary and Stellar Studies_, by John Ellard Gore, F.R.A.S.,
M.R.I.A., London, 1888.

CHAPTER XXVII

THE BEGINNING OF THINGS

LAPLACE'S NEBULAR HYPOTHESIS

Dwelling upon the fact that all the motions of revolution and rotation
in the solar system, as known in his day, took place in the same
direction and nearly in the same plane, the great French astronomer,
Laplace, about the year 1796, put forward a theory to account for the
origin and evolution of that system. He conceived that it had come into
being as a result of the gradual contraction, through cooling, of an
intensely heated gaseous lens-shaped mass, which had originally occupied
its place, and had extended outwards beyond the orbit of the furthest
planet. He did not, however, attempt to explain how such a mass might
have originated! He went on to suppose that this mass, _in some manner_,
perhaps by mutual gravitation among its parts, had acquired a motion of
rotation in the same direction as the planets now revolve. As this
nebulous mass parted with its heat by radiation, it contracted towards
the centre. Becoming smaller and smaller, it was obliged to rotate
faster and faster in order to preserve its equilibrium. Meanwhile, in
the course of contraction, rings of matter became separated from the
nucleus of the mass, and were left behind at various intervals. These
rings were swept up into subordinate masses similar to the original
nebula. These subordinate masses also contracted in the same manner,
leaving rings behind them which, in turn, were swept up to form
satellites. Saturn's ring was considered, by Laplace, as the only
portion of the system left which still showed traces of this
evolutionary process. It is even probable that it may have suggested the
whole of the idea to him.

Laplace was, however, not the first philosopher who had speculated along
these lines concerning the origin of the world.

Nearly fifty years before, in 1750 to be exact, Thomas Wright, of
Durham, had put forward a theory to account for the origin of the whole
sidereal universe. In his theory, however, the birth of our solar system
was treated merely as an incident. Shortly afterwards the subject was
taken up by the famous German philosopher, Kant, who dealt with the
question in a still more ambitious manner, and endeavoured to account in
detail for the origin of the solar system as well as of the sidereal
universe. Something of the trend of such theories may be gathered from
the remarkable lines in Tennyson's _Princess_:--

"This world was once a fluid haze of light,
Till toward the centre set the starry tides,
And eddied into suns, that wheeling cast
The planets."

The theory, as worked out by Kant, was, however, at the best merely a
_tour de force_ of philosophy. Laplace's conception was much less
ambitious, for it did not attempt to explain the origin of the entire
universe, but only of the solar system. Being thus reasonably limited in
its scope, it more easily obtained credence. The arguments of Laplace
were further founded upon a mathematical basis. The great place which
he occupied among the astronomers of that time caused his theory to
exert a preponderating influence on scientific thought during the
century which followed.

A modification of Laplace's theory is the Meteoritic Hypothesis of Sir
Norman Lockyer. According to the views of that astronomer, the material
of which the original nebula was composed is presumed to have been in
the meteoric, rather than in the gaseous, state. Sir Norman Lockyer
holds, indeed, that nebulæ are, in reality, vast swarms of meteors, and
the light they emit results from continual collisions between the
constituent particles. The French astronomer, Faye, also proposed to
modify Laplace's theory by assuming that the nebula broke up into rings
all at once, and not in detail, as Laplace had wished to suppose.

The hypothesis of Laplace fits in remarkably well with the theory put
forward in later times by Helmholtz, that the heat of the sun is kept up
by the continual contraction of its mass. It could thus have only
contracted to its present size from one very much larger.

Plausible, however, as Laplace's great hypothesis appears on the
surface, closer examination shows several vital objections, a few of
those set forth by Professor Moulton being here enumerated--

Although Laplace held that the orbits of the planets were sufficiently
near to being in the one plane to support his views, yet later
investigators consider that their very deviations from this plane are a
strong argument against the hypothesis.

Again, it is thought that if the theory were the correct explanation,
the various orbits of the planets would be much more nearly circular
than they are.

It is also thought that such interlaced paths, as those in which the
asteroids and the little planet Eros move, are most unlikely to have
been produced as a result of Laplace's nebula.

Further, while each of the rings was sweeping up its matter into a body
of respectable dimensions, its gravitative power would have been for the
time being so weak, through being thus spread out, that any lighter
elements, as, for instance, those of the gaseous order, would have
escaped into space in accordance with the principles of the kinetic
theory.

_The idea that rings would at all be left behind at certain intervals
during the contraction of the nebula is, perhaps, one of the weakest
points in Laplace's hypothesis._

Mathematical investigation does not go to show that the rings, presuming
they could be left behind during the contraction of the mass, would have
aggregated into planetary bodies. Indeed, it rather points to the
reverse.

Lastly, such a discovery as that the ninth satellite of Saturn revolves
in a _retrograde_ direction--that is to say, in a direction contrary to
the other revolutions and rotations in our solar system--appears
directly to contradict the hypothesis.

Although Laplace's hypothesis seems to break down under the keen
criticism to which it has been subjected, yet astronomers have not
relinquished the idea that our solar system has probably had its origin
from a nebulous mass. But the apparent failure of the Laplacian theory
is emphasised by the fact, that _not a single example of a nebula, in
the course of breaking up into concentric rings, is known to exist in
the entire heaven_. Indeed, as we saw in Chapter XXIV., there seems to
be no reliable example of even a "ring" nebula at all. Mr. Gore has
pointed this out very succinctly in his recently published work,
_Astronomical Essays_, where he says:--"To any one who still persists in
maintaining the hypothesis of ring formation in nebulæ, it may be said
that the whole heavens are against him."

The conclusions of Keeler already alluded to, that the spiral is the
normal type of nebula, has led during the past few years to a new theory
by the American astronomers, Professors Chamberlin and Moulton. In the
detailed account of it which they have set forth, they show that those
anomalies which were stumbling-blocks to Laplace's theory do not
contradict theirs. To deal at length with this theory, to which the name
of "Planetesimal Hypothesis" has been given, would not be possible in a
book of this kind. But it may be of interest to mention that the authors
of the theory in question remount the stream of time still further than
did Laplace, and seek to explain the _origin_ of the spiral nebulæ
themselves in the following manner:--

Having begun by assuming that the stars are moving apparently in every
direction with great velocities, they proceed to point out that sooner
or later, although the lapse of time may be extraordinarily long,
collisions or near approaches between stars are bound to occur. In the
case of collisions the chances are against the bodies striking together
centrally, it being very much more likely that they will hit each other
rather towards the side. The nebulous mass formed as a result of the
disintegration of the bodies through their furious impact would thus
come into being with a spinning movement, and a spiral would ensue.
Again, the stars may not actually collide, but merely approach near to
each other. If very close, the interaction of gravitation will give rise
to intense strains, or tides, which will entirely disintegrate the
bodies, and a spiral nebula will similarly result. As happens upon our
earth, two such tides would rise opposite to each other; and,
consequently, it is a noticeable fact that spiral nebulæ have almost
invariably two opposite branches (see Plate XXII., p 314). Even if not
so close, the gravitational strains set up would produce tremendous
eruptions of matter; and in this case, a spiral movement would also be
generated. On such an assumption the various bodies of the solar system
may be regarded as having been ejected from parent masses.

The acceptance of the Planetesimal Hypothesis in the place of the
Hypothesis of Laplace will not, as we have seen, by any means do away
with the probability that our solar system, and similar systems, have
originated from a nebulous mass. On the contrary it puts that idea on a
firmer footing than before. The spiral nebulæ which we see in the
heavens are on a vast scale, and may represent the formation of stellar
systems and globular clusters. Our solar system may have arisen from a
small spiral.

We will close these speculations concerning the origin of things with a
short sketch of certain investigations made in recent years by Sir
George H. Darwin, of Cambridge University, into the question of the
probable birth of our moon. He comes to the conclusion that at least
fifty-four millions of years ago the earth and moon formed one body,
which had a diameter of a little over 8000 miles. This body rotated on
an axis in about five hours, namely, about five times as fast as it does
at present. The rapidity of the rotation caused such a tremendous strain
that the mass was in a condition of, what is called, unstable
equilibrium; very little more, in fact, being required to rend it
asunder. The gravitational pull of the sun, which, as we have already
seen, is in part the cause of our ordinary tides, supplied this extra
strain, and a portion of the mass consequently broke off, which receded
gradually from the rest and became what we now know as the moon. Sir
George Darwin holds that the gravitational action of the sun will in
time succeed in also disturbing the present apparent harmony of the
earth-moon system, and will eventually bring the moon back towards the
earth, so that after the lapse of great ages they will re-unite once
again.

In support of this theory of the terrestrial origin of the moon,
Professor W.H. Pickering has put forward a bold hypothesis that our
satellite had its origin in the great basin of the Pacific. This ocean
is roughly circular, and contains no large land masses, except the
Australian Continent. He supposes that, prior to the moon's birth, our
globe was already covered with a slight crust. In the tearing away of
that portion which was afterwards destined to become the moon the
remaining area of the crust was rent in twain by the shock; and thus
were formed the two great continental masses of the Old and New Worlds.
These masses floated apart across the fiery ocean, and at last settled
in the positions which they now occupy. In this way Professor Pickering
explains the remarkable parallelism which exists between the opposite
shores of the Atlantic. The fact of this parallelism had, however, been
noticed before; as, for example, by the late Rev. S.J. Johnson, in his
book _Eclipses, Past and Future_, where we find the following passage:--

"If we look at our maps we shall see the parts of one Continent that jut
out agree with the indented portions of another. The prominent coast of
Africa would fit in the opposite opening between North and South
America, and so in numerous other instances. A general rending asunder
of the World would seem to have taken place when the foundations of the
great deep were broken up."

Although Professor Pickering's theory is to a certain degree anticipated
in the above words, still he has worked out the idea much more fully,
and given it an additional fascination by connecting it with the birth
of the moon. He points out, in fact, that there is a remarkable
similarity between the lunar volcanoes and those in the immediate
neighbourhood of the Pacific Ocean. He goes even further to suggest that
Australia is another portion of the primal crust which was detached out
of the region now occupied by the Indian Ocean, where it was originally
connected with the south of India or the east of Africa.

Certain objections to the theory have been put forward, one of which is
that the parallelism noticed between the opposite shores of the Atlantic
is almost too perfect to have remained through some sixty millions of
years down to our own day, in the face of all those geological movements
of upheaval and submergence, which are perpetually at work upon our
globe. Professor Pickering, however, replies to this objection by
stating that many geologists believe that the main divisions of land and
water on the earth are permanent, and that the geological alterations
which have taken place since these were formed have been merely of a
temporary and superficial nature.

CHAPTER XXVIII

THE END OF THINGS

We have been trying to picture the beginning of things. We will now try
to picture the end.

In attempting this, we find that our theories must of necessity be
limited to the earth, or at most to the solar system. The time-honoured
expression "End of the World" really applies to very little beyond the
end of our own earth. To the people of past ages it, of course, meant
very much more. For them, as we have seen, the earth was the centre of
everything; and the heavens and all around were merely a kind of minor
accompaniment, created, as they no doubt thought, for their especial
benefit. In the ancient view, therefore, the beginning of the earth
meant the beginning of the universe, and the end of the earth the
extinction of all things. The belief, too, was general that this end
would be accomplished through fire. In the modern view, however, the
birth and death of the earth, or indeed of the solar system, might pass
as incidents almost unnoticed in space. They would be but mere links in
the chain of cosmic happenings.

A number of theories have been forward from time to time prognosticating
the end of the earth, and consequently of human life. We will conclude
with a recital of a few of them, though which, if any, is the true one,
the Last Men alone can know.

Just as a living creature may at any moment die in the fulness of
strength through sudden malady or accident, or, on the other hand, may
meet with death as a mere consequence of old age, so may our globe be
destroyed by some sudden cataclysm, or end in slow processes of decay.
Barring accidents, therefore, it would seem probable that the growing
cold of the earth, or the gradual extinction of the sun, should after
many millions of years close the chapter of life, as we know it. On the
former of these suppositions, the decrease of temperature on our globe
might perhaps be accelerated by the thinning of the atmosphere, through
the slow escape into space of its constituent gases, or their gradual
chemical combination with the materials of the earth. The subterranean
heat entirely radiated away, there would no longer remain any of those
volcanic elevating forces which so far have counteracted the slow
wearing down of the land surface of our planet, and thus what water
remained would in time wash over all. If this preceded the growing cold
of the sun, certain strange evolutions of marine forms of life would be
the last to endure, but these, too, would have to go in the end.

Should, however, the actual process be the reverse of this, and the sun
cool down the quicker, then man would, as a consequence of his
scientific knowledge, tend in all probability to outlive the other forms
of terrestrial life. In such a vista we can picture the regions of the
earth towards the north and south becoming gradually more and more
uninhabitable through cold, and human beings withdrawing before the
slow march of the icy boundary, until the only regions capable of
habitation would lie within the tropics. In such a struggle between man
and destiny science would be pressed to the uttermost, in the devising
of means to counteract the slow diminution of the solar heat and the
gradual disappearance of air and water. By that time the axial rotation
of our globe might possibly have been slowed down to such an extent that
one side alone of its surface would be turned ever towards the fast
dying sun. And the mind's eye can picture the last survivors of the
human race, huddled together for warmth in a glass-house somewhere on
the equator, waiting for the end to come.

The mere idea of the decay and death of the solar system almost brings
to one a cold shudder. All that sun's light and heat, which means so
much to us, entirely a thing of the past. A dark, cold ball rushing
along in space, accompanied by several dark, cold balls circling
ceaselessly around it. One of these a mere cemetery, in which there
would be no longer any recollection of the mighty empires, the loves and
hates, and all that teeming play of life which we call History.
Tombstones of men and of deeds, whirling along forgotten in the darkness
and silence. _Sic transit gloria mundi._

In that brilliant flight of scientific fancy, the _Time Machine_, Mr.
H.G. Wells has pictured the closing years of the earth in some such
long-drawn agony as this. He has given us a vision of a desolate beach
by a salt and almost motionless sea. Foul monsters of crab-like form
crawl slowly about, beneath a huge hull of sun, red and fixed in the
sky. The rocks around are partly coated with an intensely green
vegetation, like the lichen in caves, or the plants which grow in a
perpetual twilight. And the air is now of an exceeding thinness.

He dips still further into the future, and thus predicts the final form
of life:--

"I saw again the moving thing upon the shoal--there was no mistake now
that it was a moving thing--against the red water of the sea. It was a
round thing, the size of a football perhaps, or it may be bigger, and
tentacles trailed down from it; it seemed black against the weltering
blood-red water, and it was hopping fitfully about."

What a description of the "Heir of all the Ages!"

To picture the end of our world as the result of a cataclysm of some
kind, is, on the other hand, a form of speculation as intensely dramatic
as that with which we have just been dealing is unutterably sad.

It is not so many years ago, for instance, that men feared a sudden
catastrophe from the possible collision of a comet with our earth. The
unreasoning terror with which the ancients were wont to regard these
mysterious visitants to our skies had, indeed, been replaced by an
apprehension of quite another kind. For instance, as we have seen, the
announcement in 1832 that Biela's Comet, then visible, would cut through
the orbit of the earth on a certain date threw many persons into a
veritable panic. They did not stop to find out the real facts of the
case, namely, that, at the time mentioned, the earth would be nearly a
month's journey from the point indicated!

It is, indeed, very difficult to say what form of damage the earth
would suffer from such a collision. In 1861 it passed, as we have seen,
through the tail of the comet without any noticeable result. But the
head of a comet, on the other hand, may, for aught we know, contain
within it elements of peril for us. A collision with this part might,
for instance, result in a violent bombardment of meteors. But these
meteors could not be bodies of any great size, for the masses of comets
are so very minute that one can hardly suppose them to contain any large
or dense constituent portions.

The danger, however, from a comet's head might after all be a danger to
our atmosphere. It might precipitate, into the air, gases which would
asphyxiate us or cause a general conflagration. It is scarcely necessary
to point out that dire results would follow upon any interference with
the balance of our atmosphere. For instance, the well-known French
astronomer, M. Camille Flammarion,[39] has imagined the absorption of
the nitrogen of the air in this way; and has gone on to picture men and
animals reduced to breathing only oxygen, first becoming excited, then
mad, and finally ending in a perfect saturnalia of delirium.

Lastly, though we have no proof that stars eventually become dark and
cold, for human time has so far been all too short to give us even the
smallest evidence as to whether heat and light are diminishing in our
own sun, yet it seems natural to suppose that such bodies must at last
cease their functions, like everything else which we know of. We may,
therefore, reasonably presume that there are dark bodies scattered in
the depths of space. We have, indeed, a suspicion of at least one,
though perhaps it partakes rather of a planetary nature, namely, that
"dark" body which continually eclipses Algol, and so causes the
temporary diminution of its light. As the sun rushes towards the
constellation of Lyra such an extinguished sun may chance to find itself
in his path; just as a derelict hulk may loom up out of the darkness
right beneath the bows of a vessel sailing the great ocean.

Unfortunately a collision between the sun and a body of this kind could
not occur with such merciful suddenness. A tedious warning of its
approach would be given from that region of the heavens whither our
system is known to be tending. As the dark object would become visible
only when sufficiently near our sun to be in some degree illuminated by
his rays, it might run the chance at first of being mistaken for a new
planet. If such a body were as large, for instance, as our own sun, it
should, according to Mr. Gore's calculations, reveal itself to the
telescope some fifteen years before the great catastrophe. Steadily its
disc would appear to enlarge, so that, about nine years after its
discovery, it would become visible to the naked eye. At length the
doomed inhabitants of the earth, paralysed with terror, would see their
relentless enemy shining like a second moon in the northern skies.
Rapidly increasing in apparent size, as the gravitational attractions of
the solar orb and of itself interacted more powerfully with diminishing
distance, it would at last draw quickly in towards the sun and disappear
in the glare.

It is impossible for us to conceive anything more terrible than these
closing days, for no menace of catastrophe which we can picture could
bear within it such a certainty of fulfilment. It appears, therefore,
useless to speculate on the probable actions of men in their now
terrestrial prison. Hope, which so far had buoyed them up in the direst
calamities, would here have no place. Humanity, in the fulness of its
strength, would await a wholesale execution from which there could be no
chance at all of a reprieve. Observations of the approaching body would
have enabled astronomers to calculate its path with great exactness, and
to predict the instant and character of the impact. Eight minutes after
the moment allotted for the collision the resulting tide of flame would
surge across the earth's orbit, and our globe would quickly pass away in
vapour.

And what then?

A nebula, no doubt; and after untold ages the formation possibly from it
of a new system, rising phoenix-like from the vast crematorium and
filling the place of the old one. A new central sun, perhaps, with its
attendant retinue of planets and satellites. And teeming life,
perchance, appearing once more in the fulness of time, when temperature
in one or other of these bodies had fallen within certain limits, and
other predisposing conditions had supervened.

"The world's great age begins anew,
The golden years return,
The earth doth like a snake renew
Her winter weeds outworn:
Heaven smiles, and faiths and empires gleam
Like wrecks of a dissolving dream.

A brighter Hellas rears its mountains
From waves serener far;
A new Peneus rolls his fountains
Against the morning star;
Where fairer Tempes bloom, there sleep
Young Cyclads on a sunnier deep.

A loftier Argo cleaves the main,
Fraught with a later prize;
Another Orpheus sings again,
And loves, and weeps, and dies;
A new Ulysses leaves once more
Calypso for his native shore.

* * * * *

Oh cease! must hate and death return?
Cease! must men kill and die?
Cease! drain not to its dregs the urn
Of bitter prophecy!
The world is weary of the past,--
Oh might it die or rest at last!"

[39] See his work, _La Fin du Monde_, wherein the various ways by which
our world may come to an end are dealt with at length, and in a
profoundly interesting manner.

INDEX

Achromatic telescope, 115, 116

Adams, 24, 236, 243

Aerial telescopes, 110, 111

Agathocles, Eclipse of, 85

Agrippa, Camillus, 44

Ahaz, dial of, 85

Air, 166

Airy, Sir G.B., 92

Al gûl, 307

Al Sufi, 284, 290, 296, 315

Alcor, 294

Alcyone, 284

Aldebaran, 103, 288, 290, 297

Algol, 307, 309-310, 312, 323, 347

Alpha, Centauri, 52-53, 280, 298-299, 304, 320

Alpha Crucis, 298

Alps, Lunar, 200

Altair, 295

Altitude of objects in sky, 196

Aluminium, 145

Amos viii. 9, 85

Anderson, T.D., 311-312

Andromeda (constellation), 279, 314;
Great Nebula in, 314, 316

Andromedid meteors, 272

Anglo-Saxon Chronicle, 87-88

Anighito meteorite, 277

Annular eclipse, 65-68, 80, 92, 99

Annular Nebula in Lyra, 315-316

Annulus, 68

Ansæ, 242-243

Anticipation in discovery, 236-237

Apennines, Lunar, 200

Aphelion, 274

Apparent enlargement of celestial objects, 192-196

Apparent size of celestial objects deceptive, 196, 294

Apparent sizes of sun and moon, variations in, 67, 80, 178

Aquila (constellation), 295

Arabian astronomers, 107, 307

Arago, 92, 257

Arc, degrees minutes and seconds of, 60

Arcturus, 280, 282, 290, 295

Argelander, 290

Argo (constellation), 298

Aristarchus of Samos, 171

Aristarchus (lunar crater), 205

Aristophanes, 101

Aristotle, 161, 173, 185

Arrhenius 222, 253-254

Assyrian tablet, 84

Asteroidal zone, analogy of, to Saturn's rings, 238

Asteroids (or minor planets), 30-31, 225-228, 336;
discovery of the, 23, 244;
Wolf's method of discovering, 226-227

Astrology, 56

_Astronomical Essays_, 63, 337

Astronomical Society, Royal, 144

_Astronomy, Manual of_, 166

Atlantic Ocean, parallelism of opposite shores, 340-341

Atlas, the Titan, 18

Atmosphere, absorption by earth's, 129-130;
ascertainment of, by spectroscope, 124-125, 212;
height of earth's, 167, 267;
of asteroids, 226;
of earth, 129, 130, 166-169, 218, 222, 267, 346;
of Mars, 156, 212, 216;
of Mercury, 156;
of moon, 70-71, 156, 201-203;
of Jupiter, 231;
of planets, 125;
of Saturn's rings, 239

"Atmosphere" of the stars, 331

Atmospheric layer and "glass-house" compared, 167, 203

August Meteors (Perseids), 270

Auriga (constellation), 294-296, 306, 311;
New Star in, 311

Aurigæ, [b] (Beta), 294, 297, 304

Aurora Borealis, 141, 143, 259

Australia, suggested origin of, 340

Axis, 29-30;
of earth, 163, 180;
small movement of earth's, 180-181

Babylonian tablet, 84

Babylonian idea of the moon, 185

Bacon, Roger, 108

Bacubirito meteorite, 277

Bagdad, 107

Baily, Francis, 92

"Baily's Beads," 69, 70, 91-92, 154

Bailly (lunar crater), 199

Ball, Sir Robert, 271

Barnard, E.E., 31, 224, 232-234, 237, 258

"Bay of Rainbows," 197

Bayer's classification of stars, 289, 291-292

Bayeux Tapestry, 263

Bear, Great (constellation). _See_ Ursa Major;
Little, _see_ Ursa Minor

Beehive (Præsepe), 307

Beer, 206

Belopolsky, 304

"Belt" of Orion, 297

Belt theory of Milky Way, 321

Belts of Jupiter, 230

Bergstrand, 314

Berlin star chart, 244

Bessel, 173, 280, 305

Beta ([b]) Lyræ, 307

Beta ([b]) Persei. _See_ Algol

Betelgeux, 297

Bible, eclipses in, 85

Biela's Comet, 256-257, 272-273, 345

Bielids, 270, 272-273

Billion, 51-52

Binary stars, spectroscopic, 301-306, 309;
visual, 300, 303-306

"Black Drop," 152-154

"Black Hour," 89

"Black Saturday," 89

Blood, moon in eclipse like, 102

Blue (rays of light), 121, 130

Bode's Law, 22-23, 244-245

Bolometer, 127

Bond, G.P., 236, 257

Bonpland, 270

Boötes (constellation), 295, 314

Bradley, 111

Brahe, Tycho, 290, 311

Brédikhine's theory of comets' tails, 253-254, 256

Bright eclipses of moon, 65, 102

British Association for the Advancement of Science, 318

_British Astronomical Association, Journal of_, 194

British Museum, 84

Bull (constellation). _See_ Taurus;
"Eye" of the, 297;
"Head" of the, 297

Burgos, 98

Busch, 93

Cæsar, Julius, 85, 110, 180, 259, 262, 291, 293

Calcium, 138, 145

Callisto, 233-234

Cambridge, 24, 91, 119, 243

Campbell, 305

Canali, 214

"Canals" of Mars, 214-222, 224-225

Cancer (constellation), 307

Canes Venatici (constellation), 306, 314

Canis Major (constellation), 289, 296-297;
Minor, 296-297

Canopus, 285, 298-299, 320

Capella, 280, 282, 290, 294, 297, 303, 313

Carbon, 145

Carbon dioxide. _See_ Carbonic acid gas

Carbonic acid gas, 166, 213, 221-222

Carnegie Institution, Solar Observatory of, 118

Cassegrainian telescope, 114, 118

Cassini, J.D., 236, 240

"Cassini's Division" in Saturn's ring, 236, 238

Cassiopeia (constellation), 279, 294, 311, 314

Cassiopeiæ, [ê] (Eta), 303

Cassiopeia's Chair, 294

Cassius, Dion, 86

Castor, 282, 297, 304

Catalogues of stars, 106, 290-291, 311

Centaur. _See_ Centaurus

Centaurus (constellation), 298, 306

Centre of gravity, 42, 283-284, 324

Ceres, diameter of, 30, 225

Ceti, Omicron (or Mira), 307-308

Cetus, or the Whale (constellation), 307

Chaldean astronomers, 74, 76

Challis, 243-244

Chamberlin, 337

"Chambers of the South," 299

Chandler, 308

Charles V., 261

"Charles' Wain," 291

Chemical rays, 127

Chinese and eclipses, 83

Chloride of sodium, 122

Chlorine, 122, 145

Christ, Birth of, 102

Christian Era, first recorded solar eclipse in, 85

Chromatic aberration, 110

Chromosphere, 71-72, 93-94, 130-132, 138-139

Circle, 171-173

Clark, Alvan, & Sons, 117-118, 303

Claudius, Emperor, 86

Clavius (lunar crater), 199

Clerk Maxwell, 237

"Clouds" (of Aristophanes), 101

Clustering power, 325

Clusters of stars, 300, 306, 314, 328

Coal Sacks. _See_ Holes in Milky Way

Coelostat, 119

Coggia's Comet, 254

Colour, production of, in telescopes, 109-111, 115, 121

Collision of comet with earth, 345-346;
of dark star with sun, 346-348;
of stars, 285, 312

Columbus, 103

Coma Berenices (constellation), 307, 316

Comet, first discovery of by photography, 258;
first orbit calculated, 255;
first photograph of, 257-258;
furthest distance seen, 258;
passage of among satellites of Jupiter, 250;
passage of earth and moon through tail of, 257, 346

Comet of 1000 A.D., 262;
1066, 262-264;
1680, 255, 265;
1811, 254-255;
1861, 254, 257, 346;
1881, 257-258;
1882, 251, 258, 291;
1889, 258;
1907, 258

Comets, 27-28, 58, Chaps. XIX. and XX., 345-346;
ancient view of, 259-261;
captured, 251-253;
Chinese records of, 83-84;
composition of, 252;
contrasted with planets, 247;
families of, 251-252, 256;
meteor swarms and, 274;
revealed by solar eclipses, 95-96;
tails of, 141, 182, 248, 252-254

Common, telescopes of Dr. A.A., 118

Conjunction, 209

Constellations, 105, 278-279, 285, 289

Contraction theory of sun's heat, 128-129, 335

Cook, Captain, 154

Cooke, 118

Copernican system, 20, 107, 149, 170-173, 279, 280

Copernicus, 20, 108, 149, 158, 170-172, 236

Copernicus (lunar crater), 200, 204

Copper, 145

Corder, H., 144

Corona, 70-72, 90, 92-97, 132, 140-141, 270;
earliest drawing of, 91;
earliest employment of term, 90;
earliest mention of, 86;
earliest photograph of, 93;
illumination given by, 71;
possible change in shape of during eclipse, 96-98;
structure of, 142-143;
variations in shape of, 141

Corona Borealis (constellation), 295

Coronal matter, 142;
streamers, 95-96, 141-143

Coronium, 133, 142, 317

Cotes, 91

Coudé, equatorial, 119

Cowell, P.H., 255, 264

Crabtree, 152

Crape ring of Saturn, 236-237

Craterlets on Mars, 220

Craters (ring-mountains) on moon, 197-205, 214, 340;
suggested origin of, 203-204, 214

Crawford, Earl of, 94

Crecy, supposed eclipse at battle of, 88-89

Crescent moon, 183, 185

Crommelin, A.C.D., 255, 264

Crossley Reflector, 118, 315-316

Crown glass, 115

Crucifixion, darkness of, 86

Crucis, [a] (Alpha), 298

Crux, or "Southern Cross" (constellation), 298-299, 323

Cycle, sunspot, 136-137, 141, 143-144

Cygni, 61, 173, 280

Cygnus, or the Swan (constellation), 295, 325

Daniel's Comet of 1897, 258

Danzig, 111

Dark Ages, 102, 107, 260

Dark eclipses of moon, 65, 102-103

Dark matter in space, 323

Dark meteors, 275-276

Dark stars, 309-310, 312, 323, 346-347

"Darkness behind the stars," 325

Darwin, Sir G.H., 339

Davis, 94

Dawes, 236

Dearborn Observatory, 303

Death from fright at eclipse, 73

Debonnaire, Louis le, 88, 261

Deimos, 223

Deity, symbol of the, 87

"Demon star." _See_ Algol

Denebola, 296

Denning, W.F., 269

Densities of sun and planets, 39

Density, 38

Deslandres, 140

Diameters of sun and planets, 31

Disappearance of moon in lunar eclipse, 65, 102-103

Disc, 60

"Disc" theory. _See_ "Grindstone" theory

Discoveries, independent, 236

Discovery, anticipation in, 236-237;
indirect methods of, 120

"Dipper," the, 291;
the "Little," 294

Distance of a celestial body, how ascertained, 56-58;
of sun from earth, how determined, 151, 211

Distances of planets from sun, 47

Distances of sun and moon, relative, 68

Dog, the Greater. _See_ Canis Major;
the Lesser, _see_ Canis Minor

"Dog Star," 289, 297

Dollond, John, 115-116

Donati's Comet, 254, 257

Doppler's method, 125, 136, 282, 301-302

Dorpat, 117

Double canals of Mars, 214-215, 218-220

Double planet, earth and moon a, 189

Double stars, 300

Douglass, 233

"Dreams, Lake of," 197

Dumb-bell Nebula, 316

Earth, 20, 22, 31, 39, 48, 64, Chap. XV., 267;
cooling of, 343;
diameter of, 31;
interior of, 166;
mean distance of from sun, 47;
rigidity of, 181;
rotation of, 30, 33, 161-165, 170;
shape of, 165;
"tail" to, 182

"Earthlight," or "Earthshine," 186

Earth's axis, Precessional movement of, 175-177, 295, 298-299

Earth's shadow, circular shape of, 64, 160

Eclipse, 61

Eclipse knowledge, delay of, 74

Eclipse party, work of, 73

Eclipse of sun, advance of shadow in total, 69;
animal and plant life during, 71;
earliest record of total, 84;
description of total, 69-73;
duration of total, 69, 72;
importance of total, 68

Eclipses, ascertainment of dates of past, 74;
experience a necessity in solar, 73-74;
of moon, 63-65, Chap. IX., 203;
photography in, 93;
prediction of future, 74;
recurrence of, 74-80

Eclipses of sun, 25, 65-74, Chap. VIII., 201-202, 234;
1612 A.D., 90;
1715, 88, 91;
1724, 88, 91;
1836, 92;
1842, 92-93;
1851, 81, 93;
1868, 93;
1870, 94;
1871, 94;
1878, 95;
1882, 95;
1883, 95-96;
1893, 95-96;
1896, 96, 99;
1898, 96, 98;
1900, 97;
1905, 75-76, 80-81, 97-98;
1907, 98;
1908, 98;
1914, 99;
1927, 92, 99-100

_Eclipses, Past and Future_, 340

Egenitis, 272

Electric furnace, 128

Electric light, spectrum of, 122

Elements composing sun, 144-145

Ellipses, 32, 66, 172-173, 177-178

Elliptic orbit, 66, 177

Ellipticity, 32

Elongation, Eastern, 147, 149;
Western, 147, 149

Encke's Comet, 253, 256

"End of the World," 342

England, solar eclipses visible in, 87-88, 91-92

Epsilon, ([e]) Lyræ, 302

Equator, 48

Equatorial telescope, 226

Equinoxes. _See_ Precession of

Eros, 210-211, 223, 226-227;
discovery of, 24, 210, 227;
importance of, 211;
orbit of, 32, 37, 210, 336

Eruptive prominences, 139

_Esclistre_, 89

Ether, 322-323, 331-332

Europa, 233, 235

Evans, J.E., 219

Evening star, 149-150, 241

Everest, Mount, 200

Evershed, 182

Eye-piece, 110

Fabricius, 307

Faculæ, 136, 143

Fauth, 205

Faye, 335

_Fin du Monde_, 346

First quarter, 183

"Fixed stars," 280

Flagstaff, 215-216, 220

Flammarion, Camille, 346

Flamsteed, 90

"Flash spectrum," 137

"Flat," 112

Flint glass, 115

Focus, 66, 177

"Forty-foot Telescope," 115

Foster, 102

Fraunhofer, 117

French Academy of Sciences, 115

Froissart, 89

"Full moon" of Laplace, 190

Galaxy. _See_ Milky Way.

Galilean telescope, 109

Galileo, 55, 109, 172, 197, 206, 232-235, 242

Galle, 24, 211, 244

Ganymede, 233-234

Gas light, spectrum of, 122

Gegenschein, 181-182

"Gem" of meteor ring, 271

Gemini, or the Twins (constellation), 22, 296-297

Geminorum, [z] (Zeta), 304

Geometrical groupings of stars, 292

"Giant" planet, 230, 238-239

Gibbous, 183, 185

Gill, Sir David, 211, 258, 291, 317-318

Gold, 145

Goodricke, 307

Gore, J.E., 63, 285, 303, 307-308, 310, 323-324, 331, 337, 347

Granulated structure of photosphere, 134

Gravitation (or gravity), 39, 41-45, 128, 306

Greek ideas, 18, 158, 161-162, 171, 186, 197

Green (rays of light), 121

Greenwich Observatory, 143-144, 232, 255, 303

Gregorian telescope, 113-114

Grimaldi (lunar crater), 199

"Grindstone" theory, 319-322

"Groombridge, 1830," 281-282, 326, 330

Groups of stars, 306-307

Grubb, Sir Howard, 118

_Gulliver's Travels_, 224

Hale, G.E., 119, 140

Half moon, 183, 185

Hall, Asaph, 223

Hall, Chester Moor, 115

Halley, Edmund, 91, 255, 264-265, 306

Halley's Comet, 255, 264-265

Haraden Hill, 91

Harvard, 118, 302

Harvest moon, 190-192

Hawaii, 221

Heat rays, 127

Heidelberg, 226, 232

Height of lunar mountains, how determined, 201

Height of objects in sky, estimation of, 196

Helium, 138, 145, 182

Helmholtz, 128, 335

Hercules (constellation), 295

Herod the Great, 101-102

Herodotus, 84

Herschel, A.S., 269

Herschel, Sir John, 92, 322

Herschel, Sir William, 22, 36, 114-115, 204, 213, 235, 283, 292, 308,
319-320, 326-328

Herschelian telescope, 114, 119

Hesper, 109

Hesperus, 150

Hevelius, 111

Hezekiah, 85

Hi, 83

Hindoos, 18

Hipparchus, 106, 177, 290, 311

Ho, 83

Holes in Milky Way, 321-323

Holmes, Oliver Wendell, 213

Homer, 223

Horace, Odes of, 106

Horizon, 159

Horizontal eclipse, 169

Horrox, 44, 151-152

Hour Glass Sea, 212

Huggins, Sir William, 94, 125, 317

Humboldt, 270

"Hunter's moon," 192

Huyghens, 111-112, 240, 242-243

Hyades, 296-297, 307

Hydrocarbon gas, 254

Hydrogen, 94, 131, 138, 140, 144, 156, 182, 254

Ibrahim ben Ahmed, 270

Ice-layer theory:
Mars, 219;
moon, 205, 219

Illusion theory of Martian canals, 219

Imbrium, Mare, 197

Inclination of orbits, 36-37

Indigo (rays of light), 121

Inferior conjunction, 147, 149

Inferior planets, 20, 22, Chap. XIV., 229

Instruments, pre-telescopic, 106-107, 172

International photographic survey of sky, 290-291

Intra-Mercurial planet, 25-26

_Introduction to Astronomy_, 31

Inverted view in astronomical telescope, 116-117

Io, 233-234

Iridum, Sinus, 197

Iron, 145, 254

_Is Mars Habitable?_ 221

Jansen, 108

Janssen, 94, 236, 258

Japetus, 240

Jessenius, 89

Job, Book of, 299

Johnson, S.J., 103, 340

Josephus, 101, 262

Juno, 225

Jupiter, 20, 22-23, 31, 34, 37, 42, 227-228, 230-236, 241, 272, 311;
comet family of, 251-253, 256;
discovery of eighth satellite, 26, 232;
eclipse of, by satellite, 234;
without satellites, 234-235

Jupiter, satellites of, 26, 62, 108, 189, 232-235;
their eclipses, 234-235;
their occultations, 62, 234;
their transits, 62, 234

Kant, 334

Kapteyn, 284, 313

Keeler, 315, 337

Kelvin, Lord, 129

Kepler, 44, 152, 172, 237, 242, 245, 253, 311

Kinetic theory, 156, 202, 212, 226, 231, 239, 336

King, L.W., 84

_Knowledge_, 87

Labrador, 97

Lacus Somniorum, 197

"Lake of Dreams," 197

Lalande, 244, 283

Lampland, 215, 219

Langley, 95, 127

Laplace, 190, 333

Laputa, 224

Le Maire, 115

Le Verrier, 24, 236, 243-244, 275

Lead, 145

Leibnitz Mountains (lunar), 200

Leo (constellation), 270, 295-296

Leonids, 270-272, 274-275

Lescarbault, 25

Lewis, T., 303

Lexell's Comet, 250

Lick Observatory, 31, 98, 117-118, 215, 232, 303, 305, 315;
Great Telescope of, 117, 215, 237

"Life" of an eclipse of the moon, 80;
of the sun, 77-78

Life on Mars, Lowell's views, 217-218;
Pickering's, 221;
Wallace's, 221-223

Light, no extinction of, 322-324;
rays of, 127;
velocity of, 52, 235-236;
white, 121

"Light year," 53, 280

Lindsay, Lord, 94

Linné (lunar crater), 205

Liouville, 190

Lippershey, 108

Liquid-filled lenses, 116

_Locksley Hall_, 296;
_Sixty Years After_, 109

Lockyer, Sir Norman, 73, 94, 236, 335

Loewy, 119, 206

London, eclipses visible at, 87-88, 91-92

Longfellow, 88

Lowell Observatory, 215, 219, 233-234

Lowell, Percival, 155, 212-213, 215-221

Lucifer, 150

Lynn, W.T., 219, 263

Lyra (constellation), 177, 283, 294-295, 307, 315, 347

Mädler, 206, 284

Magellanic Clouds, 317

Magnetism, disturbances of terrestrial, 143, 283

Magnitudes of stars, 287-289

Major planets, 229-230

"Man in the Moon," 197

_Manual of Astronomy_, 166

Maps of the moon, 206

Mare Imbrium, 197

Mare Serenitatis, 205

Mars, 20, 22-23, 31-32, 34, 37, 109, 155, 210-225, 234;
compared with earth and moon, 221, 225;
polar caps of, 212-214, 216;
satellites of, 26, 223-224;
temperature of, 213, 216, 221-222

Mass, 38;
of a star, how determined, 305

Masses of celestial bodies, how ascertained, 42;
of earth and moon compared, 42;
of sun and planets compared, 39

Maunder, E.W., 87, 143, 219

Maunder, Mrs., E.W., 96, 144

Maxwell, Clerk, 237

Mayer, Tobias, 206, 283

McClean, F.K., 98

Mean distance, 46

"Medicean Stars," 232

Mediterranean, eclipse tracks across, 94, 97

Melbourne telescope, 118

Melotte, P., 232

Mercator's Projection, 80-81

Mercury (the metal), 145

Mercury (the planet), 20, 22, 25-26, 31-32, 34, 37, Chap. XIV.;
markings on, 156;
possible planets within orbit of, 25-26;
transit of, 62, 151, 154

Metals in sun, 145

Meteor swarms, 268-269, 271, 274-275

Meteors, 28, 56, 167, 259, Chap. XXI.

Meteors beyond earth's atmosphere, 275-276

Meteorites, 276-277

Meteoritic Hypothesis, 335

Metius, Jacob, 108

Michell, 283, 305

Middle Ages, 102, 260, 264

Middleburgh, 108

Milky Way (or Galaxy), 285, 299, 311, 317, 319-327;
penetration of, by photography, 325

Million, 47, 51-52

Minor planets. _See_ Asteroids.

Mira Ceti, 307-308

"Mirk Monday," 89

Mirror (speculum), 111, 116

Mizar, 294, 302

Monck, W.H.S., 275

Mongol Emperors of India, 107

Moon, 26, Chap. XVI.;
appearance of, in lunar eclipse, 65, 102-103;
diameter of, 189;
distance of, how ascertained, 58;
distance of, from earth, 48;
full, 63, 86, 149, 184, 189, 190, 206;
mass of, 200, 202;
mountains on, 197-205;
how their height is determined, 201;
movement of, 40-42;
new, 86, 149, 183, 185;
origin of, 339-341;
plane of orbit of, 63;
possible changes on, 204-205, 221;
"seas" of, 197, 206;
smallest detail visible on, 207;
volume of, 200

Morning star, 149-150, 241

Moulton, F.R., 31, 118, 128, 302, 335, 337

Moye, 154

Multiple stars, 300

Musa-ben-Shakir, 44

Mythology, 105

Neap-tides, 179

Nebulæ, 314-318, 328, 335, 345;
evolution of stars from, 317-318

Nebular Hypothesis of Laplace, 333-338

Nebular hypotheses, Chap. XXVII.

Nebulium, 317

Neison, 206

Neptune, 20, 25, 31, 34, 37, 243-246, 249, 252, 274, 304;
discovery of, 23-24, 94, 210, 236, 243-244;
Lalande and, 244;
possible planets beyond, 25, 252;
satellite of, 26, 245;
"year" in, 35-36

"New" (or temporary), stars, 310-314

Newcomb, Simon, 181, 267, 281, 324, 326-327, 329

Newton, Sir Isaac, 40, 44, 91, 111-113, 115, 165, 172, 237, 255

Newtonian telescope, 112, 114, 116, 119

Nineveh Eclipse, 84-85

Nitrogen, 145, 156, 166, 346

Northern Crown, 295

Nova Aurigæ, 311

Nova Persei, 312-314

Novæ. _See_ New (or temporary) stars

Nubeculæ, 317

"Oases" of Mars, 216, 220

Object-glass, 109

Oblate spheroid, 165

Occultation, 61-62, 202, 296

_Olaf, Saga of King_, 88

Olbers, 227, 253, 256, 271

"Old moon in new moon's arms," 185

Olmsted, 271

Omicron (or "Mira") Ceti, 307-308

Opposition, 209

"Optick tube," 108-109, 232

Orange (rays of light), 121

Orbit of moon, plane of, 63

Orbits, 32, 36-37, 66, 150, 157

Oriental astronomy, 107

Orion (constellation), 195, 279, 296-297, 316;
Great Nebula in, 316, 328

Oxford, 139

Oxygen, 145, 156, 166, 346

Pacific Ocean, origin of moon in, 339

Palitzch, 255

Pallas, 225, 227

Parallax, 57, 173, 280, 305, 320, 326

Paré, Ambrose, 264-265

Peal, S.E., 205

Peary, 277

Pegasus (constellation), 306

Penumbra of sunspot, 135

Perennial full moon of Laplace, 190

Pericles, 84

Perrine, C.D., 232-233, 315

Perseids, 270, 273-275

Perseus (constellation), 273, 279, 307, 312

Phases of an inferior planet, 149, 160;
of the moon, 149, 160, 183-185

Phlegon, Eclipse of, 85-86

Phobos, 223

Phoebe, retrograde motion of, 240, 250, 336

Phosphorescent glow in sky, 323

Phosphorus (Venus), 150

Photographic survey of sky, international, 290-291

Photosphere, 130-131, 134

Piazzi, 23

Pickering, E.C., 302

Pickering, W.H., 199, 205-206, 220-221, 240, 339-341

Pictor, "runaway star" in constellation of, 281-282, 320, 330

Plane of orbit, 36, 150

Planetary nebulæ, 245, 315

_Planetary and Stellar Studies_, 331

Planetesimal hypothesis, 337-338

Planetoids. _See_ Asteroids

Planets, classification of, 229;
contrasted with comets, 247;
in Ptolemaic scheme, 171;
relative distances of, from sun, 31-32

Plato (lunar crater), 198

Pleiades, 284, 296-297, 307

Pliny, 169, 260

Plough, 284, 291-296, 302

Plutarch, 86, 89, 169, 181

"Pointers," 292

Polaris. _See_ Pole Star

Pole of earth, Precessional movement of, 176-177, 295, 298-299

Pole Star, 33, 163, 177, 292-296, 300-301

Poles, 30, 163-164;
of earth, speed of point at, 164

Pollux, 282, 297

Posidonius, 186

Powell, Sir George Baden, 96

Præsepe (the Beehive), 307

Precession of the Equinoxes, 177, 295, 298-299

Pre-telescopic notions, 55

Primaries, 26

_Princess, The_ (Tennyson), 334

Princeton Observatory, 258

Prism, 121

Prismatic colours, 111, 121

Procyon, 284, 290, 297, 303

Prominences, Solar, 72, 93, 131, 139-140, 143;
first observation of, with spectroscope, 94, 140, 236

Proper motions of stars, 126, 281-285, 326, 329-330

Ptolemæus (lunar crater), 198-199, 204

Ptolemaic idea, 319;
system, 18, 19, 158, 171-172

Ptolemy, 18, 101, 171, 290, 296

Puiseux, P., 206

Pulkowa telescope, 117

Puppis, V., 310

Quiescent prominences, 139

Radcliffe Observer, 139

"Radiant," or radiant point, 269

Radiation from sun, 130, 134

Radium, 129, 138

Rainbow, 121

"Rainbows, Bay of," 197

Rambaut, A., 139

Ramsay, Sir William, 138

Rays (on moon), 204

Recurrence of eclipses, 74-80

Red (rays of light), 121, 125, 127, 130

Red Spot, the Great, 230

Reflecting telescope, 111-116;
future of, 119

Reflector. _See_ Reflecting telescope

Refracting and reflecting telescopes contrasted, 118

Refracting telescope, 109-111, 115-117;
limits to size of, 119-120

Refraction, 121, 168-169

Refractor, _See_ Refracting telescope

Regulus, 290, 296

Retrograde motion of Phoebe, 240, 250, 336

"Reversing Layer," 94, 130, 132, 137-138

Revival of learning, 107

Revolution, 30;
of earth around sun, 170-173;
periods of sun and planets, 35

Riccioli, 198

Rice-grain structure of photosphere, 134

Rigel, 285, 297

Rills (on moon), 204

Ring-mountains of moon. _See_ Craters

"Ring" nebulæ, 315, 337

"Ring with wings," 87

Rings of Saturn, 108, 236-239, 241-243, 334

Ritchey, G.W., 118

Roberts, A.W., 308, 310

Roberts, Isaac, 325

"Roche's limit," 238

Roemer, 235

Roman history, eclipses in, 85-86

Romulus, 85

Röntgen, 120

Rosse, great telescope of Lord, 117, 314

Rotation, 30;
of earth, 33, 161-165, 170;
of sun, 34, 125, 135-136, 231;
periods of sun and planets, 35

Royal Society of London, 90-91, 111

Rubicon, Passage of the, 85

"Runaway" stars, 281, 326, 330

Sagittarius (constellation), 316

Salt, spectrum of table, 122

Samarcand, 107

"Saros," Chaldean, 76-78, 84

Satellites, 26-27, 37

Saturn, 20, 22, 34, 37, 108, 236-243, 258;
comet family of, 252;
a puzzle to the early telescope observers, 241-243;
retrograde motion of satellite Phoebe, 240, 250, 336;
ring system of, 241;
satellites of, 36, 239-240;
shadows of planet on rings and of rings on planet, 237

Schaeberle, 95-96, 303, 316

Schiaparelli, 155, 214, 223

Schickhard (lunar crater), 199

Schmidt, 206

Schönfeld, 290

Schuster, 95

Schwabe, 136

Scotland, solar eclipses visible in, 89-90, 92

Sea of Serenity, 205

"Sea of Showers," 197

"Seas" of moon, 197, 206

Seasons on earth, 174-175;
on Mars, 211

Secondary bodies, 26

Seneca, 95, 260

_Septentriones_, 291

Serenitatis, Mare, 205

"Seven Stars," 291

"Shadow Bands," 69

Shadow of earth, circular shape of, 62-64

Shadows on moon, inky blackness of, 202

Shakespeare, 259, 293

Sheepshanks Telescope, 119

"Shining fluid" of Sir W. Herschel, 328

"Shooting Stars." _See_ Meteors

Short (of Edinburgh), 114

"Showers, Sea of," 197

Sickle of Leo, 270-271, 296

Siderostat, 118

Silver, 145

Silvered mirrors for reflecting telescopes, 116

Sinus Iridum, 197

Sirius, 280, 282, 284-285, 288-290, 297, 303-304, 320;
companion of, 303;
stellar magnitude of, 289

Size of celestial bodies, how ascertained, 59

Skeleton telescopes, 110

Sky, international photographic survey of, 290-291;
light of the, 323

Slipher, E.C., 213, 222

Smithsonian Institution of Washington, 98

Snow on Mars, 213

Sodium, 122, 124, 254

Sohag, 95

Solar system, 20-21, 29-31;
centre of gravity of, 42;
decay and death of, 344

Somniorum, Lacus, 197

Sound, 125, 166, 331

South pole of heavens, 163, 285, 298-299

Southern constellations, 298-299

Southern Cross. _See_ Crux

Space, 328

Spain, early astronomy in, 107;
eclipse tracks across 93, 97-98

Spectroheliograph, 140

Spectroscope, 120, 122, 124-125, 144-145, 212, 231;
prominences first observed with, 94, 140, 236

Spectrum of chromosphere, 132-133;
of corona, 133;
of photosphere, 132;
of reversing layer, 132, 137;
solar, 122-123, 127, 132

Speculum, 111, 116;
metal, 112

Spherical bodies, 29

Spherical shape of earth, proofs of, 158-161

Spherical shapes of sun, planets, and satellites, 160

Spiral nebulæ, 314-316, 337-338

Spring balance, 166

Spring tides, 192

Spy-glass, 108

"Square of the distance," 43-44

Stannyan, Captain, 90

Star, mass of, how determined, 305;
parallax of, first ascertained, 173, 280

Stars, the, 20, 124, 126, 278 _et seq._;
brightness of, 287, 320;
distances between, 326-327;
distances of some, 173, 280, 320;
diminution of, below twelfth magnitude, 324;
evolution of, from nebulæ, 317-318;
faintest magnitude of, 288;
number of those visible altogether, 324;
number of those visible to naked eye, 288

"Steam cracks," 221

Steinheil, 118

Stellar system, estimated extent of, 325-327;
an organised whole, 327;
limited extent of, 322-328, 330;
possible disintegration of, 329

Stiklastad, eclipse of, 88

Stone Age, 285

Stoney, G.J., 202, 222

Stonyhurst Observatory, 100

_Story of the Heavens_, 271

Streams of stars, Kapteyn's two, 284

Stroobant, 196

Stukeley, 91

Sulphur, 145

Summer, 175, 178

Sun, Chaps XII. and XIII.;
as a star, 124, 278, 289;
as seen from Neptune, 246, 304;
chemical composition of, 144-145;
distance of, how ascertained, 151, 211;
equator of, 135-136, 139;
gravitation at surface of, 129, 138-139;
growing cold of, 343-344;
mean distance of, from earth, 47, 211;
motion of, through space, 282-286, 326;
not a solid body, 136;
poles of, 136;
radiations from, 130;
revolution of earth around, 170-173;
stellar magnitude of, 288-289;
variation in distance of, 66, 178

Sunspots, 34, 125, 134-137, 140-141, 143-144, 308;
influence of earth on, 144

Suns and possible systems, 50, 286

Superior conjunction, 147-149

Superior planets, 22, 146, 209-210, 229

Swan (constellation). _See_ Cygnus

Swift, Dean, 224

"Sword" of Orion, 297, 316

Syrtis Major. _See_ Hour Glass Sea

"_Systematic_ Parallax," 326

Systems, other possible, 50, 286

Tails of comets, 182

Tamerlane, 107

Taurus (constellation), 103, 296-297, 307

"Tears of St. Lawrence," 273

Tebbutt's Comet, 257-258

Telescope, 33, 55, 107-108, 149;
first eclipse of moon seen through, 104;
of sun, 90

Telescopes, direct view reflecting, 114;
gigantic, 111;
great constructors of, 117-118;
great modern, 117-118

Tempel's Comet, 274

Temperature on moon, 203;
of sun, 128

Temporary (or new) stars, 310-314

Tennyson, Lord, 109, 296, 334

Terrestrial planets, 229-230

Terrestrial telescope, 117

Thales, Eclipse of, 84

Themis, 240

"Tidal drag," 180, 188, 208, 344

Tide areas, 179-180

Tides, 178-180, 338-339

_Time Machine_, 344

Tin, 145

Titan, 240

Titius, 245

Total phase, 71-72

Totality, 72;
track of, 66

Trail of a minor planet, 226-227

Transit, 62, 150-154;
of Mercury, 62, 151, 154;
of Venus, 62, 151-152, 154, 211

Trifid Nebula, 316

Triple stars, 300

Tubeless telescopes, 110-111, 243

Tubes used by ancients, 110

Tuttle's Comet, 274

Twilight, 167, 202

Twinkling of stars, 168

Twins (constellation). _See_ Gemini

Tycho Brahe, 290, 311

Tycho (lunar crater), 204

Ulugh Beigh, 107

Umbra of sunspot, 134-135

Universe, early ideas concerning, 17-18, 158, 177, 342

Universes, possibility of other, 330-331

Uranus, 22-24, 31, 210, 243, 245, 275;
comet family of, 252;
discovery of, 22, 210, 243;
rotation period of 34, 245;
satellites of, 26, 245;
"year" in, 35-36

Ursa Major (constellation), 279, 281, 291, 295, 314;
minor, 177, 279, 293-294

Ursæ Majoris, ([z]) Zeta. _See_ Mizar

Variable stars, 307-310

Variations in apparent sizes of sun and moon, 67, 80, 178

Vault, shape of the celestial, 194-196

Vega, 177, 278, 280, 282-283, 285, 290, 294, 302, 307, 323

Vegetation on Mars, 221, 217-218;
on moon, 205

Venus, 20, 22, 31, 71, 90, 108-109, 111, Chap. XIV., 246, 311;
rotation period of, 34, 155

Very, F.W., 314

Vesta, 225, 227

Violet (rays of light), 121-122, 125

Virgil, 19

Volcanic theory of lunar craters, 203-204, 214

Volume, 38

Volumes of sun and planets compared, 38-39

"Vulcan," 25

Wallace, A.R., on Mars, 220-223

Water, lack of, on moon, 201-202

Water vapour, 202, 213, 222

Wargentin, 103

Warner and Swasey Co., 117

Weather, moon and, 206-207

Weathering, 202

Webb, Rev. T.W., 204

Weight, 43, 165-166

Wells, H.G., 344

Whale (constellation). _See_ Cetus

Whewell, 190

Willamette meteorite, 277

Wilson, Mount, 118

Wilson, W.E., 313

"Winged circle" (or "disc"), 87

Winter, 175, 178

Witt, 227

Wolf, Max, 226-227, 232

Wright, Thomas, 319, 334

Wybord, 89

Xenophon, 101

Year, 35

"Year" in Uranus and Neptune, 35-36

Year, number of eclipses in a, 68

"Year of the Stars," 270

Yellow (rays of light), 121-122, 124

Yerkes Telescope Great, 117, 303

Young, 94, 137, 166

Zenith, 174

Zinc, 145

Zodiacal light, 181

Zone of asteroids, 30-31, 227

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