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Title: Lectures in Navigation
Author: Draper, Ernest Gallaudet, 1885-1954
Language: English
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "Lectures in Navigation" ***
[Transcriber's Note:-
Words and phrases surrounded by underscores are italicised.
There are four symbols which are denoted by :-
(.) Circle surrounding a centred dot - Right Ascension of the
Meridian;
(+) Circle surrounding a cross - Earth's Central Progress;
(_) Circle with centre dot and line under - Observed altitude of sun's
lower limb;
-(-)- Circle with line through - True Altitude.
]
LECTURES IN NAVIGATION
Prepared for Use as a Text Book
at the
OFFICERS' MATERIAL SCHOOL
NAVAL AUXILIARY RESERVE
by
Lieutenant ERNEST G. DRAPER, U.S.N.R.F.
Head of the Department of Navigation
Officers' Material School, Naval Auxiliary Reserve
COPYRIGHT
BY
ERNEST G. DRAPER
FOREWORD
These Lectures have been compiled as speedily as possible to meet the
demand for some quick but fairly comprehensive method whereby large
bodies of men, divided into small classes, might learn the elements of
Navigation and thus assume, without delay, their responsibilities as
Junior Officers of the deck, Navigators and Assistant Navigators in the
United States Naval Auxiliary Reserve.
I realize that the haste with which the book has been written is
apparent in many places, and it is hoped that many evidences of this
haste will disappear in case further editions are printed. Besides
acknowledging the help and information which was secured from the list
of navigational works, mentioned on another page, I wish to mention
particularly Prof. Charles Lane Poor's book, entitled "Nautical
Science," from which was secured practically all of the information in
the Lecture on Planets and Stars (Tuesday--Week V); Commander W. C. P.
Muir's book, "Navigation and Compass Deviations," and Lieutenant W. J.
Henderson's book, "Elements of Navigation," the text of which was
followed closely in discussing Variation and Deviation and Traverse
Sailing.
I desire to express my gratitude to Lieutenant Commander R. T. Merrill,
2nd, U. S. N., for suggesting a detailed outline of the whole course; to
Lieutenant Commander B. O. Wills, U. S. N., for his valuable criticisms
and almost daily help during the preparation of these Lectures; to
Lieutenant (j. g.) C. D. Draper, U. S. N. R. F.; Lieutenant (j. g.) R.
Brush, U. S. N. R. F., and Lieutenant (j. g.) P. C. McPherson, U. S. N.
R. F., for many criticisms and suggestions; and to Captain Huntington,
Seamen's Church Institute, for suggesting helpful diagrams, particularly
the one on page 44. This opportunity is also taken for thanking the many
Instructors in the School for their opinions on various questions that
have come up in connection with the course and for assistance in
eliminating errors from the text.
E. G. D.
LIST OF BOOKS CONSULTED
AMERICAN PRACTICAL NAVIGATOR, BOWDITCH
NAVIGATION AND COMPASS DEVIATIONS, MUIR
NAUTICAL SCIENCE, POOR
ELEMENTS OF NAVIGATION, HENDERSON
WRINKLES IN PRACTICAL NAVIGATION, LECKY
WHYS AND WHEREFORES OF NAVIGATION, BRADFORD
EPITOME OF NAVIGATION, NORIE
NAVIGATION, HOSMER
FINDING A SHIP'S POSITION AT SEA, SUMNER
GENERAL ASTRONOMY, YOUNG
PREFACE
TO THOSE TAKING THIS COURSE IN NAVIGATION:
These lectures have been written with the idea of explaining, in as
simple language as possible, the fundamental elements of Navigation as
set forth in Bowditch's American Practical Navigator. They will be given
you during the time at the Training School devoted to this subject. At
present this time includes two morning periods of one and a half hours
each, separated by a recess of fifteen minutes. In general the plan is
to devote the first period to the lecture and the second period to
practical work.
Not many examples for practical work have been included in this book,
but one example, illustrating each new method, has been worked out. If
you understand these examples you should be able to understand others
similar to them.
Toward the end of the course a portion of each second period will be
devoted to handling the sextant, work with charts, taking sights, etc.
In short, every effort will be made to duplicate, as nearly as possible,
navigating conditions on board a modern merchant ship.
DEPARTMENT OF NAVIGATION,
_Officers' Material School_,
_Naval Auxiliary Reserve_
CONTENTS
WEEK I--PILOTING
_Tuesday_--The Compass 1
_Wednesday_--Pelorus; Parallel Rulers; The Lead,
Sounding Machine, Dividers and Log 6
_Thursday_--The Chart 10
_Friday_--The Protractor and Sextant 13
_Saturday_--Fixes, Angles by Bearings and Sextant 16
WEEK II--DEAD RECKONING
_Tuesday_--Latitude and Longitude 20
_Wednesday_--Useful Tables--Plane and Traverse Sailing 23
_Thursday_--Examples on Plane and Traverse Sailing (Continued) 27
_Friday_--Mercator Sailing 28
_Saturday_--Great Circle Sailing--The Chronometer 30
WEEK III--CELESTIAL NAVIGATION
_Tuesday_--Celestial Co-ordinates, Equinoctial System, etc. 34
_Wednesday_--Time by the Sun--Mean Time, Solar Time,
Conversion, etc. 36
_Thursday_--Sidereal Time--Right Ascension 43
_Friday_--The Nautical Almanac 47
_Saturday_--Correction of Observed Altitudes 52
WEEK IV--NAVIGATION
_Tuesday_--The Line of Position 55
_Wednesday_--Latitude by Meridian Altitude 58
_Thursday_--Azimuths of the Sun 61
_Friday_---Marc St. Hilaire Method by a Sun Sight 63
_Saturday_--Examples on Marc St. Hilaire Method by a Sun Sight 66
WEEK V--NAVIGATION
_Tuesday_--A Short Talk on the Planets and Stars--Identification of
Stars--Time of Meridian Passage of a Star 66
_Wednesday_--Latitude by Meridian Altitude of a Star--Latitude by
Polaris 73
_Thursday_--Marc St. Hilaire Method by a Star Sight 74
_Friday_--Examples: Latitude by Meridian Altitude of a Star;
Latitude by Polaris; Marc St. Hilaire Method by a Star Sight 75
_Saturday_--Longitude by Chronometer Sight of the Sun 76
WEEK VI--NAVIGATION
_Tuesday_--Longitude by Chronometer Sight of a Star 79
_Wednesday_--Examples on Longitude by Chronometer Sight of a Star 80
_Thursday_--Latitude by Ex-Meridian Altitude of the Sun 81
_Friday_--Examples: Latitude by Ex-Meridian Altitude of the Sun 83
_Saturday_--Finding the Watch Time of Local Apparent Noon 83
WEEK VII--NAVIGATION
_Tuesday_--Compass Error by an Azimuth 88
_Wednesday_--Correcting Longitude by a Factor 89
_Thursday_--The Navigator's Routine--A Day's Work at Sea 91
_Friday_--Day's Work 105
_Saturday_--Day's Work 105
WEEK VIII--NAVIGATION
_Monday to Thursday_--Day's Work 107-108
_Additional Lecture_--Compass Adjustment 109
WEEK I--PILOTING
TUESDAY LECTURE
THE COMPASS
Everyone is supposed to know what a compass looks like. It is marked in
two ways--the old way and the new way. Put in your Note-Book this
diagram:
[Illustration]
The new way marked on the outside of the diagram, starts at North with
0°, increases toward the right through East at 90°, South at 180°, West
at 270° and back to North again at 360° or 0°.
The old way, marked on the inside of the diagram, starts at North with
0°, goes to the right to 90° at East and to the left to 90° at West. It
also starts at South with 0°, goes to the right to East at 90° and to
the left to West at 90°.
A Compass Course can be named in degrees, according to either the new or
old way. For instance, the new way is just 45°. The old way for the same
course is N 45° E. New way--100°. Old way for same course--S 80° E.
There is another way to name a compass course. It is by using the name
of the point toward which the ship is heading. On every ship the compass
is placed with the lubber line (a vertical black line on the compass
bowl) vertical and in the keel line of the ship. The lubber line,
therefore, will always represent the bow of the ship, and the point on
the compass card nearest the lubber line will be the point toward which
the ship is heading.
The compass card of 360° is divided into 32 points. Each point,
therefore, represents 11-1/4°. The four principal points are called
_cardinal_ points. They are--North, East, South, West. Each cardinal
point is 90° from the one immediately adjacent to it. It is also 8
points from the one adjacent to it, as 90° is 8 points, i.e., 11-1/4°
(one point) times 8. Midway between the cardinal points are the
inter-cardinal points. They are--N E, S E, S W, N W, and are 45° or 4
points from the nearest cardinal point. Midway between each cardinal and
inter-cardinal point--at an angular distance of 22-1/2° or 2 points, is
a point named by combining a cardinal point with an inter-cardinal
point. For instance, NNE, ENE, ESE, SSE, SSW, WSW, WNW, NNW. Midway
between the last points named and a cardinal or inter-cardinal point, at
an angular distance of 11-1/4°, is a point which bears the name of that
cardinal or inter-cardinal point joined by the word _by_ to that of the
cardinal point nearest to it. As, for instance, N by E, E by N, E by S,
S by E, S by W, W by S, W by N, N by W. Also NE x N, NE x E, SE x E, SE
x S, SW x S, SW x W, NW x W, NW x N. The angular distance between each
and every whole point is divided into 4 parts called half and quarter
points and each representing an angular measure of approximately 2° 49'.
In mentioning fractional points, the U. S. Navy regulations are to name
each point from North and South toward East and West except that
divisions adjacent to a cardinal or inter-cardinal point are always
referred to that point: For instance, N 1/2 E, N x E 1/2 E, NE 1/2 N, NW
1/2 N, NW 1/4 W, NW 3/4 W, NW 1/4 N.
Boxing the compass is naming each point and quarter-point in rotation,
i.e., starting at North and going around to the right back to North
again. Every man should be able to identify and name any point or
quarter-point on the compass card.
In changing a point course into a degree course, for either new or old
compass, a guide is herewith furnished you. This should be pasted into
the front of your Bowditch Epitome. It shows, from left to right, the
name of the point course, its angular measure in the new compass and its
angular measure in the old compass. It also shows at the bottom, the
angular measure of each division of one point. In understanding this
guide, remember that each course is expressed in degrees or degrees and
minutes.
Put in your Note-Book:
In Navigation, each degree is written thus °. Each fraction of a degree
is expressed in minutes and written thus '. There are 60' in each
degree. Each fraction of a minute is expressed in seconds and is written
". There are 60" in each minute. Four degrees, ten minutes and thirty
seconds would be written thus: 4° 10' 30".
Although this guide just given you is given as an aid to quickly
transfer a point course into a new or old compass course--or vice
versa--you should learn to do this yourself, after awhile, without the
guide.
Put in your Note-Book:
--------------+-----------+------------+--------------
Ship's Head | New | Old | By Point
--------------+-----------+------------+--------------
NE | 45° | N 45° E | NE
90° | 90° | N 90° E | EAST
SE × E | 123° 45' | S 56° E | SE × E
S 20° E | 160° | S 20° E | S x E 3/4 E
S 2 pts. E | 157° 30' | S 22° 30' E| SSE
NW 3/4 W | 306° 34' | N 53° W | NW 3/4 W
289° 41' | 289° 41' | N 70° W | WNW 1/4 W
--------------+-----------+------------+--------------
I will show you just how each one of these courses is secured from the
guide just given you.
Note to Instructor: After explaining these courses in detail, assign for
reading _in the class room_ the following articles in Bowditch: Arts.
25-26-27-28-29-30-31-32, 74-75-76-77-78-79-80-81-82.
Every compass, if correct, would have its needle point directly to the
real or _true_ North. But practically no compass with which you will
become familiar will be correct. It will have an error in it due to the
magnetism of the earth. This is called Variation. It will also have an
error in it due to the magnetism of the iron in the ship. This is called
Deviation. You are undoubtedly familiar with the fact that the earth is
a huge magnet and that the magnets in a compass are affected thereby. In
other words, the North and South magnetic poles, running through the
center of the earth, do not point true North and South. They point at an
angle either East or West of the North and South. The amount of this
angle in any one spot on the earth is the amount of Variation at that
spot. In navigating a ship you must take into account the amount of this
Variation. The amount of allowance to be made and the direction (i.e.
either East or West) in which it is to be applied are usually indicated
on the chart. On large charts, such as those of the North Atlantic, will
be found irregular lines running over the chart, and having beside them
such notations as 10° W, 15° W, etc. Some lines are marked "No
Variation." In such cases no allowance need be made. On harbor charts or
other small charts, the Variation is shown by the compass-card printed
on the chart. The North point of this card will be found slewed around
from the point marking True North and in the compass card will be some
such inscription as this: "Variation 9° West in 1914. Increasing 6' per
year."
Now let us see how we apply this Variation so that although our compass
needle does not point to true North, we can make a correction which will
give us our true course in spite of the compass reading. Note these
diagrams:
[Illustration]
The outer circle represents the sea horizon with the long arrow pointing
to true North. The inner circle represents the compass card. In the
diagram to the left, the compass needle is pointing three whole points
to the left or West of True North. In other words, if your compass said
you were heading NE x N, you would not actually be heading NE x N. You
would be heading true North.
[Illustration]
In other words, standing in the center of the compass and looking toward
the circumference, you would find that every true course you sailed
would be three points to the _left_ of the compass course. That is
called Westerly Variation.
Now look at the diagram to the right. The compass needle is pointing
three whole points to the right or East of True North. In other words,
standing in the center of the compass and looking toward the
circumference, you would find that every true course you sailed would be
three points to the _right_ of the compass course. That is called
Easterly Variation.
Hence we have these rules, which put in your Note-Book:
_To convert a compass course into a true course_
When the Variation is westerly, the true course will be as many points
to the left of the compass course as there are points or degrees of
Variation. When the Variation is easterly, the true course will be as
many points or degrees to the right of the compass course.
_To convert a true course into a compass course_
The converse of the above rule is true. In other words, Variation
westerly, compass to the right of true course; variation easterly,
compass course to the left.
DEVIATION
As stated before, Deviation causes an error in the Compass due to the
magnetism of the iron in the ship. When a ship turns, the compass card
does not turn, but the relation of the iron's magnetism to the magnets
in the compass is altered. Hence, every change in course causes a new
amount of Deviation which must be allowed for in correcting the compass
reading. It is customary in merchant vessels to have the compasses
adjusted while the ship is in port. The adjuster tries to counteract the
Deviation all he can by magnets, and then gives the master of the ship a
table of the Deviation errors remaining. These tables are not to be
depended upon, as they are only accurate for a short time. Ways will be
taught you to find the Deviation yourself, and those ways are the only
ones you can depend upon.
Put in your Note-Book:
Westerly Deviation is applied exactly as westerly Variation. Easterly
Deviation is applied exactly as easterly Variation.
The amount of Variation plus the amount of Deviation is called the
Compass Error. For instance, a Variation of 10° W plus a Deviation of 5°
W equals a compass error of 15° W, or a Variation of 10° W plus a
Deviation of 5° E leaves a net compass error of 5° W.
LEEWAY
Leeway is not an error of the compass, but it has to be compensated for
in steaming any distance. Hence it is mentioned here. A ship steaming
with a strong wind or current abeam, will slide off to the leeward more
or less. Hence, her course will have to be corrected for Leeway as well
as for Variation and Deviation.
Put in your Note-Book:
Leeway on the starboard tack is the same as westerly Variation. Leeway
on the port tack is the same as easterly Variation. This is apparent
from the following diagram:
[Illustration]
As the wind, blowing from the North, hits the left hand ship, for
instance, on her starboard side, it shoves the ship to the left of her
true course by the number of points or degrees of leeway.
Leave a space and put the following heading in your Note-Book:
I. Complete rule for converting a compass course into a true course:
1. Change the compass course into a new compass reading.
2. Apply Easterly Variation and Deviation +.
3. Apply Westerly Variation and Deviation -.
4. Apply port tack Leeway +.
5. Apply starboard tack Leeway -.
II. Complete rule for converting a true course into a compass course:
1. Reverse the above signs in applying each correction.
I will now correct a few courses, and these are to be put into your
Note-Book:
---------+------+-------------+--------+--------+-------+---------
C Cos | Wind | Leeway | Dev. | Var. | New | Old
---------+------+-------------+--------+--------+-------+---------
N x E | NW | 1/2 pt. | 5° E | 10° W | 12° | N 12° E
S 67° E | S | 1 pt. | 3° W | 5° E | 104° | S 76° E
E x N | SE | 1/2 pt. | 5° W | 10° E | 78° | N 78° E
W x N | NW | 1-1/2 pts. | 1° E | 15° E | 280° | N 80° W
---------+------+-------------+--------+--------+-------+---------
Assign for Night Work the following arts. in Bowditch:
36-8-10-13-14-15-16-17-18-19-20-21-22-23-24.
WEDNESDAY LECTURE
PELORUS, PARALLEL RULERS, THE LEAD, SOUNDING MACHINE, DIVIDERS AND LOG
_I. The Pelorus_
This is an instrument for taking bearings of distant objects, and for
taking bearings of celestial bodies such as the sun, stars, etc. It
consists of a circular, flat metallic ring, mounted on gimbals, upon a
vertical standard. The best point to mount it is in the bow or on the
bridge of the ship, where a clear view for taking bearings can be had.
The center line of the pelorus should also be directly over the keel
line of the ship. The inner edge of the metallic ring is engraved in
degrees--the 0° or 360° and the 180° marks indicating a fore-and-aft
line parallel to the keel of the ship. Within this ring a ground glass
dial is pivoted. This ground glass dial has painted upon it a compass
card divided into points and sub-divisions and into 360°. This dial is
capable of being moved around, but can also be clamped to the outside
ring. Pivoted with the glass dial and flat ring is a horizontal bar
carrying at both of its extremes a sight vane. This sight vane can be
clamped in any position independently of the ground glass dial, which
can be moved freely beneath it. An indicator showing the direction the
sight vane points can be read upon the compass card on the glass dial.
If the glass dial be revolved until the degree of demarcation, which is
coincident with the right ahead marking on the flat ring, is the same as
that which points to the lubber's line of the ship's compass, then all
directions indicated by the glass dial will be parallel to the
corresponding directions of the ship's compass, and all bearings taken
will be compass bearings, i.e., as though taken from the compass itself.
In other words, it is just as though you took the compass out of its
place in the pilot house, or wherever it is regularly situated, put it
down where the pelorus is, and took a bearing from it of any object
desired.
In taking a bearing by pelorus, two facts must be kept in mind. First,
that when the bearing is taken, the exact heading, as shown by the
ship's compass, is the heading shown by the pelorus. In other words, if
the ship is heading NW, the pelorus must be set with the NW point on the
lubber line when the bearing is taken of any object. Second, it must be
remembered that the bearing of any object obtained from the pelorus is
the bearing _by compass_. To get the true bearing of the same object you
must make the proper corrections for Variation and Deviation. This can
be compensated for by setting the glass dial at a point to the right or
left of the compass heading to correspond with the compass error; then
the bearing of any object will be the true bearing. But naturally, you
will not be able to make compensation for these errors unless you have
immediately before found the correct amount of the compass error.
_Parallel Rulers_
The parallel rulers need no explanation except for the way in which they
are used on a chart. Supposing, for instance, you wish to steam from
Pelham Bay to the red buoy off the westerly end of Great Captain's
Island. Take your chart, mark by a pencil point the place left and the
place to go to and draw a straight line intersecting these two points.
Now place the parallel rulers along that line and slide them over until
the nearest edge intersects the center of the compass rose at the bottom
or side of the chart. Look along the ruler's edge to find where it cuts
the circumference of the compass rose. That point on the compass rose
will be the _true_ compass course, and can be expressed in either the
new or old compass, as, for instance, 60° or N 60° E. Remember, however,
that this is the _true_ course. In order to change it into the compass
course of your ship, you must make the proper corrections for the
compass error, i.e., Variation and Deviation and for Leeway, if any.
_The Lead and Sounding Machine_
The lead, as you know, is used to ascertain the depth of the water and,
when necessary, the character of the bottom. There are two kinds of
leads: the hand lead and deep-sea lead. The first weighs from 7 to 14
pounds and has markings to 25 fathoms. The second weighs from 30 to 100
pounds and is used in depths up to and over 100 fathoms. Put in your
Note-Book:
Fathoms which correspond with the depths marked are called _marks_. All
other depths are called _deeps_. The hand lead is marked as follows:
2 fathoms--2 strips of leather.
3 fathoms--3 strips of leather or blue rag.
5 fathoms--A white rag.
7 fathoms--A red rag.
10 fathoms--A piece of leather with one hole in it.
13 fathoms--Same as at 3.
15 fathoms--Same as at 5.
17 fathoms--Same as at 7.
20 fathoms--2 knots or piece of leather with 2 holes.
25 fathoms--1 knot.
30 fathoms--3 knots.
35 fathoms--1 knot.
40 fathoms--4 knots.
And so on up to 100 fathoms.
The large hand leads are hollowed out on the lower end so that an
"arming" of tallow can be put in. This will bring up a specimen of the
bottom, which should be compared with the description found on the
chart.
All up-to-date sea-going ships should be fitted with Sir William
Thompson's Sounding Machine (see picture in B. J. Manual). This machine
consists of a cylinder around which are wound about 300 fathoms of piano
wire. To the end of this is attached a heavy lead. An index on the side
of the instrument records the number of fathoms of wire paid out. Above
the lead is a copper cylindrical case in which is placed a glass tube
open only at the bottom and chemically colored inside. The pressure of
the sea forces water up into this tube, as it goes down, a distance
proportionate to the depth, and the color is removed. When hoisted, the
tube is laid upon a prepared scale, and the height to which the water
has been forced inside shows the depth in fathoms on the scale.
DIVIDERS
The dividers are nothing but an instrument for measuring distances,
etc., on the chart.
THE LOG
There are two kinds of logs--the chip log, used for measuring the speed
of the ship, and the patent log, used for measuring distance run.
The chip log consists of a reel, line, toggle and chip. Usually a second
glass is used for measuring time. The chip is the triangular piece of
wood ballasted with lead to ride point up. The toggle is a little wooden
case into which a peg, joining the ends of the two lower lines of the
bridle, is set in such a way that a jerk on the line will free it,
causing the log to lie flat so that it can be hauled in. The first 10 or
15 fathoms of line from the log-chip are called "stray line," and the
end of this is distinguished by a mark of red bunting. Its purpose is to
let the chip get clear of the vessel's wake. The marks on the line
(called knots) are pieces of fish line running through the strands of
the reel line to the number of two, three, four, etc. A piece of white
bunting marks every two-tenths of a knot. This is because the run of the
ship is recorded in knots and tenths. The knots of fish line are 47 feet
4 inches from each other.
The log glass measures 28 seconds in time. For high rates of speed, a 14
second glass is used. Then the number of knots shown by the log line
must be doubled. The principle of the chip log is that each division of
the log line bears the same ratio to a nautical mile that the log glass
does to the hour. In other words, if 10 knots or divisions of the log
line run out while the 28 second glass empties itself, the ship's speed
is 10 knots per hour. If ten knots or divisions run out while the 14
second glass empties itself, the ship's speed is 20 knots per hour.
The patent or towing log consists of a dial, line and rotator. The large
circle of the dial records the knots and the small circle tenths of
knots. When changing course, read the log and enter it in the log book.
When changing course again, read the log again. The difference between
the two readings will be the distance run.
Both logs are liable to error. A following sea makes them under-rate, a
head sea over-rate. With both logs you must allow for currents. If a
current is against you--and you know its rate--you must deduct its rate
from that recorded in the log and vice versa. The reason for this is
that your log measures your speed through the water. What you must find
is your actual distance made good over the earth's surface.
Put in your Note-Book:
Between Sandy Hook and Fort Hamilton, bound due North, speed by chip-log
was 10 knots, tidal current setting North 2 knots per hour; what did the
ship make per hour? Answer: 12 knots.
At sea in North Sea ship heading S x W, patent log bet. 8 A.M. and 12 M.
registered 32 miles, current running N x E 2 knots per hour; what was
the actual distance made good? Answer: 24 miles.
Directions for allowing for a current setting diagonally across a ship's
course will be given in the proper place.
Assign for Night Work the following articles in Bowditch: Arts.
161-162-163-164-165.
THURSDAY LECTURE
THE CHART
_Aids to Navigation_
A chart is a map of an ocean, bay, sound or other navigable water. It
shows the character of the coast, heights of mountains, depths at low
water, direction and velocity of tidal currents, location, character,
height and radius of visibility of all beacon lights, location of rocks,
shoals, buoys, and nature of the bottom wherever soundings can be
obtained.
The top of the chart is North unless otherwise noted. When in doubt as
to where North is, consult the compass card printed somewhere on the
chart. On sea charts, such as those of the North Atlantic, only the true
compass is printed, with the amount and direction of Variation indicated
by lines on the chart.
Parallels of latitude are shown by straight lines running parallel to
each other across the chart. The degrees and minutes of these parallels
are given on the perpendicular border of the chart. Meridians of
longitude are shown by straight lines running up and down, perpendicular
to the parallels of latitude, and the degrees and minutes of these
meridians are given on the horizontal border of the chart.
Put in your Note-Book:
A minute of latitude is always a mile, because parallels of latitude are
equidistant at all places. A minute of longitude is a mile only on the
equator, for the meridians are coming closer to each other as they
converge toward either pole. They come together at the North and South
poles, and here there is no longitude.
* * * * *
I can explain this very easily by reference to the following
illustration:
[Illustration]
As every parallel of latitude is a circle of 360° the distance from A to
B will be the same number of degrees, minutes and seconds whether
measured upon parallel AA' or EE', but it will not be the same number of
miles as the meridians of longitude are gradually converging toward the
poles. On the other hand, the distances from A to C, C to D, D to E,
etc., must be the same because the lines AA', CC', DD', EE' are all
parallel. That is why the distance is always measured on the latitude
scale (i.e. on the vertical border of the chart), and a minute of
latitude is always a mile on the chart, no matter in what locality your
ship happens to be.
You should be able to understand any kind of information given you on a
chart. For instance, what are the various kinds of buoys and how are
they marked?
Put in your Note-Book:
1. In coming from seaward, red buoys mark the starboard side of the
channel, and black buoys the port side.
2. Dangers and obstructions which may be passed on either hand are
marked by buoys with red and black horizontal stripes.
3. Buoys indicating the fairway are marked with black and white vertical
stripes and should be passed close to.
4. Sunken wrecks are marked by red and black striped buoys described in
No. 2. In foreign countries green buoys are frequently used to mark
sunken wrecks.
5. Quarantine buoys are yellow.
6. As white buoys have no especial significance, they are frequently
used for special purposes not connected with Navigation.
7. Starboard and port buoys are numbered from the seaward end of the
channel, the black bearing the odd and red bearing the even numbers.
8. Perches with balls, cages, etc., will, when placed on buoys, be at
turning points, the color and number indicating on which side they shall
be passed.
9. Soundings in plain white are in fathoms; those on shaded parts are in
feet. On large ocean charts fathom curves, showing the range of
soundings of 10, 20, 30, 40, etc., fathoms are shown.
10. A light is indicated by a red and yellow spot. F. means fixed, Fl.,
flashing; Int., intermittent; Rev., revolving, etc.
11. An arrow indicates a current and its direction. The speed is always
given.
12. Rocks just under water are shown by a cross surrounded by a dotted
circle; rocks above water, by a dotted circle with dots inside it.
Practically all charts you will use will be called Mercator charts. Just
how they are constructed is a difficult mathematical affair but,
roughly, the idea of their construction is based upon the earth being a
cylinder, instead of a sphere. Hence, the meridians of longitude,
instead of converging at the poles, are parallel lines. This compels
the parallels of latitude to be adjusted correspondingly. Although such
a chart in any one locality is out of proportion compared with some
distant part of the earth's surface, it is nevertheless in proportion
for the distance you can travel in a day or possibly a week--and that is
all you desire. The Hydrographic Office publishes blank Mercator charts
for all latitudes in which they can be used for plotting your position.
It makes no difference what longitude you are in for, on a Mercator
chart, meridians of longitude are all marked parallel. It makes a great
difference, however, what latitude you are in, as in each a mile is of
different length on the chart. Hence, it will be impossible for you to
correctly plot your course and distance sailed unless you have a chart
which shows on it the degrees of latitude in which you are. For
instance, if your Mercator chart shows parallels of latitude from 30° to
40° that chart must be used when you are in one of those latitudes. When
you move into 41° or 29°, you must be sure to change your plotting chart
accordingly. In very high latitudes and near the North pole, the
Mercator chart is worthless. How can you steer for the North pole when
the meridians of your chart never come together at any pole? For the
same reason, bearings of distant objects may be slightly off when laid
down on this chart in a straight line. On the whole, however, the
Mercator chart answers the mariner's needs so far as all practical
purposes are concerned.
The instruments used in consulting a chart, i.e., parallel rulers,
dividers, etc. have already been described. The only way to lay down a
course and read it is by practice.
The one important thing to remember in laying down a course, is that
what you lay down is a true course. To steam this course yourself, you
must make the proper correction for your compass error.
Assign for Night Work in Bowditch, Arts.
9-239-240-241-243-244-245-246-247-248-249-251-252-253-254-255-256-257-258.
If any time in class room is left, spend it in laying down courses on
the chart and reading them; also in answering such questions as these:
1. I desire to sail a true course of NE. My compass error is 2 points
Westerly Variation and 1 point Easterly Deviation. What compass course
shall I sail?
2. I desire to sail a true course of SW x W. My Variation is 11° W,
Deviation 2 pts. W and Leeway 1 pt. starboard. What compass course shall
I sail?
3. I desire to sail a true course of 235°. My compass error is 4 pts. E
Variation, 27° W Deviation, Leeway 1 pt. port. What compass course shall
I sail?
4. I desire to sail a true course of S 65° W. My compass error is 10° E
Variation, 3° E Deviation, Leeway 1/4 point starboard. What compass
course shall I sail?
FRIDAY LECTURE
THE PROTRACTOR AND SEXTANT
The protractor is an instrument used to shape long courses. There are
many kinds. The simplest and the one most in use is merely a piece of
transparent celluloid with a compass card printed on it and a string
attached to the center of the compass card. To find your course by
protractor, put the protractor down on the chart so that the North and
South line on the compass card of the protractor will be immediately
over a meridian of longitude on the chart, or be exactly parallel to
one, and will intersect the point from which you intend to depart. Then
stretch your string along the course you desire to steam. Where this
string cuts the compass card, will be the direction of your course.
Remember, however, that this will be the _true_ course to sail. In order
to convert this true course into your compass course, allow for
Variation and Deviation according to the rules already given you.
In case you know the exact amount of Variation and Deviation at the time
you lay down the course--and your course is not far--you can get your
compass course in one operation by setting the North point of your
protractor as far East or West of the meridian as the amount of your
compass error is. By then proceeding as before, the course indicated on
the compass card will be the compass course to sail. This method should
not be used where your course in one direction is long or where your
course is short but in two or more directions. The reason for this is
that in both cases, either your Variation or Deviation may change and
throw you off.
Practically all navigation in strange waters in sight of land and in all
waters out of sight of land depends upon the determination of angles.
The angle at which a lighthouse is seen from your ship will give you
much information that may be absolutely necessary for your safety. The
angular altitude of the sun, star or planet does the same. The very
heart of Navigation is based upon dealing with angles of all kinds. The
instrument, therefore, that measures these angles is the most important
of any used in Navigation and you must become thoroughly familiar with
it. It is the sextant or some member of the sextant family--such as the
quadrant, octant, etc. The sextant is the one most in use and so will be
described first.
Put in your Note-Book:
The sextant has the following parts: (Instructor points to each.)
1. Mirror
2. Telescope
3. Horizon Glass
4. Shade Glasses
5. Back Shade Glasses
6. Handle
7. Sliding Limb
8. Reading Glass
9. Tangent Screw
10. Arc
In getting angles of land-marks or buoys, the sextant is held by the
handle No. 6 in a horizontal position. The vernier arrow in the sliding
limb is set on zero. Now, suppose you wish to get the angular distance
between two lighthouses as seen from the bridge of your ship. (Draw
diagram.)
[Illustration]
Look at one lighthouse through the line of sight and true horizon part
of the horizon glass. Now, move the sliding limb along the arc gradually
until you see the other lighthouse in the reflected horizon of the
horizon glass. When one lighthouse in the true horizon is directly on
top of the other lighthouse in the reflected horizon, clamp the sliding
limb. If any additional adjustment must be made, make it with the
tangent screw No. 9.
Now look through the reading glass No. 8. You should see that the arc is
divided into degrees and sixths of degrees in the following manner:
11° 10°
| |
-------------+-----------------|
| |
----|--+--+--+--+--+--|--+--+--+
| | | | | | | | | |
----+--+--+--+--+--+--+--+--+--+
Now, as every degree is divided into sixty minutes, one-sixth of a
degree is 10 minutes. In other words, each of the divisions of a degree
on this arc represents 10 minutes.
Now on the vernier in the sliding limb, directly under the arc, is the
same kind of a division. But these divisions on the vernier represent
minutes and sixths of a minute, or 10 seconds.
To read the angle, the zero point on the vernier is used as a starting
point. If it exactly coincides with one of the lines on the scale of the
arc, that line gives the measurement of the angle. In the following
illustration the angle is 10-1/2 degrees or 10° 30':
10° 9°
----------------+-----------------------+
| |
---------|---+---+---|---+---+---|---+---+---|
| | | | | | | | | |
-+---+---+---+---+---+---+---+---+---+---+---+
| | ^
----+---+---+
0
If however, you find the zero on the vernier has passed a line of the
arc, your angle is more than 10° 30' as in this:
11° 10°
------------+-----------------------+
| |
------------|---+---+---|---+---+---|
| | | | | | |
------------+---+---+---+---+---+---+
| | | ^
------------+---+---+---+
0
You must then look along the vernier to the left until you find the
point where the lines do coincide. Then add the number of minutes and
sixths of a minute shown on the vernier between zero and the point where
the lines coincide to the number of degrees and minutes shown on the arc
at the line which the vernier zero has passed, and the sum will be the
angle measured by the instrument.
Now in measuring the altitude of the sun or other celestial body,
exactly the same process is gone through except that the sextant is held
vertically instead of horizontally. You look through the telescope
toward that part of the sea directly beneath the celestial body to be
observed. You then move the sliding limb until the image of the
celestial body appears in the horizon glass, and is made to "kiss" the
horizon, i.e., its lowest point just touching the horizon. The sliding
limb is then screwed down and the angle read. More about this will be
mentioned when we come to Celestial Navigation.
Every sextant is liable to be in error. To detect this error there are
four adjustments to be made. These adjustments do not need to be learned
by heart, but I will mention them:
1. The mirror must be perpendicular to the plane of the arc. To prove
whether it is or not, set the vernier on about 60°, and look slantingly
through the mirror. If the true and reflected images of the arc
coincide, no adjustment is necessary. If not, the glass must be
straightened by turning the screws at the back.
2. The horizon glass must be perpendicular to the plane of the arc. Set
the vernier on zero and look slantingly through the horizon glass. If
the true and reflected horizons show one unbroken line, no adjustment is
necessary. If not, turn the screw at the back until they do.
3. Horizon glass and mirror must be parallel. Set the vernier on zero.
Hold the instrument vertically and look through the line of sight and
horizon glass. If the true and reflected horizons coincide, no
adjustment is necessary. If they do not, adjust the horizon glass.
4. The line of sight (telescope) must be parallel to the plane of the
arc. This adjustment is verified by observing two stars in a certain way
and then performing other operations that are described in Bowditch,
Art. 247.
Do not try to adjust your sextant yourself. Have it adjusted by an
expert on shore. Then, if there is any error, allow for it. An error
after adjustment is called the Index Error.
Put in your Note-Book:
How to find and apply the IE (Index Error):
Set the sliding limb at zero on the arc, hold the instrument
perpendicularly and look at the horizon. Move the sliding limb forward
or backward slowly until the true horizon and reflected horizon form one
unbroken line. Clamp the limb and read the angle. This is the IE. If the
vernier zero is to the left of the zero on the arc, the IE is minus and
it is to be subtracted from any angle you read, to get the correct
angle. If the vernier zero is to the right of the zero on the arc, the
IE is plus and is to be added to any angle you read to get the correct
angle. Index error is expressed thus: IE + 2' 30" or IE - 2' 30".
Quadrants, octants and quintants work on exactly the same principles as
the sextant, except that the divisions on the arc and the vernier differ
in number from the sixth divisions on the arc and vernier of the
sextant.
If any time is left, spend it in marking courses with the protractor and
handling the sextant.
Assign for Night Work the following Arts. in Bowditch:
134-135-136-138-142-144-145-151-152-157-158-159-160-161-162-163.
SATURDAY LECTURE
FIXES, ANGLES BY BEARINGS AND SEXTANT
There are five good ways of fixing your position (obtaining a "fix," as
it is called) providing you are within sight of landmarks which you can
identify or in comparatively shoal water.
Put in your Note-Book:
1. Cross bearings of two known objects.
2. Bearing and distance of a known object, the height of which is known.
3. Two bearings of a known object separated by an interval of time, with
a run during that interval.
4. Sextant angles between three known objects.
5. Using the compass, log and lead in a fog or in unfamiliar waters.
1. Cross bearings of two known objects.
Select two objects marked on the chart, so far apart that each will bear
about 45° off your bow but in opposite directions. These bearings will
be secured in the best way by the use of your pelorus. Correct each
bearing for Variation and Deviation so that it will be a true bearing.
Then with the parallel rulers carry the bearing of one object from the
chart compass card until you can intersect the object itself and draw a
line through it. Do exactly the same with the other object. Where the
two lines intersect, will be the position of the ship at the time the
bearings were taken.
[Illustration]
Now supposing you wish to find the latitude and longitude of that
position of the ship. For the latitude, measure the distance of the
place from the nearest parallel with the dividers. Take the dividers to
the latitude scale at the side of the chart and put one point of them on
the same parallel. Where the other point touches on the latitude scale,
will be the latitude desired. For the longitude, do exactly the same
thing, but use a meridian of longitude instead of a parallel of latitude
and read from the longitude scale at the top or bottom of the chart
instead of from the side.
2. Bearing and distance of a known object, the height of which is known.
Take a bearing of, say, a lighthouse the height of which is known. The
height of all lighthouses on the Atlantic Coast can be found in a book
published by the U.S. Dept. of Commerce. Correct the bearing, as
mentioned in case No. 1. Now read the angle of the height of that light
by using your sextant. Do this by putting the vernier 0 on the arc 0,
sliding the limb slowly forward until the top of the lighthouse in the
reflected horizon just touches the bottom of the lighthouse in the true
horizon. With this angle and the known height of the light, enter Table
33 in Bowditch. At the left of the Table will be found the distance off
in knots. This method can be used with any fairly perpendicular object,
the height of which is known and which is not more than 5 knots away, as
Table 33 is not made out for greater distances.
3. Two bearings of the same object, separated by an interval of time and
with a run during that interval.
Take a compass bearing of some prominent object when it is either 2, 3
or 4 points off the bow. Take another bearing of the same object when it
is either 4, 6 or 8 points off the bow. The distance run by the ship
between the two bearings will be her distance from the observed object
at the second bearing. "The distance run is the distance off."
A diagram will show clearly just why this is so:
[Illustration]
The ship at A finds the light bearing NNW 2 points off her bow. At B,
when the light bears NW and 4 points off, the log registers the distance
from A to B 9 miles. 9 miles, then, will be the distance from the light
itself when the ship is at B. The mathematical reason for this is that
the distance run is one side of an isosceles triangle. Such triangles
have their two sides of equal length. For that reason, the distance run
is the distance off. Now the same fact holds true in running from B,
which is 4 points off the bow, to C, which is 8 points off the bow, or
directly abeam. The log shows the distance run between B and C is 6.3
miles. Hence, the ship is 6.3 miles from the light when directly abeam
of it. This last 4 and 8 point bearing is what is known as the "bow and
beam" bearing, and is the standard method used in coastwise navigation.
Any one of these methods is of great value in fixing your position with
relation to the land, when you are about to go to sea.
4. Sextant angles between three known objects.
This method is the most accurate of all. Because of its precision it is
the one used by the Government in placing buoys, etc. Take three known
objects such as A, B and C which are from 30° to 60° from each other.
[Illustration]
With a sextant, read the angle from A to B and from B to C. Place a
piece of transparent paper over the compass card and draw three lines
from the center of the compass card to the circumference in such a way
that the angles secured by the sextant will be formed by the three lines
drawn. Now take this paper with the angles on it and fit it on the chart
so that the three objects of which angles were taken will be intersected
by the three lines on the paper. Where the point S is (in my diagram)
will be the point of the ship's position at the time of sight. To secure
greater accuracy the two angles should be taken at the same time by two
observers.
5. Using a compass, log and lead when you are in a fog or unfamiliar
waters.
Supposing that you are near land and want to fix your position but have
no landmarks which you can recognize. Here is a method to help you out:
Take a piece of tracing paper and rule a vertical line on it. This will
represent a meridian of longitude. Take casts of the lead at regular
intervals, noting the time at which each is taken, and the distance
logged between each two. The compass corrected for Variation and
Deviation will show your course. Rule a line on the tracing paper in
the direction of your course, using the vertical line as a N and S
meridian. Measure off on the course line by the scale of miles in your
chart, the distance run between casts and opposite each one note the
time, depth ascertained and, if possible, nature of the bottom. Now lay
this paper down on the chart which can be seen under it, in about the
position you believe yourself in when you made the first cast. If your
chain of soundings agrees with those on the chart, you are all right. If
not, move the paper about, keeping the vertical line due N and S, till
you find the place on the chart that does agree with you. That is your
line of position. You will never find in that locality any other place
where the chain of soundings are the same on the same course you are
steaming. This is the only method by soundings that you can use in thick
weather and it is an invaluable one.
Put in your Note-Book this diagram:
10 \ 8.30A.M. |
12 \ 9.00A.M. |
13 \ 10A.M. |
13-1/2 \ 10.30A.M. |
14 \ 11A.M. |
14-3/4 \ 11.30A.M. |
Assign for Night Work, Review for Weekly Examination to be held on
Monday.
Add an explanation of the Deviation Card in Bowditch, page 41.
Put in your Note-Book:
Entering New York Harbor, ship heading W 3/4 N, Variation 9° W. Observed
by pelorus the following objects:
Buoy No. 1--ENE 1/4 E
" " 2--E 1/2 N
" " 3--NE 1/4 E
" " 4--NW 1/4 N
Required true bearings of objects observed.
Answer:
From Deviation Card in Bowditch, p. 41, Deviation on W 3/4 N course is 5°
E. Hence, Compass Error is 5° E (Dev.) + 9° W (Var.) = 4° W.
C. B. C. E. T. B.
ENE 1/4 E 70° 4° W 66°
E 1/2 N 84° 4° W 80°
NE 1/4 E 48° 4° W 44°
NW 1/4 N 318° 4° W 314°
WEEK II--DEAD RECKONING
TUESDAY LECTURE
LATITUDE AND LONGITUDE
[Illustration]
We have been using the words Latitude and Longitude a good deal since
this course began. Let us see just what the words mean. Before doing
that, there are a few facts to keep in mind about the earth itself. The
earth is a spheroid slightly flattened at the poles. The axis of the
earth is a line running through the center of the earth and intersecting
the surface of the earth at the poles. The equator is the great circle,
formed by the intersection of the earth's surface with a plane
perpendicular to the earth's axis and equidistant from the poles. Every
point on the equator is, therefore, 90° from each pole.
Meridians are great circles formed by the intersection with the earth´s
surface of planes perpendicular to the equator.
Parallels of latitude are small circles parallel to the equator.
The Latitude of a place on the surface of the earth is the arc of the
meridian intercepted between the equator and that place. It is measured
by the angle running from the equator to the center of the earth and
back through the place in question. Latitude is reckoned from the
equator (0°) to the North Pole (90°) and from the equator (0°) to the
South Pole (90°). The difference of Latitude between any two places is
the arc of the meridian intercepted between the parallels of Latitude of
the places and is marked N or S according to the direction in which you
steam (T n´).
The Longitude of a place on the surface of the earth is the arc of the
equator intercepted between the meridian of the place and the meridian
at Greenwich, England, called the Prime Meridian. Longitude is reckoned
East or West through 180° from the Meridian at Greenwich. Difference of
Longitude between any two places is the arc of the equator intercepted
between their meridians, and is called East or West according to
direction. Example: Diff. Lo. T and T´ = E´ M, and E or W according as
to which way you go.
Departure is the actual linear distance measured on a parallel of
Latitude between two meridians. Difference of Latitude is reckoned in
minutes because miles and minutes of Latitude are always the same.
Departure, however, is only reckoned in _miles_, because while a mile is
equal to 1´ of longitude on the equator, it is equal to more than 1´ as
the latitude increases; the reason being, of course, that the meridians
of Lo. converge toward the pole, and the distance between the same two
meridians grows less and less as you leave the equator and go toward
either pole. Example: TN, N´n´. 10 mi. departure on the equator = 10´
difference in Lo. 10 mi. departure in Lat. 55° equals something like 18´
difference in Lo.
The curved line which joins any two places on the earth´s surface,
cutting all the meridians at the same angle, is called the Rhumb Line.
The angle which this line makes with the meridian of Lo. intersecting
any point in question is the Course, and the length of the line between
any two places is called the distance between them. Example: T or T´.
_Chart Projections_
The earth is projected, so to speak, upon a chart in three different
ways--the Mercator Projection, the Polyconic Projection and the Gnomonic
Projection.
_The Mercator Projection_
You already know something about the Mercator Projection and a Mercator
chart. As explained before, it is constructed on the theory that the
earth is a cylinder instead of a sphere. The meridians of longitude,
therefore, run parallel instead of converging, and the parallels of
latitude are lengthened out to correspond to the widening out of the Lo.
meridians. Just how this Mercator chart is constructed is explained in
detail in the Arts. in Bowditch you were given to read last night. You
do not have to actually construct such a chart, as the Government has
for sale blank Mercator charts for every parallel of latitude in which
they can be used. It is well to remember, however, that since a mile or
minute of latitude has a different value in every latitude, there is an
appearance of distortion in every Mercator chart which covers any large
extent of surface. For instance, an island near the pole, will be
represented as being much larger than one of the same size near the
equator, due to the different scale used to preserve the accurate
character of the projection.
_The Polyconic Projection_
The theory of the Polyconic Projection is based upon conceiving the
earth´s surface as a series of cones, each one having the parallel as
its base and its vertex in the point where a tangent to the earth at
that latitude intersects the earth´s axis. The degrees of latitude and
longitude on this chart are projected in their true length and the
general distortion of the earth´s surface is less than in any other
method of projection.
[Illustration]
A straight line on the polyconic chart represents a near approach to a
great circle, making a slightly different angle with each meridian of
longitude as they converge toward the poles. The parallels of latitude
are also shown as curved lines, this being apparent on all but large
scale charts. The Polyconic Projection is especially adapted to
surveying, but is also employed to some extent in charts of the U. S.
Coast & Geodetic Survey.
_Gnomonic Projection_
The theory of this projection is to make a curved line appear and be a
straight line on the chart, i.e., as though you were at the center of
the earth and looking out toward the circumference. The Gnomonic
Projection is of particular value in sailing long distance courses where
following a curved line over the earth´s surface is the shortest
distance between two points that are widely separated. This is called
Great Circle Sailing and will be talked about in more detail later on.
The point to remember here is that the Hydrographic Office prints Great
Circle Sailing Charts covering all the navigable waters of the globe.
Since all these charts are constructed on the Gnomonic Projection, it is
only necessary to join any two points by a straight line to get the
_curved_ line or great circle track which your ship is to follow. The
courses to sail and the distance between each course are easily
ascertained from the information on the chart. This is the way it is
done:
(Note to Instructor: Provide yourself with a chart and explain from the
chart explanation just how these courses are laid down.)
Spend the rest of the time in having pupils lay down courses on the
different kinds of charts. If these charts are not available assign for
night work the following articles in Bowditch, part of which reading can
be done immediately in the class room--so that as much time as possible
can be given to the reading on Dead Reckoning:
167-168-169-172-173-174-175-176--first two sentences
178-202-203-204-205-206-207-208.
Note to pupils: In reading articles 167-178, disregard the formulæ and
the examples worked out by logarithms. Just try to get a clear idea of
the different sailings mentioned and the theory of Dead Reckoning in
Arts. 202-209.
WEDNESDAY LECTURE
USEFUL TABLES--PLANE AND TRAVERSE SAILING
The whole subject of Navigation is divided into two parts, i.e., finding
your position by what is called Dead Reckoning and finding your position
by observation of celestial bodies such as the sun, stars, planets, etc.
To find your position by dead reckoning, you go on the theory that small
sections of the earth are flat. The whole affair then simply resolves
itself into solving the length of right-angled triangles except, of
course, when you are going due East and West or due North and South. For
instance, any courses you sail like these will be the hypotenuses of a
series of right-angled triangles. The problem you have to solve is,
having left a point on land, the latitude and longitude of which you
know, and sailed so many miles in a certain direction, in what latitude
and longitude have you arrived?
[Illustration]
If you sail due North or South, the problem is merely one of arithmetic.
Suppose your position at noon today is Latitude 39° 15' N, Longitude 40°
W, and up to noon tomorrow you steam due North 300 miles. Now you have
already learned that a minute of latitude is always equal to a nautical
mile. Hence, you have sailed 300 minutes of latitude or 5°. This 5° is
called difference of latitude, and as you are in North latitude and
going North, the difference of latitude, 5°, should be added to the
latitude left, making your new position 44° 15' N and your Longitude the
same 40° W, since you have not changed your longitude at all.
In sailing East or West, however, your problem is more difficult. Only
on the equator is a minute of longitude and a nautical mile of the same
length. As the meridians of longitude converge toward the poles, the
lengths between each lessen. We now have to rely on tables to tell us
the number of miles in a degree of longitude at every distance North or
South of the equator, i.e., in every latitude. Longitude, then, is
reckoned in _miles_. The number of miles a ship makes East or West is
called Departure, and it must be converted into degrees, minutes and
seconds to find the difference of longitude.
A ship, however, seldom goes due North or South or due East or West. She
usually steams a diagonal course. Suppose, for instance, a vessel in
Latitude 40° 30' N, Longitude 70° 25' W, sails SSW 50 miles. What is the
new latitude and longitude she arrives in? She sails a course like this:
[Illustration]
Now suppose we draw a perpendicular line to represent a meridian of
longitude and a horizontal one to represent a parallel of latitude. Then
we have a right-angled triangle in which the line AC represents the
course and distance sailed, and the angle at A is the angle of the
course with a meridian of longitude. If we can ascertain the length of
AB, or the distance South the ship has sailed, we shall have the
difference of latitude, and if we can get the length of the line BC, we
shall have the Departure and from it the difference of longitude. This
is a simple problem in trigonometry, i.e., knowing the angle and the
length of one side of a right triangle, what is the length of the other
two sides? But you do not have to use trigonometry. The whole problem is
worked out for you in Table 2 of Bowditch. Find the angle of the course
SSW, i.e., S 22° W in the old or 202° in the new compass reading. Look
down the distance column to the left for the distance sailed, i.e., 50
miles. Opposite this you find the difference of latitude 46-4/10 (46.4)
and the departure 18-7/10 (18.7). Now the position we were in at the
start was Lat. 40° 30' N, Longitude 70° 25' W. In sailing SSW 50 miles,
we made a difference of latitude of 46' 24" (46.4), and as we went
South--toward the equator--we should subtract this 46' 24" from our
latitude left to give us our latitude in.
Now we must find our difference of longitude and from it the new or
Longitude in. The first thing to do is to find the _average_ or middle
latitude in which you have been sailing. Do this by adding the latitude
left and the latitude in and dividing by 2.
40° 30' 00"
39 43 36
-----------
2)80 13 36
-----------
40° 06' 48" Mid. Lat.
Take the nearest degree, i.e., 40°, as your answer. With this 40° enter
the same Table 2 and look for your departure, i.e., 18.7 in the
_difference of latitude_ column. 18.4 is the nearest to it. Now look to
the left in the distance column opposite 18.4 and you will find 24,
which means that in Lat. 40° a departure of 18.7 miles is equivalent to
24' of difference of Longitude. We were in 70° 25' West Longitude and we
sailed South and West, so this difference of Longitude should be added
to the Longitude left to get the Longitude in:
Lo. left 70° 25' W
Diff. Lo. 24
-----------
Lo. in 70° 49' W
The whole problem therefore would look like this:
Lat. left 40° 30' N Lo. left 70° 25' W
Diff. Lat. 46 24 Diff. Lo. 24
------------- ----------
Lat. in 39° 43' 36" N Lo. in 70° 49' W
There is one more fact to explain. When the course is 45° or less (old
compass reading) you read from the top of the page of Table 2 down. When
the course is more than 45° (old compass reading) you read from the
bottom of the page up. The distance is taken out in exactly the same way
in both cases, but the difference of Latitude and the Departure, you
will notice, are reversed. (Instructor: Read a few courses to thoroughly
explain this.) From all this explanation we get the following rules,
which put in your Note-Book:
To find the new or Lat. in: Enter Table 2 with the true course at the
top or bottom of the page according as to whether it is less or greater
than 45° (old compass reading). Take out the difference of Latitude and
Departure and mark the difference of Latitude minutes ('). When the
Latitude left and the difference of Latitude are both North or both
South, add them. When one is North and the other South, subtract the
less from the greater and the remainder, named North or South after the
greater, will be the new Latitude, known as the Latitude in.
To find the new or Lo. in: Find the middle latitude by adding the
latitude left to the latitude in and dividing by 2. With this middle
latitude, enter Table 2. Seek for the departure in the difference of
latitude column. Opposite to it in the distance column will be the
figures indicating the number of minutes in the difference of longitude.
With this difference of Longitude, apply it in the same way to the
Longitude left as you applied the difference of Latitude to the Latitude
left. The result will be the new or Longitude in.
Now if a ship steamed a whole day on the same course, you would be able
to get her Dead Reckoning position without any further work, but a ship
does not usually sail the same course 24 hours straight. She usually
changes her course several times, and as a ship's position by D.R. is
only computed once a day--at noon--it becomes necessary to have a method
of obtaining the result after several courses have been sailed. This is
called working a traverse and sailing on various courses in this fashion
is called Traverse Sailing.
Put in your Note-Book the following example and the way in which it is
worked:
Departure taken from Barnegat Light in Lat. 39° 46' N, Lo. 74° 06' W,
bearing by compass NNW, 15 knots away. Ship heading South with a
Deviation of 4° W. She sailed on the following courses:
--------+----+-------+---------+--------+------------------------------
Course |Wind| Leeway|Deviation|Distance| Remarks
--------+----+-------+---------+--------+------------------------------
SE 3/4 E| NE | 1 pt. | 3° E | 30 |Variation throughout day 8° W.
S 11° W | NE | 0 | 6° E | 55 | A current set NE magnetic
NNW | NE | 0 | 2° W | 14 | 1/2 mi. per hr. for the day.
S 87°E | NE | 0 | 3° E | 50 | Required Lat. and Lo. in
| | | | | and course and distance
| | | | | made good.
--------+----+-------+---------+--------+------------------------------
-----------------------------------------------------------------------------
C. Cos. |Wind|Leeway| Dev.| Var.| NEW | OLD |Dist.|Diff. Lat. |Departure
| | | | |T. Cos.|T Cos. | +-----+-----+-----+----
| | | | | | | | N | S | E | W
--------+----+------+-----+-----+-------+-------+-----+-----+-----+-----+----
SSE | .. | .. | 4° W| 8° W| 145° | S 35°E| 15 | .. |12.3 | 8.6 | ..
SE 3/4 E| NE | 1 pt.| 3° E| 8° W| 133° | S 47°E| 30 | .. |20.5 |21.9 | ..
S 11° W | NE | 0 | 6° E| 8° W| 189° | S 9°W| 55 | .. |54.3 | .. | 8.6
NNW | NE | 0 | 2° W| 8° W| 327° | N 33°W| 14 |11.7 |.. | .. | 7.6
S 87° E | NE | 0 | 3° E| 8° W| 88° | N 88°E| 50 | 1.7 |.. |50 | ..
NE | .. | .. | mg | 8° W| 3° | N 3° E| 12 | 9.6 |.. | 7.2 | ..
--------+----+------+-----+-----+-------+-------+-----+-----+-----------+----
23.0 |87.1 |87.7 |16.2
.. |23.0 |16.2 |..
+-----+-----+----
.. |.. |.. |..
.. |64.1 |71.5 |..
+-----+-----+----
.. S E
Lat. left 39°-46'-00" N Mid. Lat. 39°
Diff. Lat. 1 -04 -06 S Dep. 71.5
----------
Lat. in. 38 -41 -54 N Table 2--Under 39° Dep. in
39 -46 -00 Diff. Lat. col. = 92' = 1° 32' Diff. Lo.
----------
2)78 -27 -54
----------
Mid. Lat. 39 -13 -57
Lo. left 74°-06'-00" W
Diff. Lo. 1 -32 -00 E
--------------
Lo. in. 72°-34'-00" W
Table 2--Diff. Lat. 64.1, Dep. 71.5.
Course S 48° E--Distance 96 miles.
The rule covering all these operations is as follows:
1. Write out the various courses with their corrections for Leeway,
Deviation, Variation and the distance run on each.
2. In four adjoining columns headed N, S, E, W respectively, put down
the Difference of Latitude and Departure for each course.
3. Add together all the northings, all the southings, all the eastings
and all the westings. Subtract to find the difference between northings
and southings and you will get the whole difference of Latitude. The
difference between the eastings and westings will be the whole
departure.
4. Find the latitude in, as already explained.
5. Find the Lo. in, as already explained.
6. With the whole difference of Latitude and whole Departure, seek in
Table 2 for the page where the nearest agreement of Difference of
Latitude and Departure can be found. The number of degrees at the top or
bottom of the page (according as to whether the Diff. of Lat. or Dep. is
greater) will give you the true course made good, and the number in the
distance column opposite the proper Difference of Latitude and Departure
will give you the distance made.
It is often convenient to use the reverse of the above method, i.e.,
being given the latitude and longitude of the position left and the
latitude and longitude of the position arrived in, to find the course
and distance between them by Middle Latitude Sailing. The full rule is
as follows:
1. Find the algebraic difference between the latitudes and longitudes
respectively.
2. Using the middle (or average) latitude as a course, find in Table 2
of Bowditch the Diff. of Lo. in the distance column. Opposite, in the
Diff. of Lat. column, will be the correct Departure.
3. With the Diff. of Lat. between the position left and the position
arrived in, and the Departure, just secured, seek in Table 2 for the
page where the nearest agreement to these values can be found. On this
page will be secured the true course and distance made, as explained in
the preceding method.
4. Use this method only when steaming approximately an East and West
course.
For an example of this method, see Bowditch, p. 77, example 3.
THURSDAY LECTURE
EXAMPLES ON PLANE AND TRAVERSE SAILING (_Continued_)
1. Departure taken from Cape Horn. Lat. 55° 58' 41" S, Lo. 67° 16' 15"
W, bearing by compass SSW 20 knots. Ship heading SW x S, Deviation 4° E,
steamed the following courses:
---------+----------+------------+-----------+----------
C. Cos. | Wind | Leeway | Deviation | Distance
---------+----------+------------+-----------+----------
SW x S | SE | 1 pt. | 4° E | 40
WNW | N | 2 pts. | 5° E | 25
S 40° E | NE | 2 pts. | 4° W | 20
---------+----------+------------+-----------+----------
_Remarks_
Variation 18° E throughout. Current set NW magnetic 30 mi. for the day.
Required Latitude and Longitude in and course and distance made good.
2. Departure taken from St. Agnes Lighthouse, Scilly Islands, Lat. 49°
53' S, Lo. 6° 20' W, bearing by compass E x S, distance 18 knots,
Deviation 10° W, Variation 23° W. Ship headed N steamed on the following
courses:
-------+------+-------+-------+------+------------------------------
C. Cos.| Wind |Leeway |Devia- |Dis- | Remarks
| | |tion |tance |
-------+------+-------+-------+------+------------------------------
N | | .. | 10° W | 60 |Variation 23° W. Current set
S 1/2 E| W | 3 pts.| 10° E | 40 |SE mg 1-1/2 miles for 24 hrs.
NNE | NNW | 2 pts.| 8° W | 45 |Req. Lat. and Lo. in and
| | | | |course and distance made
| | | | |good,
-------+------+-------+-------+------+------------------------------
Assign for Night Work the following articles in Bowditch:
179-180-181-182. Also additional problems in Dead Reckoning.
FRIDAY LECTURE
MERCATOR SAILING
This is a method to find the true course and distance between two
points. The method can be used in two ways, i.e., by the use of Tables 2
and 3 (called the inspection method) and by the use of logarithms. The
first method is the quicker and will do for short distances. The second
method, however, is more accurate in all cases, and particularly where
the distances are great. The inspection method is as follows (Put in
your Note-Book):
Find the algebraic difference between the meridional parts corresponding
to the Lat. in and Lat. sought by Table 3. Call this Meridional
difference of Latitude. Find the algebraic difference between Longitude
in and Longitude sought and call this difference of Longitude. With the
Meridional difference of Latitude and the difference of Longitude, find
the course by searching in Table 2 for the page where they stand
opposite each other in the latitude and departure columns. Now find the
real difference of latitude. Under the course just found and opposite
the _real_ difference of Latitude, will be found the distance sailed in
the distance column. Example:
What is the course and distance from Lat. 40° 28' N, Lo. 73° 50' W, to
Lat. 39° 51' N, Lo. 72° 45' W?
Lat. in 40° 28' N Meridional pts. 2644.2
Lat. sought 39 51 N Meridional pts. 2596.0
--------- ------
0° 37' Mer. diff. Lat. 48.2
Lo. in 73° 50' W
Lo. sought 72 45 W
---------
1° 05' = 65'
On page 604 Bowditch you will find 48.7 and 64.7 opposite each other,
and as 48.7 is in the Lat. column only when you read from the bottom,
the course is S 53° E. The real difference of Lat. under this course is
opposite 62 in the distance column. Hence the distance to be sailed is
62 miles.
If distances are too great, divide meridional difference of Lat., real
difference of Latitude and difference of Longitude by 10 or any other
number to bring them within the scope of the distances in Table 2. When
distance to be sailed is found, it must be multiplied by the same
number. For instance, if the difference of Lat., difference of Lo.,
etc., are divided by 10 to bring them in the scope of Table 2, and with
these figures 219 is the distance found, the real distance would be 10
times 219 or 2190.
Now let us work out the same problem by logarithms. This will acquaint
us with two new Tables, i.e., Tables 42 and 44. Put this in your
Note-Book:
Lat. in 40° 28' N Mer. pts. 2644.2 Lo. in 73° 50'
Lat. sought 39 51 Mer. pts. 2596.0 Lo. sought 72 45
--------- ------ -------
Real diff. 0° 37' 48.2 1° 05'
60
--
60
5
--
(Table 42) log (+ 10) 65 = 11.81291
Log 48.2 = 1.68305
--------
Log tan TC (Table 44) 10.12986
TC = S 53° 26' E
Log sec TC (53° 26') = 10.22493
Log real diff. Lat. = 1.56820 +
--------
11.79313
- 10.
--------
1.79313
Distance (Table 42) = 62.11 miles
Find algebraically the real difference of latitude, meridional
difference of latitude and the difference of longitude. Reduce real
difference of latitude and difference of longitude to minutes. Take log
of the difference of longitude (Table 42) and add 10. From this log
subtract the log of difference of meridional parts. The result will be
the log tan of the True Course, which find in Table 44. On the same page
find the log sec of true course. Add to this the log of the real
difference of latitude, and if the result is more than 10, subtract 10.
This result will be the log of the distance sailed. This method should
be used only when steaming approximately a North and South course.
Note.--For detailed explanation of Tables 42 and 44 see Bowditch, pp.
271-276.
Assign for Night Reading Arts, in Bowditch:
183-184-185-186-187-188-189-194-259-260-261-262-263-264-265-266-267-268.
Also, one of the examples of Mercator sailing to be done by both the
Inspection and Logarithmic method.
SATURDAY LECTURE
GREAT CIRCLE SAILING--THE CHRONOMETER
In Tuesday's Lecture of this week, I explained how a Great Circle track
was laid down on one of the Great Circle Sailing Charts which are
prepared by the Hydrographic Office.
Supposing, however, you do not have these charts on hand. There is an
easy way to construct a great circle track yourself. Turn to Art. 194,
page 82, in Bowditch. Here is a table with an explanation as to how to
use it. Take, for instance, the same two points between which you just
drew a line on the great circle track. Find the center of this line and
the latitude of that point. At this point draw a line perpendicular to
the course to be sailed, the other end of which must intersect the
corresponding parallel of latitude given in the table. With this point
as the center of a circle, sweep an arc which will intersect the point
left and the point sought. This arc will be the great circle track to
follow.
To find the courses to be sailed, get the difference between the course
at starting and that at the middle of the circle, and find how many
quarter points are contained in it. Now divide the distance from the
starting point to the middle of the circle by the number of quarter
points. That will give the number of miles to sail on each quarter point
course. See this illustration:
[Illustration]
Difference between ENE and E = 2 pts. = 8 quarter points. Say distance
is 1600 miles measured by dividers or secured by Mercator Sailing
Method. Divide 1600 by 8 = 200. Every 200 miles you should change your
course 1/4 point East.
_The Chronometer_
The chronometer is nothing more than a very finely regulated clock. With
it we ascertain Greenwich Mean Time, i.e., the mean time at Greenwich
Observatory, England. Just what the words "Greenwich Mean Time" signify,
will be explained in more detail later on. What you should remember here
is that practically every method of finding your exact position at sea
is dependent upon knowing Greenwich Mean Time, and the only way to find
it is by means of the chronometer.
It is essential to keep the chronometer as quiet as possible. For that
reason, when you take an observation you will probably note the time by
your watch. Just before taking the observation, you will compare your
watch with the chronometer to notice the exact difference between the
two. When you take your observation, note the watch time, apply the
difference between the chronometer and watch, and the result will be the
CT.
For instance, suppose the chronometer read 3h 25m 10s, and your watch,
at the same instant, read 1h 10m 5s. C--W would be:
3h -- 25m -- 10s
-- 1 -- 10 -- 05
----------------
2h -- 15m -- 05s
Now suppose you took an observation which, according to your watch, was
at 2h 10m 05s. What would be the corresponding C T? It would be
WT 2h -- 10m -- 05s
C -- W 2 -- 15 -- 05
--------------------
CT 4h -- 25m -- 10s
If the chronometer time is less than the W T add 12 hours to the C T, so
that it will always be the larger and so that the amount to be added to
W T will always be +. For instance, CT 1h--25m--45s, WT 4h--13m--25s,
what is the C-W?
CT 13h--25m--45s
WT 4 --13 --25
----------------
C--W 9h--12m--20s
Now, suppose an observation was taken at 6h 13m 25s according to watch
time. What would be the corresponding CT?
WT 6h--13m--25s
C--W 9 --12 --20
----------------
15h--25m--45s
--12
----------------
CT 3h--25m--45s
Put in your Note-Book: CT = WT + C - W.
If, in finding C-W, C is less than W, add 12 hours to C, subtracting
same after CT is secured.
Example No. 1:
CT 3h--25m--10s
WT 1 --10 --05
----------------
C--W 2h--15m--05s
WT 2h--10m--05s
+ C--W 2 --15 --05
----------------
CT 4h--25m--10s
Example No. 2:
CT 1h--25m--45s
WT 4h--13m--25s
(+12 hrs.) CT 13h--25m--45s
WT 4 --13 --25
----------------
C-W 9h--12m--20s
WT 6h--13m--25s
+ C-W 9 --12 --20
----------------
15h--25m--45s
(-12 hrs.) 12
----------------
CT 3h--25m--45s
There is one more very important fact to know about the chronometer. It
is physically impossible to keep it absolutely accurate over a long
period of time. Instead of continually fussing with its adjustment and
hands, the daily rate of error is ascertained, and from this the exact
time for any given day. It is an invariable practice among good mariners
to _leave the chronometer alone_. When you are in port, you can find out
from a time ball or from some chronometer maker what your error is. With
this in mind, you can apply the new correction from day to day. Here is
an example (Put in your Note-Book):
On June 1st, CT 7h--20m--15s, CC 2m--40s fast. On June 16th, (same CT)
CC 1m--30s fast. What was the corresponding G.M.T. on June 10th?
June 1st 2m--40s fast
16th 1m--30s fast
----------------
1m--10s
60
--
60
10
--
15) 70s (4.6 sec. Daily Rate of error losing
June 1st-10th, 9 days times 4.6 sec. = 41 sec. losing
June 1st 2m--40s fast
June 10th 41s losing
---------
June 10th 1m--59s fast
CT 7h--20m--15s
CC -- 1 --59
------------
G.M.T. 7h--18m--16s on June 10th
If CC is fast, subtract from CT
If CC is slow, add to CT
WEEK III--CELESTIAL NAVIGATION
TUESDAY LECTURE
CELESTIAL CO-ORDINATES, EQUINOCTIAL SYSTEM, ETC.
We have already discussed the way in which the earth is divided so as to
aid us in finding our position at sea, i.e., with an equator, parallels
of latitude, meridians of longitude starting at the Greenwich meridian,
etc. We now take up the way in which the celestial sphere is
correspondingly divided and also simple explanations of some of the more
important terms used in Celestial Navigation.
As you stand on any point of the earth and look up, the heavenly bodies
appear as though they were situated upon the surface of a vast hollow
sphere, of which your eye is the center. Of course this apparent concave
vault has no existence and we cannot accurately measure the distance of
the heavenly bodies from us or from each other. We can, however, measure
the direction of some of these bodies and that information is of
tremendous value to us in helping us to fix our position.
Now we could use our eye as the center of the celestial sphere but more
accurate than that is to use the center of the earth. Suppose we do use
the center of the earth as the place from which to observe these
celestial bodies and, in imagination, transfer our eye there. Then we
will find projected on the celestial sphere not only the heavenly bodies
but the imaginary points and circles of the earth's surface. Parallels
of latitude, meridians of longitude, the equator, etc., will have the
same imaginary position on the celestial sphere that they have on the
earth. Your actual position on the earth will be projected in a point
called your zenith, i.e., the point directly overhead.
[Illustration]
From this we get the definition that the Zenith of an observer on the
earth's surface is the point in the celestial sphere directly overhead.
It would be a simple matter to fix your position if your position never
changed. But it is always changing with relation to these celestial
bodies. First, the earth is revolving on its own axis. Second, the
earth is moving in an elliptic track around the sun, and third, certain
celestial bodies themselves are moving in a track of their own. The
changes produced by the daily rotation of the earth on its axis are
different for observers at different points on the earth and, therefore,
depend upon the latitude and longitude of the observer. But the changes
arising from the earth's motion in its orbit and the motion of various
celestial bodies in their orbits, are true no matter on what point of
the earth you happen to be. These changes, therefore, in their relation
to the center of the earth, may be accurately gauged at any instant. To
this end the facts necessary for any calculation have been collected and
are available in the Nautical Almanac, which we will take up in more
detail later.
Now with these facts in mind, let us explain in simple words the meaning
of some of the terms you will have to become acquainted with in
Celestial Navigation.
In the illustration (Bowditch p. 88) the earth is supposed to be
projected upon the celestial sphere N E S W. The Zenith of the observer
is projected at Z and the pole of the earth which is above the horizon
is projected at P. The other pole is not given.
The Celestial Equator is marked here E Q W and like all other points and
lines previously mentioned, it is the projection of the Equator until it
intersects the celestial sphere. Another name for the Celestial Equator
is the Equinoctial.
All celestial meridians of longitude corresponding to longitude
meridians on the earth are perpendicular to the equinoctial and likewise
P S, the meridian of the observer, since it passes through the
observer's zenith at Z, is formed by the extension of the earth's
meridian of the observer and hence intersects the horizon at its N and S
points. This makes clear again just what is the meridian of the
observer. It is the meridian of longitude which passes through the N and
S poles and the observer's zenith. In other words, when the sun or any
other heavenly body is on your meridian, a line stretched due N and S,
intersecting the N and S poles, will pass through your zenith and the
center of the sun or other celestial body. To understand this is
important, for no sight with the sextant is of value except with
relation to your meridian.
The Declination of any point in the celestial sphere is its distance in
arc, North or South of the celestial equator, i.e., N or S of the
Equinoctial.
North declinations, i.e., declinations north of the equinoctial are
always marked, +; those south of the equinoctial, -. For instance, in
the Nautical Almanac, you will never see a declination of the sun or
other celestial body marked, N 18° 28' 30". It will always be marked
+18° 28' 30" and a south declination will be marked -18° 28' 30".
Another fact to remember is that Declination on the celestial sphere
corresponds to latitude on the earth. If, for instance, the Sun's
declination is +18° 28' 30" at noon, Greenwich, then at that instant,
i.e., noon at Greenwich, the sun will be directly overhead a point on
earth which is in latitude N 18° 28' 30".
The Polar Distance of any point is its distance in arc from either pole.
It must, therefore, equal 90° minus the declination, if measured from
the pole of the same name as the declination or 90° plus the declination
if measured from the pole of the opposite name.
P M is the polar distance of M from P, or P B the polar distance of B
from P.
The true altitude of a celestial body is its angular height from the
true horizon.
The zenith distance of any point or celestial body is its angular
distance from the zenith of the observer.
The Ecliptic is the great circle representing the path in which the sun
appears to move in the celestial sphere. As a matter of fact, you know
that the earth moves around the sun, but as you observe the sun from
some spot on the earth, it appears to move around the earth. This
apparent track is called the Ecliptic as stated before, and in the
illustration the Ecliptic is represented by the curved line, C V T. The
plane of the Ecliptic is inclined to that of the Equinoctial at an angle
of 23° 27-1/2', and this inclination is called the obliquity of the
Ecliptic.
The Equinoxes are those points at which the Ecliptic and Equinoctial
intersect, and when the sun occupies either of these two positions, the
days and nights are of equal length. The Vernal Equinox is that one
which the sun passes through or intersects in going from S to N
declination, and the Autumnal Equinox that which it passes through or
intersects in going from N to S declination. The Vernal Equinox (V in
the illustration) is also designated as the First Point of Aries which
is of use in reckoning star time and will be mentioned in more detail
later.
The Solstitial Points, or Solstices, are points of the Ecliptic at a
distance of 90° from the Equinoxes, at which the sun attains its highest
declination in each hemisphere. They are called the Summer and Winter
Solstice according to the season in which the sun appears to pass these
points in its path.
To sum up: The way to find any point on the earth is to find the
distance of this point N or S of the equator (i.e., its Latitude) and
its distance E or W of the meridian at Greenwich (i.e., its longitude).
In the celestial sphere, the way to find the location of a point or
celestial body such as the sun is to find its declination (i.e.,
distance in arc N or S of the equator) and its hour angle. By hour
angle, I mean the distance in time from your meridian to the meridian of
the point or celestial body in question.
Assign for Night reading, Arts, in Bowditch:
270-271-272-273-274-275-277-278-279-280-282-283-284.
WEDNESDAY LECTURE
TIME BY THE SUN--MEAN TIME, SOLAR TIME, CONVERSION, ETC.
There is nothing more important in all Navigation than the subject of
Time. Every calculation for determining the position of your ship at sea
must take into consideration some kind of time. Put in your Note-Book:
There are three kinds of time:
1. Apparent or solar time, i.e., time by the sun.
2. Mean Time, i.e., clock time.
3. Sidereal Time, or time by the stars.
So far as this lecture is concerned, we will omit any mention of
sidereal time, i.e., time by the stars. We will devote this morning to
sun time, i.e., apparent time, and mean time.
Apparent or Solar Time is, as stated before, nothing more than sun time
or time by the sun. The hour angle of the center of the sun is the
measure of apparent or solar time. An apparent or solar day is the
interval of time it takes for the earth to revolve completely around on
its axis every 24 hours. It is apparent noon at the place where you are
when the center of the sun is directly on your meridian, i.e., on the
meridian of longitude which runs through the North and South poles and
also intersects your zenith. This is the most natural and the most
accurate measure of time for the navigator at sea and the unit of time
adopted by the mariner is the apparent solar day. Apparent noon is the
time when the latitude of your position can be most easily and most
exactly determined and on the latitude by observation just secured we
can get data which will be of great value to us for longitude sights
taken later in the day.
Now it would be very easy for the mariner if he could measure apparent
time directly so that his clock or other instrument would always tell
him just what the sun time was. It is impossible, however, to do this
because the earth does not revolve at a uniform rate of speed.
Consequently the sun is sometimes a little ahead and sometimes a little
behind any average time. You cannot manufacture a clock which will run
that way because the hours of a clock must be all of exactly the same
length and it must make noon at precisely 12 o'clock every day. Hence we
distinguish clock time from sun time by calling clock time, mean (or
average) time and sun time, apparent or solar time. From this
explanation you are ready to understand such expressions as Local Mean
Time, which, in untechnical language, signifies clock time at the place
where you are; Greenwich Mean Time which signifies clock time at
Greenwich; Local Apparent Time, which signifies sun time at the place
where you are; Greenwich Apparent Time, which signifies sun time at
Greenwich.
Now the difference between apparent time and mean time can be found for
any minute of the day by reference to the Nautical Almanac which we will
take up later in more detail. This difference is called the Equation of
Time.
There is one more fact to remember in regard to apparent and mean time.
It is the relation of the sun's hour angle to apparent time. In the
first place, what is a definition of the sun's HA? It is the angle at
the celestial pole between the meridian intersecting any given point and
the meridian intersecting the center of the sun. It is measured by the
arc of the celestial equator intersected between the meridian of any
point and the meridian intersecting the center of the sun.
[Illustration]
For instance, in the above diagram, suppose PG is the meridian at
Greenwich, and PS the meridian intersecting the sun. Then the angle at
the pole GPS, measured by the arc GS would be the Hour Angle of
Greenwich, or the Greenwich Hour Angle. And now you notice that this
angular measure is exactly the same as apparent time at Greenwich or
Greenwich Apparent Time, for Greenwich Apparent Time is nothing more
than the distance in time Greenwich, England, or the meridian at
Greenwich is from the sun, i.e., the time it takes the earth to revolve
from Greenwich to the sun; and that distance is exactly measured by the
Greenwich Hour Angle or the arc on the celestial equator, GS.
The same is correspondingly true of Local Apparent Time and the ship's
Hour Angle. Suppose, for instance, PL is the meridian intersecting the
place where your ship is. Then your ship's hour angle would be the angle
at the pole intersecting the meridian of your ship and the meridian of
the sun or LPS and measured by the arc LS. And you will note that this
distance is exactly the same as apparent time at the ship, for Apparent
Time at ship is nothing more than the distance in time which the ship is
from the sun. We can sum up all this information in a few simple rules,
which put in your Note-Book:
Mean Time = Clock Time.
G.M.T. = Greenwich Mean Time.
L.M.T. = Local Mean Time.
Apparent Time = Actual or Sun Time.
G.A.T. (G.H.A.) = Greenwich Apparent Time or Greenwich Hour Angle.
L.A.T. (S.H.A.) = Local Apparent Time or Ship's Hour Angle.
Difference between apparent and mean time or mean and apparent
time--Equation of Time.
Right under this in your Note-Book put the following diagram, which I
will explain:
[Illustration]
You will see from this diagram that civil time commences at midnight and
runs through 12 hours to noon. It then commences again and runs through
12 hours to midnight. The Civil Day, then, is from midnight to midnight,
divided into two periods of 12 hours each.
The astronomical day commences at noon of the civil day of the same
date. It comprises 24 hours, reckoned from O to 24, from noon of one day
to noon of the next. Astronomical time, either apparent or mean, is the
hour angle of the true or mean sun respectively, measured to the
westward throughout its entire daily circuit.
Since the civil day begins 12 hours before the astronomical day and ends
12 hours before it, A.M. of a new civil day is P.M. of the astronomical
day preceding. For instance, 6 hours A.M., April 15th civil time is
equivalent to 18 hours April 14th, astronomical time.
Now, all astronomical calculations in which time is a necessary fact to
be known, must be expressed in astronomical time. As chronometers have
their face marked only from 0 to 12 as in the case of an ordinary watch,
it is necessary to transpose this watch or chronometer time into
astronomical time. No transposing is necessary if the time is P.M., as
you can see from the diagram that both civil and astronomical times up
to 12 P.M. are the same. But in A.M. time, such transposing is
necessary. Put in your Note-Book:
Whenever local or chronometer time is A.M., deduct 12 hours from such
time to get the correct astronomical time:
CT 15d-- 9h--10m--30s A.M.
--12
------------------------
CT 14d--21h--10m--30s
------
L.M.T. 10d-- 4h--40m--16s A.M.
--12
------------------------
L.M.T. 9d--16h--40m--16s
Now we come to a very important application of time. You will remember
that in one of the former lectures we stated that to find our latitude,
we had to find how far North or South of the equator we were, and to
find our longitude, we had to find how far East or West of the meridian
at Greenwich we were. Never mind about latitude for the present. We can
find our longitude exactly if we know our Greenwich time and our time at
ship. For instance, in the accompanying diagram:
[Illustration]
Suppose PG is the meridian at Greenwich, then anything to the west of PG
is West longitude and anything to the East of PG is East longitude. Now
suppose GPS is the H.A. of G. or G.A.T.--i.e., the distance in time G.
is from the sun. And L P S is the H.A. of the ship or L.A.T.--i.e., the
distance in time the ship is from the sun. Then the difference between G
P S and L P S is G P L, measured by the arc L G, and that is the
difference that the ship, represented by its meridian PL, is from the
Greenwich meridian PG. In other words, that is the ship's longitude for,
as mentioned before, longitude is the distance East or West of Greenwich
that any point is, measured on the arc of the celestial equator. The
longitude is West, for you can see LPG or the arc LG is west of the
meridian PG.
Likewise if P E is the meridian of your ship, the Longitude in time is
the S.H.A. or L.A.T., E P S (the distance your ship is from the sun)
less the G.H.A. or G.A.T., G P S (the distance Greenwich is from the
sun) which is the angle G P E measured by the arc G E. And this
Longitude is East for you can see G P E, measured by G E, is east of the
Greenwich meridian, P G.
In both these cases, however, the longitude is expressed in time, i.e.,
so many hours, minutes and seconds from the Greenwich meridian and we
wish to express this distance in degrees, minutes and seconds of arc.
The earth describes a circle of 360° every 24 hours. Then if you are 1
hour from Greenwich, you are 1/24 of 360° or 15° from Greenwich and if
you are 12 hours from Greenwich, you are 1/2 of 360° or 180° from
Greenwich. By keeping this in mind, you should be able to transpose time
into degrees, minutes and seconds of arc for any fraction of time. It
is, however, all worked out in Table 7 of Bowditch which turn to. (Note
to Instructor: Explain this table carefully). Put in your Note-Book:
89° 24' 26" = (89°) 5h--56m
(24') 1m--36s
(26") 1--44/60s
--------------------
5h--57m--37s 44/60s = 38s
4h--42m--26s
4h--40m = 70°
2m--24s = 36'
2s = 30"
-----------
70° 36' 30"
Also put in your Note-Book this diagram and these formulas: (For diagram
use illustration on p. 40.)
L.M.T. + West Lo. = G.M.T. L.A.T. + West Lo. = G.A.T.
L.M.T. - East Lo. = G.M.T. L.A.T. - East Lo. = G.A.T.
G.M.T. - West Lo. = L.M.T. G.A.T. - West Lo. = L.A.T.
G.M.T. + East Lo. = L.M.T. G.A.T. + East Lo. = L.A.T.
If G.M.T. or G.A.T. is greater than L.M.T. or L.A.T. respectively, Lo.
is West.
If G.M.T. or G.A.T. is less than L.M.T. or L.A.T. respectively, Lo. is
East.
Example:
In longitude 81° 15' W, L.M.T. is April 15d--10h--17m--30s A.M. What is
G.M.T.?
L.M.T. 15d--10h--17m--30s A.M.
--12
------------------
L.M.T. 14d--22h--17m--30s
5 --25 W +
------------------
G.M.T. 15d-- 3h--42m--30s
--------
G.M.T. April 15d-- 3h--42m--30s
L.M.T. April 15d--10h--17m--30s A.M.
In what Lo. is ship?
G.M.T. 15d 3h--42m--30s
L.M.T. 14d 22h--17m--30s
------------------
Lo. in T 5h--25m--00s W
Lo. = 81° 15'W
Assign also for Night Work reading the following articles in Bowditch:
276-278-279-226-228-286-287-288-290-291-294 (omitting everything on page
114.)
THURSDAY LECTURE
SIDEREAL TIME--RIGHT ASCENSION
Our last lecture was devoted to a discussion of sun time. Today we are
going to talk about star time, or, using the more common words, sidereal
time.
Now, just one word of review. You remember that we have learned that
astronomical time is reckoned from noon of one day to noon of the next
and hence the astronomical day corresponds to the 24 hours of a ship's
run. The hours are counted from 0 to 24, so that 10 o'clock in the
morning of October 25th is astronomically October 24th, 22 hours or 22
o'clock of October 24th.
Now Right Ascension is different from both astronomical and civil time.
Right Ascension is practically celestial longitude. For instance, the
position of a place on the earth is fixed by its latitude and longitude;
the position of a heavenly body is fixed by its declination and right
ascension. But Right Ascension is not measured in degrees and minutes
nor is it measured East and West. It is reckoned in hours and minutes
all the way around the sky, eastward from a certain point, through the
approximate 24 hours. The point from which this celestial longitude
begins is not at Greenwich, but the point where the celestial equator
intersects the ecliptic in the spring of the year, i.e., the point where
the sun, coming North in the Spring, crosses the celestial equator. This
point is called the First Point of Aries. You will frequently hear me
speak of a star having, for instance, a Right Ascension of 5h 16m 32s. I
mean by that, that starting at the celestial meridian, i.e., the
meridian passing through the First Point of Aries, it will take a spot
on the earth 5h 16m 32s to travel until it reaches the meridian of the
star in question.
Roughly speaking then, just as Greenwich Apparent Time means the
distance East or West the Greenwich meridian is from the sun and Local
Apparent Time means the distance East or West your ship is from the sun,
so R.A.M.G. means the distance in time the Meridian of Greenwich is from
the First Point of Aries, measured eastward in a circle. And this
distance is the same as Greenwich Sidereal Time, i.e., Sidereal Time at
Greenwich or the distance in time the meridian of Greenwich is from the
First Point of Aries.
Now, what is the star time that corresponds to local time? It is called
the Right Ascension of the Meridian, which means the R. A. of the
meridian which intersects your zenith. Just as L.A.T. is the distance in
time your meridian is from the sun, so Local Sidereal Time is the R. A.
of your meridian, i.e., the distance in time your meridian is from the
First Point of Aries. Put in your Note-Book:
G.S.T. and R.A.M.G. are one and the same thing.
L.S.T. and R.A.M. are one and the same thing.
G.M.T. + (.).R.A. + (+).C.P. = G.S.T. (R.A.M.G.) If the result is
more than 24 hours, subtract 24 hours.
G.S.T. - (.).R.A. - (+).C.P. = G.M.T.
G.S.T. - W.Lo. = L.S.T.
+ E.Lo.
L.S.T. + W.Lo. = G.S.T.
- E.Lo.
I can explain all these formulas very easily by the following
illustration which put in your Note-Book: (Note to Instructor: If
possible have copies of this illustration mimeographed and distributed
to each student.)
[Illustration: FIRST POINT OF ARIES.]
There is one term I have used which does not appear in the illustration.
It is the Earth's Central Progress ((+).C.P.). The astronomical day
based on the sun, is 24 hours long, as said before. The sidereal day,
however, is only 23h 56m 04s long. This is due to the fact that whereas
the earth is moving in its ecliptic track around the sun while revolving
on its own axis, the First Point of Aries is a fixed point and hence
never moves. The correction, then, for the difference in the length of
time between a sidereal day and a mean solar day is called the Earth's
Central Progress and, of course, has to be figured for all amounts of
time after mean noon at Greenwich, since the Sun's Right Ascension
tables in the Nautical Almanac are based on time at mean noon at
Greenwich.
Now you have a formula for practically all kinds of conversion except
for converting L.M.T. into L.S.T. You could do it by the formula
L.M.T. + W.Lo. = G.M.T. + (.).R.A. + (+).C.P. = G.S.T. - W.Lo. = L.S.T.
- E.Lo. + E.Lo.
But that involves too many operations.
A shorter way, though not so simple perhaps, is as follows: L.M.T. +
Reduction page 2 N.A. for time after local mean noon + (.).R.A. of
Greenwich mean noon ± Reduction page 2 N.A. for Lo. in T. (W+, E-) =
L.S.T.
Note to Instructor:
Explain this formula by turning to page 107 N.A. and work it out by the
formula L.M.T. + Lo. in T (W) = G.M.T. + (.).R.A. + (+).C.P. = G.S.T. -
Lo. in T (W) = L.S.T. Example:
L.M.T. 10h--40m--30s
Lo. in T 4 --56 W +
-------------
G.M.T. 15 --36 --30
(.).R.A. 5 --11 --10
(+).C.P. -- 2 --34
-------------
G.S.T. 20 --50 --14
Lo. W - 4 --56
-------------
L.S.T. 15h--54m--14s
Now Bowditch gets this L.S.T. in still another way. Turn to page 110,
Article 290. There the formula used is L.M.T. + (.).R.A. + (+).C.P. =
L.S.T, and in order to get the correct (.).R.A. and (+).C.P. the G.M.T.
has to be secured by the formula
L.M.T. + W.Lo. = G.M.T.
- E.Lo.
Let us work this same example in Bowditch by the other two methods.
First by the formula
L.M.T. + W.Lo. = G.M.T. + (.).R.A. + (+).C.P. = G.S.T. - W.Lo. = L.S.T.
- E.Lo. + E.Lo.
L.M.T. 22d-- 2h--00m--00s
+ W. Lo. 5 --25
----------------------
G.M.T. 22d-- 7h--25m--00s
(.).R.A. 1 --57 --59
(+).C.P. 1 --13
----------------------
G.S.T. 22d-- 9h--24m--12s
- W. Lo. 5 --25
----------------------
L.S.T. 22d-- 3h--59m--12s
The small difference between this answer and that of Bowditch's is that
the (.).R.A. for 1916 is slightly different from that of 1919. Bowditch
used the 1916 Almanac, whereas we are working from the 1919 Almanac. Now
turn to page 107 of the N.A. and let us work the same example in
Bowditch by the method used here:
L.M.T. 2h - 00m - 00s
Red. for 2h 0 - 20
(.).R.A. 0h 1 - 57 - 59
Red. Lo. 5h - 25m 0 - 53
---------------
L.S.T. 3h - 59m - 12s
The reason I am going so much into detail in explaining methods of
finding L.S.T. is because, by a very simple calculation which will be
explained later, we can get our latitude at night if we know the
altitude of Polaris (The North Star) and if we know the L.S.T. at the
time of observation. Some of you may think that the N.A. way is the
simplest. It is given in the N.A., and in an examination it would be
permissible for you to use the N.A. as a guide because, in an
examination, I propose to let you have at hand the same books you would
have in the chart house of a ship. On the other hand, the method given
in the N.A. is not as clear to my mind as the method which starts with
L.M.T., then finds with the Longitude the G.M.T. That gives you, roughly
speaking, the distance in time Greenwich is from the sun. Add to that
the sun's R.A. or the distance in time the sun is from the First Point
of Aries at Greenwich Mean Noon. Add to that the correction for the time
past noon. The result is G.S.T. Now all you have to do is to apply the
longitude correctly to find the L.S.T., just as when you have G.M.T. and
apply the longitude correctly you get L.M.T. That is a method which does
not seem easy to forget, for it depends more upon simple reasoning where
the others, for a beginner, depend more upon memory. However, any of the
three methods is correct and can be used by you. Perhaps the best way is
to work a problem by two of the three that seem easiest. In this way you
can check your figures. When I give you a problem that involves finding
the L.S.T. I do not care how you get the L.S.T. providing it is correct
when you get it.
Assign for Night Reading in Bowditch the following Arts.:
282-283-284-285. Also the following questions:
1. Given the G.M.T. and the longitude in T which is W, what is the
formula for L.S.T.?
2. Given the L.A.T. and longitude in T which is E, what is the formula
for G.S.T.?
3. Given the L.S.T. and longitude in T which is W. Required G.M.T. Etc.
FRIDAY LECTURE
THE NAUTICAL ALMANAC
For the last two days we have been discussing Time--sun time or solar
time and star time or sidereal time. Now let us examine the Nautical
Almanac to see how that time is registered and how we read the various
kinds of time for any instant of the day or night. Before starting in,
put a large cross on pages 4 and 5. For any calculations you are going
to make, these pages are unnecessary and they are liable to lead to
confusion.
Sun time of the mean sun at Greenwich is given for every minute of the
day in the year 1919 in the pages from 6 to 30. This is indicated by the
column to the left headed G.M.T. Turn to page 6 under Wednesday, Jan.
1st. You can see that the even hours are given from 0 to 24. Remember
that these are expressed in astronomical time, so that if you had Jan.
2nd--10 hours A.M., you would not look in the column under Jan. 2nd but
under the column for Jan. 1st, 22 hours, since 10 A.M. Jan. 2nd is 22
o'clock Jan. 1st, and no reading is used in this Almanac except a
reading expressed in astronomical time. Now at the bottom of the column
under Jan. 1st you see the letters H.D. That stands for "hourly
difference" and represents the amount to be added or subtracted for an
odd hour from the nearest even hour. In this instance it is .2. You note
that even hours 2, 4, 6, etc., are given. To find an odd hour during
this astronomical day, subtract .2 from the preceding even hour. For any
fraction of an hour you simply take the corresponding fraction of the
H.D. and subtract it from the preceding even hour. For instance, the
declination for Jan. 1st--12 hours would be 23° 1.8' or 23°--1'--48", 13
hours would be 23° 1.6' or 23°--1'--36", 12-1/2 hours would be 23° 1.7'
or 23°--1'--42", and 13-1/2 hours would be 23° 1.5' or 23°--1'--30".
Now to the right of the hours you note there is given the corresponding
amount of Declination and the Equation of Time. Before going further,
let us review a few facts about Declination. The declination of a
celestial body is its angular distance N or S of the celestial equator
or equinoctial. Now get clearly in your mind how we measure the angular
distance from the celestial equator of any heavenly body. It is measured
by the angle one of whose sides is an imaginary line drawn to the center
of the earth and the other of whose sides is an imaginary line passing
from the center of the earth into the celestial sphere through the
center of the heavenly body whose declination you desire. Now as you
stand on any part of the earth, you are standing at right angles to the
earth itself. Hence if this imaginary line passed through you it would
intersect the celestial sphere at your zenith, i.e., the point in the
celestial sphere which is directly above you. Now suppose you happen to
be standing at a certain point on the earth and suppose that point was
in 15° N latitude. And suppose at noon the center of the sun was
directly over you, i.e., the center of the sun and your zenith were one
and the same point. Then the declination of the sun at that moment would
be 15° N. In other words, your angular distance from the earth's equator
(which is another way of expressing your latitude) would be precisely
the same as the angular distance of the center of the sun from the
celestial equator. Suppose you were standing directly on the equator
and the center of the sun was directly over you, then the declination of
the sun would be 0°. Now if the axis of the earth were always
perpendicular to the plane of the sun's orbit, then the sun would always
be immediately over the equator and the sun's declination would always
be 0°. But you know that the axis of the earth is inclined to the plane
of the sun's orbit. As the earth, then, revolves around the sun, the
amount of the declination increases and then decreases according to the
location of the earth at any one time with relation to the sun. On March
21st and Sept. 23rd, 1919, the sun is directly over the equator and the
declination is 0°. From March 21st to June 21st the sun is coming North
and the declination is increasing until on June 21st--12 hours--it
reaches its highest declination. From then on the sun starts to travel
South, crosses the equator on Sept. 23d and reaches its highest
declination in South latitude on Dec. 22nd, when it starts to come North
again. This explains easily the length of days. When the sun is in North
latitude, it is nearer our zenith, i.e., higher in the heavens. It can,
therefore, be seen for a longer time during the 24 hours that it takes
the earth to revolve on its axis. Hence, when the sun reaches its
highest declination in North latitude--June 21st--i.e., when it is
farthest North from the equator and nearest our zenith (which is in 40°
N latitude) it can be seen for the longest length of time. In other
words, that day is the longest of the year. For the same reason, Dec.
22nd, when the sun reaches its highest declination in South latitude,
i.e., when it is farthest away to the South, is the shortest day in the
year for us; for on that day, the sun being farthest away from our
zenith and hence lowest down toward the horizon, can be seen for the
shortest length of time.
Put in your Note-Book:
North Declination is expressed +.
South Declination is expressed -.
Now turn to page 6 of the Nautical Almanac. You will see opposite Jan.
1st 0h, a declination of 23° 4.2'. Every calculation in this Almanac is
based on time at Greenwich, i.e., G.M.T. So at 0h Jan. 1st at
Greenwich--that is at noon--the Sun's declination is S 23° 4.2'.
You learned in the lecture the other day on solar time, that the
difference between mean time and apparent time was called the equation
of time. This equation of time, with the sign showing in which way it is
to be applied, is given for any minute of any day in the column marked
"Equation of Time." You will also notice that there is an H.D. for
equations of time just as there is for each declination, and this H.D.
should be used when finding the equation of time for an odd hour.
Put in your Note-Book:
1. The equation of time is to be applied as given in the Nautical
Almanac when changing Mean Time into Apparent Time.
2. When changing Apparent Time into Mean Time, reverse the sign as given
in the Nautical Almanac.
That is all there is to finding sun time, either mean or apparent, for
any instant of any day in the year 1919. Do not forget, however, that
all this data is based upon Greenwich Mean Time. To find Local Mean Time
you must apply the Longitude you are in. To find Local Apparent Time you
must first secure G.A.T. from G.M.T. and then apply the Longitude.
(Note to Instructor: Make the class work out conversions here if you
have time to do so and can finish the rest of the lecture by the end of
the period.)
So much for time by the sun. Now let us examine time by the
stars--sidereal time. Turn to pages 2-3. There you find the Right
Ascension of the Mean Sun at Greenwich Mean Noon for every day in the
year. You remember that, roughly speaking, the Sun's Right Ascension was
the distance in time the sun was from the First Point of Aries. So these
tables give that distance (expressed in time) for noon at Greenwich of
every day. For the correction to be applied for all time after noon at
Greenwich (i.e., (+).C.P.), use the table at the bottom of the page. For
instance, the (.).R.A. at Greenwich 9h 24m on Jan. 1st would be
(.).R.A. 18h--40m--21s
(+).C.P. 1 33
--------------
18h--41m--54s
Now we must go back to some of the formulas we learned when discussing
star time and apply them with the information we now have from the
Nautical Almanac. If the G.M.T. on April 20th is 4h--16m--30s, what is
the G.S.T. for the same moment? That is, when Greenwich is 4h--16m--30s
from the sun, how far is Greenwich from the First Point of Aries? You
remember the formula was G.S.T. = G.M.T. + (.).R.A. + (+).C.P.
G.M.T. 4h--16m--30s
(.).R.A. 1 --50 -- 6
(+).C.P. 0--42
--------------
G.S.T. 6h--07m--18s
Suppose you were in Lo. 74° W. What would be the R.A.M. (L.S.T.)? You
remember the formula for L.S.T. from G.S.T. was the same relatively as
L.M.T. from G.M.T., i.e.,
L.S.T. = G.S.T. - W. Lo.
+ E. Lo,
Here it would be
G.S.T. 6h--07m--18s
(74° W) - 4 --56 --00
---------------
L.S.T. 1h--11m--18s
Now these are not a collection of abstruse formulas that you are
learning just for the sake of practice. They are used every clear night
on board ship, or should be, and are just as vital to know as time by
the sun.
Suppose you are at sea in Lo. 70° W and your CT is October 20th
6h--4m--30s A.M., CC 2m--30s fast. You wish to get the R.A. of your M,
i.e., the L.S.T. How would you go about it? The first thing to do would
be to get your G.M.T. It is CT--CC.
20d--06h--04m--30s A.M.
--12
------------------
CT 19d--18h--04m--30s
CC --02 --30
------------------
G.M.T. 19d--18h--02m--00s
Then get your G.S.T.
Oct. 19d--18h--02m--00s
(.).R.A. 13 --47 --38.5
(+).C.P 2 --57.7
--------------------
19d--31h--52m--36.2s
--24
--------------------
G.S.T. 19d-- 7h--52m--36.2s
Then get your L.S.T.
G.S.T. 7h--52m--36.2s
W.Lo (--) 4 --40
--------------
L.S.T. 3h--12m--36.2s
The last fact to know at this time about the Almanac is found on pages
94-95. Here is given a list of the brighter stars with their positions
respectively in the heavens, i.e., their celestial longitude or R.A. on
page 94 and their celestial latitude or declination on page 95. These
stars have very little apparent motion. They are practically fixed.
Hence, their position in the heavens is almost the same from January to
December though, of course, their position with relation to you is
constantly changing, since you on the earth are constantly moving.
The relationship between these various kinds of time is clearly
expressed by the following diagram, which put in your Note Book:
[Illustration: Going with arrow, add. Going against arrow, subtract.]
Assign for reading in Bowditch, Articles
294-295-296-297-299-300-301-302-303-304-305-306-307.
If any time is left, have the class work out such examples as these:
1. G.M.T. June 20th, 1919, 5h--14m--39s. In Lo. 68° 49' W. Required
L.S.T., G.S.T., L.M.T., L.A.T.
2. L.M.T. Oct. 15th, 1919, 6h--30m--20s A.M. In Lo. 49° 35' 16" E.
Required L.S.T.
3. L.M.T. May 14th, 1919, 10h--15m.--20s A.M. Lo. 56° 21' 39" W.
Required L.A.T.
4. W.T. April 20th, 1919, 11h--30m--14s C-W 2h--14m--59s CC 4m--30s
slow. In Lo. 89° 48' 30" W. Required G.M.T., G.A.T., L.M.T., L.A.T.,
G.S.T., L.S.T.
5. What is Declination and R.A. on May 15th, 1919, of Polaris, Arcturus,
Capella, Regulus, Altair, Deneb, Vega, Aldebaran?
6. What is the sun's declination and R.A., Time at Greenwich, July 30th:
7h--14m--39s A.M.
4h--29m--14s A.M.
3h--04m--06s
11h--49m--59s
2h--14m--30s A.M.?
SATURDAY LECTURE
CORRECTION OF OBSERVED ALTITUDES
The true altitude of a heavenly body is the angular distance of its
center as measured from the center of the earth. The observed altitude
of a heavenly body as seen at sea by the sextant may be converted to the
true altitude by the application of the following four corrections: Dip,
Refraction, Parallax and Semi-diameter.
Dip of the horizon means an increase in the altitude caused by the
elevation of the eye above the level of the sea. The following diagram
illustrates this clearly:
[Illustration]
If the eye is on the level of the sea at A, it is in the plane of the
horizon CD, and the angles EAC and EAD are right angles or 90° each. If
the eye is elevated above A, say to B, it is plain that the angles EBC
and EBD are greater than right angles, or in other words, that the
observer sees more than a semi-circle of sky. Hence all measurements
made by the sextant are too large. In other words, the elevation of the
eye makes the angle too great and therefore the correction for dip is
always subtracted.
Refraction is a curving of the rays of light caused by their entering
the earth's atmosphere, which is a denser medium than the very light
ether of the outer sky. The effect of refraction is seen when an oar is
thrust into the water and looks as if it were bent. Refraction always
causes a celestial object to appear higher than it really is. This
refraction is greatest at the horizon and diminishes toward the zenith,
where it disappears. Table 20A in Bowditch gives the correction for mean
refraction. It is always subtracted from the altitude. In the higher
altitudes, select the correction for the nearest degree.
You should avoid taking low altitudes (15° or less) when the atmosphere
is not perfectly clear. Haziness increases refraction.
Parallax is simply the difference in angular altitude of a heavenly body
as measured from the center of the earth and as measured from the
corresponding point on the surface of the earth. Parallax is greatest
when the body is in the horizon, and disappears when it is at the
zenith.
[Illustration]
When the angular altitude of the sun in this diagram is 0, the parallax
ABC is greatest. When the altitude is highest there is no parallax. The
sun is so far away that its parallax never exceeds 9". The stars have
practically none at all from the earth's surface. Parallax is always to
be added in the case of the sun.
The semi-diameter of a heavenly body is half the angle subtended by the
diameter of the visible disk at the eye of the observer. For the same
body, the SD varies with the distance. Thus, the difference of the sun's
SD at different times of the year is due to the change of the earth's
distance from the sun.
[Illustration]
The SD is to be added to the observed altitude in case the lower limb is
brought in contact with the horizon, and subtracted if the upper limb is
used. Probably most of the sights you take will be of the sun's lower
limb, i.e., when the lower limb is brought in contact with the horizon,
so all you need to remember is that in that event the SD is additive.
Now at first we will correct altitudes by applying each correction
separately, but as soon as you get the idea, there is a short way to
apply all four corrections at once. This is done in Table 46. However,
disregard that for the moment. Put this in your Note-Book:
Dip is -. Table 14 Bowditch
Refraction is -. " 20 A Bowditch
Parallax is +. " 16 Bowditch
S.D. is +. Nautical Almanac
Observed altitude of Sun's lower limb is expressed (_).
True altitude is expressed -(-)-.
Remember that before an observation is at all accurate, it must be
corrected to make it a true altitude. Remember also that the IE must be
applied, in addition to these other corrections, in order to make the
observed altitude a -(-)- altitude. So there are really five corrections
to make instead of four, providing, of course, your sextant has an IE.
Examples:
1. June 20th, 1919, observed altitude of (_) 69° 25' 30". IE + 2' 30".
HE 16 ft. Required -(-)-.
2. April 15th, 1919, observed altitude of (_) 58° 29' 40". IE - 2' 30".
HE 18 ft. Required -(-)-.
3. March 4th, 1919, observed altitude of (_) 44° 44' 10". IE - 4' 20".
HE 20 ft. Required -(-)-.
Etc.
WEEK IV--NAVIGATION
TUESDAY LECTURE
THE LINE OF POSITION
It is practically impossible to fix your position exactly by one
observation of any celestial body. The most you can expect from one
sight is to fix your line of position, i.e., the line somewhere along
which you are. If, for instance, you can get a sight by sextant of the
sun, you may be able to work out from this sight a very accurate
calculation of what your latitude is. Say it is 50° N. You are
practically certain, then, that you are somewhere in latitude 50° N, but
just where you are you cannot tell until you get another sight for your
longitude. Similarly, you may be able to fix your longitude, but not be
able to fix your latitude until another sight is made. Celestial
Navigation, then, reduces itself to securing lines of position and by
manipulating these lines of position in a way to be described later, so
that they intersect. If, for instance, you know you are on one line
running North and South and on another line running East and West, the
only spot where you _can_ be on _both_ lines is where they intersect.
This diagram will make that clear:
[Illustration]
[Illustration]
Just what a line of position is will now be explained. Wherever the sun
is, it must be perpendicularly above the same spot on the surface of the
earth marked in the accompanying diagram by S and suppose a circle be
drawn around this spot as ABCDE. Then if a man at A takes an altitude,
he will get precisely the same one as men at B, C, D, and E, because
they are all at equal distances from the sun, and hence on the
circumference of a circle whose center is S. Conversely, if several
observers situated at different parts of the earth's surface take
simultaneous altitudes, and these altitudes are all the same, then the
observers must all be on the circumference of a circle and _only one_
circle. If they are not on that circle, the altitude they take will be
greater or less than the one in question.
[Illustration]
Now such a circle on the surface of the earth would be very large--so
large that a small arc of its circumference, say 25 or 30 miles, would
be practically a straight line.
Suppose S to be the point over which the sun is vertical and GF part of
the circumference of a circle drawn around the point. Suppose you were
at B and from an altitude of the sun, taken by sextant, you worked out
your position. You would find yourself on a little arc ABC which, for
all purposes in Navigation, is a straight line at right angles to the
true bearing of the sun from the point S. You can readily see this from
the above diagram. Suppose your observer is at H. His line is GHI, which
is again a straight line at right angles to the true bearing of the sun.
He is not certain he is at H. He may be at G or I. He knows, however, he
is somewhere on the line GHI, though where he is on that line he cannot
tell exactly. That line GHI or ABC or DEF is the line of position and
such a line is called a Sumner Line, after Capt. Thomas Sumner, who
explained the theory some 45 years ago. Put in your Note-Book:
Any person taking an altitude of a celestial body must be, for all
practical purposes, on a straight line which is at right angles to the
true bearing of the body observed.
It should be perfectly clear now that if the sun bears due North or
South of the observer, i.e., if the sun is on the observer's meridian,
the resulting line of position _must_ run due East and West. In other
words it is a parallel of latitude. And that explains why a noon
observation is the best of the day for getting your latitude accurately.
Again, if the sun bears due East or West the line of position must bear
due North and South. And that explains why a morning or afternoon
sight--about 8-9 A.M. or 3-4 P.M., if the sun bears either East or West
respectively, is the best time for determining your North and South
line, or longitude.
Now suppose you take an observation at 8 A.M. and you are not sure of
your D.R. latitude. Your 8 A.M. position when the sun was nearly due
East, will give, you an almost accurate North and South line and
longitude. Suppose that from 8 A.M. to noon you sailed NE 60 miles.
Suppose at noon you get another observation. That will give you an East
and West line, for then the sun bears true North and South. An East and
West line is your correct latitude. Now you have an 8 A.M. observation
which is nearly correct for longitude and a noon position which is
correct for latitude. How can you combine the two so as to get
accurately both your latitude and longitude? Put in your Note-Book:
Through the 8 A.M. position, draw a line on the chart at right angles to
the sun's true bearing. Suppose the sun bore true E 1/2 S. Then your
line of position would run N 1/2 E. Mark it 1st Position Line.
[Illustration]
Now draw a line running due East and West at right angles to the N-S
noon bearing of the sun and mark this line Second Position Line. Advance
your First Position Line the true course and distance sailed from 8 A.M.
to noon, and through the extremity draw a third line exactly parallel to
the first line of position. Where a third line (the First Position Line
advanced) intersects the Second Position Line, will be your position at
noon. It cannot be any other if your calculations are correct. You knew
you were somewhere on your 8 A.M. line, you know you are somewhere on
your noon line, and the only spot where you can be on both at once is
the point where they intersect. You don't necessarily have to wait until
noon to work two lines. You can do it at any time if a sufficient
interval of time between sights is allowed. The whole matter simply
resolves itself into getting your two lines of position, having them
intersect and taking the point of intersection as the position of your
ship.
There is one other way to get two lines to intersect and it is one of
the best of all for fixing your position accurately. It is by getting
lines of position by observation of two stars. If, for instance, you can
get two stars, one East and the other West of you, you can take
observations of both so closely together as to be practically
simultaneous. Then your Easterly star would give you a line like AA' and
the westerly star the line BB' and you would be at the intersection S.
[Illustration]
Assign for reading: Articles in Bowditch 321-322-323-324. Spend the rest
of the period in getting times from the N. A., getting true altitudes
from observed altitudes, working examples in Mercator sailing, etc.
WEDNESDAY LECTURE
LATITUDE BY MERIDIAN ALTITUDE
A meridian altitude is an altitude taken when the sun or other celestial
body observed bears true South or North of the observer or directly
overhead. In other words, when the celestial body is on your meridian
and you take an altitude of the body by sextant at that instant, the
altitude you get is called a meridian altitude. In the case of the sun,
such a meridian altitude is at apparent noon. Now latitude is always
secured most accurately at noon by means of your meridian altitude. The
reason for this was explained in yesterday's lecture. The general
formula for latitude by meridian altitude is (Put in your Note-Book):
Latitude by meridian altitude = Zenith Distance (ZD) ± Declination
(Dec).
Zenith distance is the distance in degrees, minutes and seconds from
your zenith to the center of the observed body. For simplicity's sake,
we will consider the sun only as the observed body. Then the zenith
distance is the distance from your zenith to the center of the sun. Now
suppose that you and the sun are both North of the equator and you are
North of the sun. If you can determine exactly how far North you are of
the sun and how far North the sun is of the equator, you will, by adding
these two measurements together, know how far North of the equator you
are, i.e., your latitude. As already explained, the declination of the
sun is its distance in degrees, minutes and seconds from the equator and
the exact amount of declination is, of course, corrected to the proper
G.M.T. Your zenith distance is the distance in the celestial sphere you
are from the sun. You know that it is 90° from your zenith to the
horizon. Your zenith distance, therefore, is the difference between the
true meridian altitude of the sun, obtained by your sextant, and 90°.
Hence, having secured the true meridian altitude of the sun, you have
only to subtract it from 90° to find your zenith distance, i.e., how far
you are from the sun. This diagram will make the whole matter clear:
[Illustration:
A
|
| B (Sun)
| /
| /
| /
| /
|<-/-
| / \ 90°
|/ |
+------v---------------------- C
A = Zenith, B = Sun, C = Horizon.]
The arc ABC measures 90°. That is the distance from your zenith to the
horizon. Now if BC is the true meridian altitude of the sun at noon,
90°-BC or AB is your zenith distance. If BC measures by sextant 60°, AB
measures 90°-60° or 30°. This 30° is your Zenith Distance. Now suppose
that from the Nautical Almanac we find that the G.M.T. corresponding to
the time at which we measured the meridian altitude of the sun shows the
sun's declination to be 10° N. Well, if you are 30° North of the sun,
and the sun is 10° North of the equator, you must be 40° North of the
equator or in latitude 40° N. For that is all latitude is, namely, the
distance in degrees, minutes and seconds you are due North or South of
the equator. That is the first and simplest case.
Another case is when you are somewhere in North latitude and the sun's
declination is South. Then the situation would, roughly, look like this:
[Illustration: BC = Altitude of the sun, AB = Zenith Distance and DB =
Sun's Declination.]
In this case, your distance North of the equator AD would be your zenith
distance AB minus the sun's declination DB. This diagram is not strictly
correct, for the observer's position on the earth 0 appears to be South
of the equator instead of North of the equator. That is because the
diagram is on a flat piece of paper instead of on a globe. So far as
illustrating the Zenith Distance minus the Declination, however, the
diagram is correct. The last case is where you are, say, 10° N of the
sun (your zenith Distance is 10°) and the sun is in 20° S declination.
In that case you would have to subtract your zenith distance from the
sun's declination to get your latitude, for the sun's latitude (its
declination) is greater than yours.
Now from these three cases we deduce the following directions, which put
in your Note-Book:
Begin to measure the altitude of the sun shortly before noon. By
bringing its image down to the horizon, you can detect when its altitude
stops increasing and starts to decrease. At that instant the sun is on
your meridian, it is noon at the ship, and the angle you read from your
sextant is the meridian altitude of the sun. To work out your latitude,
name the meridian altitude S if the sun is south of you and N if north
of you.
Correct the observed altitude to a true altitude by Table 46. If the
altitude is S, the Zenith Distance is N or vice versa. (Note to
Instructor: If the sun is South of you, you are North of the sun and
vice versa.)
Correct the declination for the proper G.M.T. as shown by chronometer
(corrected). If zenith distance and declination are both North or both
South, add them and the sum will be the latitude, N or S as indicated.
If one is N, and the other S, subtract the less from the greater and the
result will be the latitude in, named N or S after the greater. Example:
At sea June 15th, observed altitude of (_) 71° 15´ S, IE--47´, HE 25 ft.
CT 3h--34m--15s P.M. Required latitude of ship.
(_) 71° 15' 00 S IE -- 47'
Corr. -- 36 24 HE + 10 36
------------------- ---------
-(-)- 70° 38' 36" S Corr. -- 36' 24"
-- 90 00 00
-------------------
ZD 19° 21' 24" N
Dec. 23 17 15 N (G.M.T. June 15--3h 34m 15s)
-------------------
Lat. 42° 38' 39" N
-------------------
Assign for Night Work or to be worked in class room such examples as the
following:
1. June 1st, 1919. (_) 33° 50' 00" S. G.M.T. 8h 55m 44s. HE 20 ft. IE +
4' 3". Required latitude in at noon.
2. April 2nd, 1919. (_) 12° 44' 30" N. CT was 2d 5h 14m 39s A.M., which
was 1m 40s slow on March 1st (same CT) and 4m 29s fast on March 15th
(same CT). IE -- 2' 20". HE 22 ft. Required latitude in at noon.
Assign for Night Work reading also, the following Articles in Bowditch:
344 and 223.
THURSDAY LECTURE
AZIMUTHS OF THE SUN
This is a peculiar word to spell and pronounce but its definition is
really very simple. Put in your Note-Book:
The azimuth of a heavenly body is the angle at the zenith of the
observer formed by the observer's meridian and a line drawn to the
center of the body observed. Azimuths are named from the latitude in and
toward the E in the A.M. and from the latitude in and toward the W in
the P.M.
All this definition means is that, no matter where you are in N
latitude, for instance, if you face N, the azimuth of the sun will be
the true bearing of the sun from you. The same holds true for moon, star
or planet, but in this lecture we will say nothing of the star azimuths
for, in some other respects, they are found somewhat differently from
the sun azimuths. Put this in your Note-Book:
To find an azimuth of the sun: Note the time of taking the azimuth by
chronometer. Apply chronometer correction, if any, to get the G.M.T.
Convert G.M.T. into G.A.T. by applying the equation of time. Convert
G.A.T. into L.A.T. by applying the longitude in time. The result is
L.A.T. or S.H.A. With the correct L.A.T., latitude and declination,
enter the azimuth tables to get the sun's true bearing, i.e., its
azimuth. Example:
March 15th, 1919. CT 10h -- 4m -- 32s. D.R. latitude 40° 10' N,
longitude 74° W. Find the TZ.
G.M.T. 10h--04m--32s
Eq. T. --09 --10
G.A.T. 9h--55m--22s
G.A.T. 9h--55m--22s
Lo. in T. 4 --56 --00 (W--)
L.A.T. 4h--59m--22s
Latitude and Declination opp. name.
TZ = N 101° 30'W
We will take up later a further use of azimuths to find the error of
your compass. Right now all you have to keep in mind is what an azimuth
is and how you apply the formulas already given you to get the
information necessary to enter the Azimuth Tables for the sun's true
bearing at any time of the astronomical day when the sun can be seen. In
consulting these tables it must be remembered that if your L.A.T. or
S.H.A. is, astronomically, 20h (A.M.), you must subtract 12 hours in
order to bring the time within the scope of these tables which are
arranged from apparent six o'clock A.M. to noon and from apparent noon
to 6 P.M. respectively.
We are taking up sun azimuths today in order to get a thorough
understanding of them before beginning a discussion of the Marc St.
Hilaire Method which we will have tomorrow. You must get clearly in your
minds just what a line of position is and how it is found. Yesterday I
tried to explain what a line of position was, i.e., a line at right
angles to the sun's or other celestial body's true bearing--in other
words, a line at right angles to the sun's or other celestial body's
azimuth. Today I tried to show you how to find your azimuth from the
azimuth tables for any hour of the day. Tomorrow we will start to use
azimuths in working out sights for lines of position by the Marc St.
Hilaire Method.
Note to Instructor: Spend the rest of the time in finding sun azimuths
in the tables by working out such examples as these:
1. April 29th, 1919. D.R. latitude 40° 40' N, Longitude 74° 55' 14" W.
CT 10h--14m--24s. CC 4m--30s slow. Find TZ.
2. May 15th, 1919. D.R. latitude 19° 20' S, Longitude 40° 15' 44" E. CT
10h--44m--55s A.M. CC 3m--10s fast. Find TZ.
Note to Instructor:
If possible, give more examples to find TZ and also some examples on
latitude by meridian altitude.
Assign for Night Work reading the following Articles in Bowditch:
371-372-373-374-375. Also, examples to find TZ.
FRIDAY LECTURE
MARC ST. HILAIRE METHOD BY A SUN SIGHT
You have learned how to get your latitude by an observation at noon. By
the Marc St. Hilaire Method, which we are to take up today, you will
learn how to get a line of position, at any hour of the day. By having
this line of position intersect your parallel of latitude, you will be
able to establish the position of your ship, both as to its latitude and
longitude.
Now you have already learned that in order to get your latitude
accurately, you must wait until the sun is on your meridian, i.e., bears
due North or South of you, and then you apply a certain formula to get
your latitude. When the sun is on or near the prime vertical (i.e., due
East or West) you might apply another set of rules, which you have not
yet learned, to get your longitude. By the Marc St. Hilaire method, the
same set of rules apply for getting a line of position at any time of
the day, no matter what the position of the observed body in the heavens
may be. Just one condition is necessary, and this condition is necessary
in all calculations of this character, i.e., an accurate measurement of
the observed body's altitude is essential.
What we do in working out the Marc St. Hilaire method, is to assume our
Dead Reckoning position to be correct. With this D. R. position as a
basis, we compute an altitude of the body observed. Now this altitude
would be correct if our D. R. position were correct and vice versa. At
the same time we measure by sextant the altitude of the celestial body
observed, say, the sun. If the computed altitude and the actual observed
altitude coincide, the D. R. position is correct. If they do not, the
computed altitude must be corrected and the D. R. position corrected to
coincide with the observed altitude. Just how this is done will be
explained in a moment. Put in your Note-Book:
_Formula for obtaining Line of Position by M. St. H. Method._
I. Three quantities must be known either from observation or from Dead
Reckoning.
1. The S. H. A., marked "t."
Note: The method for finding S. H. A. (t) differs when the sun or
star is used as follows:
(a) For the Sun:
Get G.M.T. from the corrected chronometer time. Apply
the equation of time to find the G.A.T. Apply the D.R. Lo.
(-W)
(+E) and the result is L.A.T. or S.H.A. as required.
(b) For a Star:
(Note to pupils: Leave this blank to be filled in when we
take up stars in more detail.)
2. The Latitude, marked "L."
3. The Declination of the observed body, marked "D."
II. Add together the log haversine of the S.H.A. (Table 45), the log
cosine of the Lat. (Table 44), and the log cosine of the Dec. (Table 44)
and call the sum S. S is a log haversine and must always be less than
10. If greater than 10, subtract 10 or 20 to bring it less than 10.
III. With the log haversine S enter table 45 in the adjacent parallel
column, take out the corresponding Natural Haversine, which mark N_{S}.
IV. Find the algebraic difference of the Latitude and Declination, and
from Table 45 take out the Natural Haversine of this algebraic
difference angle. Mark it N_{D±L}
V. Add the N_{S} to the N_{D±L}, and the result will be the Natural
Haversine of the calculated zenith distance. Formula N_{ZD} = N_{S} +
N_{D±L}
VI. Subtract this calculated zenith distance from 90° to get the
calculated altitude.
VII. Find the difference between the calculated altitude and the true
altitude and call it the altitude difference.
VIII. In your Azimuth Table, find the azimuth for the proper "t," L and
D.
IX. Lay off the altitude difference along the azimuth either away from
or toward the body observed, according as to whether the true altitude,
observed by sextant, is less or greater than the calculated altitude.
[Illustration]
X. Through the point thus reached, draw a line at right angles to the
azimuth. This line will be your Line of Position, and the point thus
reached, which may be read from the chart or obtained by use of Table 2
from the D. R. Position, is the nearest to the actual position of the
observer which you can obtain by the use of any method from one sight
only.
Example:
At sea, May 18th, 1919, A.M. (_) 29° 41' 00". D.R. Latitude 41° 30' N,
Longitude 33° 38' 45" W. WT 7h 20m 45s A.M. C-W 2h 17m 06s CC + 4m 59s.
IE--30". HE 23 ft. Required Line of Position and most probable position
of ship.
WT 18d -- 7h -- 20m -- 45s A.M.
-- 12
------------------------
WT 17d -- 19h -- 20m -- 45s
C-W 2 -- 17 -- 06 Corr. + 9' 34"
------------------------ IE -- 30
CT 17d -- 21h -- 37m -- 51s ------------
CC + 4 -- 59 + 9' 04"
------------------------
G.M.T. 17d -- 21h -- 42m -- 50s (_) 29° 41' 00"
Eq. T. + 3 -- 47 + 9 04
------------------------ ------------
G.A.T. 17d -- 21h -- 46m -- 37s -(-)- 29° 50' 04"
Lo. in T 2 -- 14 -- 35 (W--)
------------------------ log hav 9.48368
L.A.T.(t) 17d -- 19h -- 32m -- 02s log cos 9.87446
Lat. 41° 30' N log cos 9.97473
Dec. 19° 21' 25" N --------
log hav S 9.33287
N s .21521
L - D 22° 08' 35" N D ± L .03687
--------
Calc. ZD 60° 16' 30" N ZD .25208
-- 90° 00 00
------------- TZ found from table to be
Cal. Alt. 29° 43' 30" N 90° E.
-(-)- 29° 50' 04"
-------------
Alt. Diff 6' 34" Toward.
_ Course. Dist. Diff. Lat. Dep. Diff. Lo._
90° 6' 34" 0 6.5 8.6
D.R. Lat. 41° 30' N D.R. Lo. 33° 38' 45" W
Diff. Lat. -- Diff. Lo. 8 36 E
---------- -------------
Most probable fix Lat. 41° 30' N Lo. 33° 30' 09" W
As azimuth is N 90° E, Line of Position runs due N & S (360°) through
Lat. 41° 30' N. Lo. 33° 30' 09" W.
Assign for work in class and for Night Work examples such as the
following:
1. July 11th, 1919. (_) 45° 35' 30", Lat. by D. R. 50° 00' N, Lo. 40° 04'
W. HE 15 ft. IE--4'. CT (corrected) 5h. 38m 00s P.M. Required Line
of Position by Marc St. Hilaire Method and most probable fix of ship.
2. May 16th, 1919, A.M. (_) 64° 01' 15", D. R. Lat. 39° 45' N, Lo.
60° 29' W. HE 36 ft. IE + 2' 30". CT 2h 44m 19s. Required Line of
Position by Marc St. Hilaire Method and most probable fix of ship.
Etc.
SATURDAY LECTURE
EXAMPLES ON MARC ST. HILAIRE METHOD BY A SUN SIGHT
1. Nov. 1st, 1919. A.M. at ship. WT 9h 40m 15s. C--W 4h 54m 00s.
D. R. Lat. 40° 50' N, Lo. 73° 50' W. (_) 27° 59'. HE 14 ft. Required Line
of Position by Marc St. Hilaire Method and most probable position of ship.
2. May 30th, 1919. P.M. at ship. D. R. Lat. 38º 14' 33" N, Lo. 15° 38'
49' W. The mean of a series of observations of (_) was 39° 05' 40°. IE--01'
00". HE 27 ft. WT 3h 4m 49s. C--W 1h 39m 55s. C.C. fast, 01m 52s.
Required Line of Position by Marc St. Hilaire Method, and most probable
position of ship.
3. Oct. 21st, 1919, A.M. D. R. Lat. 40° 12' 38" N, Lo. 69° 48' 54" W.
The mean of a series of observations of (_) was 19° 21' 20". IE + 02' 10".
HE 26 ft. WT 7h 58m 49s. C--W 4h 51m 45s. C. slow, 03m 03s. Required Line
of Position by Marc St. Hilaire Method and most probable position of ship.
4. June 1st, 1919, P.M. at ship. Lat. D. R. 35° 26' 15" S, Lo. 10° 19'
50" W. W.T. 3h 30m 00s. C--W 0h 20m 38s. CC 1m 16s slow. (_) 16°
15' 40". IE + 2' 10". HE 26 ft. Required Line of Position and most probable
fix of ship.
5. Jan. 5th, 1919. A.M. D. R. Lat. 36° 29' 38" N, Lo. 51° 07' 44" W.
The mean of a series of observations of (_) was 23° 17' 20". IE + 01' 50".
HE 19 ft. WT 7h 11m 37s. C--W 5h 59m 49s. C. slow 58s. Required Line of
Position and most probable fix of ship.
WEEK V--NAVIGATION
TUESDAY LECTURE
A SHORT TALK ON THE PLANETS AND STARS IDENTIFICATION OF STARS
_1. The Planets_
You should acquaint yourself with the names of the planets and their
symbols. These can be found opposite Page 1 in the Nautical Almanac. All
the planets differ greatly in size and in physical condition. Three of
them--Mercury, Venus and Mars--are somewhat like the earth in size and
in general characteristics. So far as we know, they are solid, cool
bodies similar to the earth and like the earth, surrounded by
atmospheres of cool vapors. The outer planets on the other hand, i.e.,
Jupiter, Saturn, Uranus, and Neptune, are tremendously large--many times
the size of the earth, and resemble the sun more than the earth in
their physical appearance and condition. They are globes of gases and
vapors so hot as to be practically self luminous. They probably contain
a small solid nucleus, but the greater part of them is nothing but an
immense gaseous atmosphere filled with minute liquid particles and
heated to an almost unbelievably high temperature.
Of the actual surface conditions on Venus and Mercury, little is
definitely known. Mercury is a very difficult object to observe on
account of its proximity to the sun. It is never visible at night; it
must be examined in the twilight just before sunrise or just after
sunset, or in the full daylight. In either case the glare of the sun
renders the planet indistinct, and the heat of the sun disturbs our
atmosphere so as to make accurate visibility almost impossible. The
surface of Mercury is probably rough and irregular and much like the
moon. Like the moon, too, it has practically no atmosphere. Mercury
rotates on its axis once in 88 days. Its day and year are of the same
length. Thus the planet always presents the same face toward the sun and
on that side there is perpetual day while on the other side is
night--unbroken and cold beyond all imagination.
Venus resembles the earth more nearly than any other heavenly body. Its
diameter is within 120 miles of the earth's diameter. The exasperating
fact about Venus, however, is that it is shrouded in deep banks of
clouds and vapors which make it impossible for us to secure any definite
facts about it. The atmosphere about Venus is so dense that sunlight is
reflected from the upper surface of the clouds around the planet and so
reaches our telescopes without having penetrated to the surface at all.
From time to time markings have been discovered that at first seemed
real but whether they are just clouds or tops of mountains has never
really been established.
Of all the planets, we know more about Mars than any other. And yet
practically nothing is actually known in regard to conditions on the
surface of this planet. We do know, however, that Mars more nearly
resembles a miniature of our earth than any other celestial body. The
diameter of Mars is 4,210 miles--almost exactly half the earth's
diameter. The surface area of Mars is just about equal to the total area
of dry land on the earth. Like the earth, Mars rotates about an axis
inclined to the plane of its orbit, and the length of a Martian day is
very nearly equal to our own. The latest determinations give the length
of a Martian solar day as 24h 39m 35s. Fortunately for us, Mars is
surrounded by a very light and transparent atmosphere through which we
are able to discover with our telescopes, many permanent facts.
The most noticeable of these are the dazzling white "polar caps" first
identified by Sir William Herschel in 1784. During the long winter in
the northern hemisphere, the cap at the North pole steadily increases in
size, only to diminish during the next summer under the hot rays of the
sun. These discoveries establish without doubt the presence of vapors in
the Martian atmosphere which precipitate with cold and evaporate with
heat. The polar caps, then, are some form of snow and ice or possible
hoar frost. Outside the polar caps the surface of Mars is rough, uneven
and of different colors. Some of the darker markings appear to be long,
straight hollows. They are the so-called "canals" discovered by
Schiaparelli in 1877. The term "canal" is an unfortunate one. The word
implies the existence of water and the presence of beings of sufficient
intelligence and mechanical ability to construct elaborate works.
Flammarion in France and Lowell in the United States claim the word is
correctly used, i.e., that these markings are really canals and that
Mars is actually inhabited. The consensus of opinion among the most
celebrated astronomers is contrary to this view. Most astronomers agree
that these canals may not exist as drawn--that they are to great extent
due to defective vision. There is no conclusive proof of man-made work
on Mars, nor of the existence of conscious life of any kind. It may be
there but conclusive proof of it is still lacking.
_2. The Stars_
The planets are often called wanderers in the sky because of their ever
changing position. Sharply distinguished from them, therefore, are the
"fixed" stars. These appear as mere points of light and always maintain
the same relative positions in the heavens. Thousands of years ago the
"Great Dipper" hung in the northern sky just as it will hang tonight and
as it will hang for thousands of years to come. Yet these bodies are not
actually fixed in space. In reality they are all in rapid motion, some
moving one way and some another. It is their tremendous distance from us
that makes this motion inappreciable. The sun seems far away from us,
but the nearest star is 200,000 times as far away from us as is the sun.
Expressed in miles, the figure is so huge as to be incomprehensible. A
special unit has, therefore, been invented--a unit represented by the
distance traversed by light in one year. In one second, light travels
over 186,000 miles. In 8-1/3 minutes, light reaches us from the sun and,
in doing so, covers the distance that would take the Vaterland over four
centuries to travel. Yet the nearest star is over four "light years"
distant--it is so far away that it requires over four years for its
light to reach us. When you look at the stars tonight you see them, not
as they are, but as they were, even centuries ago. Polaris, for
instance, is distant some sixty "light years." Had it disappeared from
the heavens at the time Lee surrendered to Grant, we should still be
seeing it and entirely unaware of its disappearance.
Now each star in the heavens is in reality a sun, i.e., a vast globe of
gas and vapor, intensely hot and in a continuous state of violent
agitation, radiating forth heat and light, every pulsation of which is
felt throughout the universe. So closely indeed do many of the stars
resemble the sun, that the light which they emit cannot be distinguished
from sunlight. Some of them are larger and hotter than the sun--some
smaller and cooler. Yet the sun we see can be regarded as a typical star
and from our knowledge of it we can form a fairly correct idea of the
nature and characteristics of these other stars.
Anyone knows that the stars vary in brightness. Some of this variation
is due partly to actual differences among the stars themselves and
partly to varying distances. If all the stars were alike, then those
which were farthest away would be faintest and we could judge a star's
distance by its brilliancy. This is not the case, however. Some of the
more brilliant stars are far more distant than some of the fainter ones.
There are stars near and remote and an apparently faint star may in
reality be larger and more brilliant than a star of the first magnitude.
Vega, for instance, is infinitely farther away from us than the sun, yet
its brightness is more than 50 times that of the sun. Polaris, still
farther away, has 100 times the light and heat of the sun. In fact the
sun, considered as a star, is relatively small and feeble.
_3. Identification of Stars_
Only the brighter stars can be used in navigation. So much light is lost
in the double reflection in the mirrors of the sextant, that stars
fainter than the third magnitude can seldom be observed. This reduces
the number of stars available for navigation to within very narrow
limits, for there are only 142 stars all told which are of the third
magnitude or brighter. The Nautical Almanac gives a list of some 150
stars which may be used, but as a matter of fact, the list might be
reduced to some 50 or 60 without serious detriment to the practical
navigator. About 30 of these are of the second magnitude or greater and
hence easily found. It is not difficult to learn to know 30 or 40 of the
brighter stars, so that they can be recognized at any time. To aid in
locating the stars, many different star charts and atlases have been
published, but most of them are so elaborate that they confuse as much
as they help. The simpler the chart, the fewer stars it pretends to
locate, the better for practical purposes. Also, all charts are of
necessity printed on a flat surface and such a surface can never
represent in their true values, all parts of a sphere. A chart,
therefore, which covers a large part of the heavens, is bound to give a
distorted idea of distances or directions in some part of the sky and
must be used with caution.
There are a few stars which form striking figures of one kind or
another. These can always be easily located and form a starting point,
so to speak, from which to begin a search for other stars. Of these
groups the Great Dipper is the most prominent in the northern sky and
beginning with this the other constellations can be located one by one.
When the groups or constellations are not known, then any individual
star can be readily found by means of its Right Ascension, and
Declination. As you have already learned, Declination is equivalent to
latitude on the earth and Right Ascension practically equivalent to
longitude on the earth, except that whereas longitude on the earth is
measured E. and W. from Greenwich, Right Ascension is measured to the
east all the way around the sky, from the First Point of Aries. With
this in mind, you can easily see that if a star's R.A. is less than
yours, i.e., less than L.S.T. or the R.A. of your Meridian, the star is
not as far eastward in the heavens, as is your zenith. In other words it
is to the west of you. And vice versa, if the Star's R.A. is greater
than yours, the Star is more to the eastward than you and hence to the
east of you. Moreover, as R.A. is reckoned all around a circle and in
hours, each hour's difference between the Star's R.A. and yours is 1/24
of 360° or 15°. Hence if a star's R.A. is, for instance, 2 hours greater
than yours, the star will be found to the east of your meridian and
approximately 30° from your meridian, providing the star is in
approximately the same vertical east and west plane as is your zenith.
When the general east or west direction of any star has been determined,
its north or south position can at once be found from its declination.
If you are in Latitude 40° N. your celestial horizon to the South will
be 90° from 40° N. or 50° S. and to the North it will be 90° + 40° N. =
130° or 40° N. (below the N. pole). The general position of the equator
in the sky is always readily found according to the latitude you are in.
If you are in 40° N. latitude, the celestial equator would intersect the
celestial sphere at a point 40° South of you. Knowing this, the angular
distance of a star North or South of the equator (which is its
declination) should be easily found. Remember, however, that the equator
in the sky is a curved line and hence a star in the East or West which
looks to be slightly North of you may actually be South of you.
Put in your Note-Book:
If the star is west of you its R.A. is less than yours. If east of you,
its R.A. is greater than yours. Star will be found approximately 15° to
east or west of you for each hourly difference between the star's R.A.
and your R.A. (L.S.T.).
Having established the star's general east and west direction, its north
and south position can be found from its declination.
_4. Time of Meridian Passage of a Star_
It is often invaluable to know first, when a certain star will be on
your meridian or second, what star will be on your meridian at a certain
specified time. Here is the formula for each case, which put in your
Note Book:
1. To find when a certain star will cross your meridian, take from the
Nautical Almanac, the R.A. of the Mean Sun for Greenwich Mean Noon of
the proper astronomical day. Apply to it the correction for longitude in
time (West +, East -) as per Table at bottom of page 2, Nautical
Almanac, and the result will be the R.A. of the Mean Sun at local mean
noon, i.e., the distance in sidereal time the mean sun is from the First
Point of Aries when it is on your meridian. Subtract this from the
star's R.A., i.e., the distance in sidereal time the star is from the
First Point of Aries (adding 24 hours to the star's R.A., if necessary
to make the subtraction possible). The result will be the distance in
sidereal time the star is from your meridian i.e., the time interval
from local mean noon expressed in units of sidereal time. Convert this
sidereal time interval into a mean time interval by always subtracting
the reduction for the proper number of hours, minutes and seconds as
given in Table 8, Bowditch. The result will be the local mean time of
the star's meridian passage.
Example:
April 22nd, 1919, A.M. at ship. In Lo. 75° E. What is the local mean
time of the Star Etamin's meridian passage?
R.A.M.S. Gr. 21d--0h 1h--54m--02s
Red. for 75° E (--5h) --49.3
________________
R.A.M.S. local mean noon 1h--53m--12.7s
Star's R.A. 17h--54m--44s
-- 1 --53 --12.7
___________________
Sidereal interval from L.M. Noon 16h--01m--31.3s
Red. for Sid. Int. (Table 8) -- 2 --37.3
___________________
L.M.T. 21 d 15h--58m--54s
Hence, star will cross meridian at 3h--58m--54s A.M. April 22nd.
2. To find at any hour desired what star will cross your meridian, take
the R.A. of the Mean Sun for Greenwich Mean Noon of the proper
astronomical day. Apply to it the correction for longitude in time (West
+, East -) as per table at bottom of page 2, Nautical Almanac, and the
result will be the R.A. of the Mean Sun at local mean noon; i.e., the
distance in sidereal time the mean sun is from the First Point of Aries
when it is on your meridian. Suppose you wish to find the star at 10
P.M. Add 10 sidereal hours to the sun's R.A. just found. The result will
be the R.A. of your meridian at approximately 10 P.M.
Select in the table on p. 94 the R.A. of the star nearest in time to
your R.A. just secured. Subtract the R.A. of the Mean Sun at local mean
noon from the star's R.A. just found on p. 94 of the N.A. and the result
will be the exact distance in sidereal time the star you have just
identified is from your meridian, i.e., the time interval from local
mean noon expressed in units of sidereal time. Convert this sidereal
time interval into a mean time interval by always subtracting for the
proper number of hours, minutes and seconds as per Table 8, Bowditch.
You will then have secured the name of the star desired and the exact
local mean time of the star's meridian passage.
Example No. 2: At sea Dec. 14, 1919. Desired to get a star on my
meridian at 11 P.M. Lo. by D.R. 74° W.
(.).R.A.G.M.N. 17h--28m--26s
Corr. 74° W. (4th - 56m W + ) + 0 --48.6
___________________
(.).R.A. your M. 17h--29m--14.6s
+ 11
___________________
R.A.M. 28h--29m--14.6s
-- 24
___________________
4h--29m--14.6s
R.A. of Star Aldebaran 4h--31m--18.5s
Star R.A. 28h--31m--18.5
(.).R.A. your M. 17 -- 29--14.6
----------------
Sid. Int. from L.M. Noon 11h--02m--03.9s
Red for Sid. Int. (Table 8) -- 1 --48
----------------
L.M.T. 11h--00m--15.9s
Aldebaran, then, is the star and the exact L.M.T. of its meridian
passage will be 11h 00m 15.9s
Note: If the R.A.M. is more than 24 hours, deduct 24 hours. You will
know whether the star is North or South of you by its declination. If
you are in North latitude, the star will be south of you if its
declination is South or if its declination is North and less than your
latitude. If its declination and your latitude are both North and its
declination is greater, the star will be north of you. The same
principle applies if you are in South latitude.
Assign any of the following to be worked in the class room or at night:
1. At sea, November 1st, 1919. In Latitude 40° N., Longitude 74° W.
WT 8h 30m P.M. Observed unknown star about 80° east of my meridian and
25° south of me. What was the star?
2. At sea, December 1st, 1919. CT 10h 45m 01s. CC 20m 16s slow.
In D.R. Latitude 30° N., Longitude 60° 30' W. Observed unknown star
about 60° west of meridian and about 22° S. What was the star?
3. March 15th, 1919. In D.R. Latitude 10° 42' N, Longitude 150°
14' 28" W. CT 5h 14m 28s. CC--2m 10s. Observed unknown star almost
on my meridian and about 28° north of me. What was the star?
4. Aug. 3, 1919, P.M. at ship. In D.R. Latitude 37° 37' N. Longitude
38° 37' W. At what local mean time will the Star Antares be on the
meridian?
5. What star will transit at about 4:10 A.M. on Aug. 3rd, 1919? In
D.R. position Latitude 38° 10' N, Longitude 34° 38' W.
6. At what local mean time will the Star Arcturus transit on July 17th,
1919, in Latitude 45° 35' N., Longitude 28° 06' W.?
WEDNESDAY LECTURE
LATITUDE BY MERIDIAN ALTITUDE OF A STAR--LATITUDE BY POLARIS (POLE OR
NORTH STAR)
To find your latitude by taking an altitude of a star when it is on your
meridian, is one of the quickest and easiest of calculations in all
Navigation. The formula is exactly the same as for latitude by meridian
altitude of the sun. In using a star, however, you do not have to
consult your Nautical Almanac to get the G.M.T. and from that the
declination. All you have to do is to turn to page 95 of the Nautical
Almanac, on which is given the declination for every month of the year,
of any star you desire. The rest of the computation is, as said before,
the same as for latitude by the sun and follows the formula Lat. = Dec.
± Z.D. (90° - true altitude). As when working latitude by the sun, you
subtract the Z.D. and Dec. when of opposite name and add them when of
the same name. Put in your Note-Book:
Formula: Lat. = Dec. ± Z.D. (90° - true altitude).
At sea, Dec. 24th, 1919. Meridian altitude Star Aldebaran 52° 36' S. HE
20 ft. Required latitude of ship.
Obs. Alt. 52° 36' S
Corr. - 5 08
----------
True Alt. 52° 30' 52" S
- 90 00 00
----------
Z.D. 37° 29' 08" N
Dec. 16 21 00 N
----------
Lat. 53° 50' 08" N
Note to Instructor:
Have class work examples such as the following before taking up Latitude
by Pole Star:
1. At sea, May 5th, 1919. Meridian altitude Star Capella, 70° 29' S. HE
32 ft. Required latitude of ship.
2. At sea, August 14th, 1919. Meridian altitude Star Vega, 60° 15' 45"
N. HE 28 ft. Required latitude of ship.
Etc.
_Latitude by Polaris_ (_Pole or North Star_)
You remember we examined the formula in the N.A. for Lat. by the pole
star when we were discussing sidereal time some weeks ago. We will now
take up a practical case of securing your latitude by this method.
Before doing so, however, it may be of benefit to understand how we can
get our latitude by the pole star. In the first place, imagine that the
Pole Star is directly over the N pole of the earth and is fixed. If that
were so, and imagine for a minute that it is so, then it would be
exactly 90° from the Pole Star to the celestial equator. Now, no matter
where you stand, it is 90° from your zenith to your true horizon. Hence
if you stood at the equator, your zenith would be in the celestial
equator and your true horizon would exactly cut the Pole Star. Now,
supposing you went 10° N of the equator. Then your northerly horizon
would drop by 10° and the Pole Star would have an altitude of 10°. In
other words, when you were in 10° N latitude, the pole star would
measure 10° high by sextant. And so on up to 90°, where the Pole Star
would be directly over you and you would be at the North Pole. Now all
this is based upon the Pole Star being in the celestial sphere exactly
over the North Pole of the earth. It is not, however. Owing to the
revolution of the earth, the star appears to move in an orbit of a
maximum of 1° 08'. Just what part of that 1° 08' is to be applied to the
true altitude of the star for any time of the sidereal day, has been
figured out in the table on page 107 of the Nautical Almanac. What you
have to get first is the L.S.T. Find from the table the correction
corresponding to the L.S.T. and apply this correction with the proper
sign to the true altitude of Polaris. The result is the latitude in. Put
in your Note-Book:
To get latitude by pole star, first get L.S.T. This can be secured by
using any one of the three formulas given you in Week III--Thursday's
Lecture on Sidereal Time and Right Ascension. Then proceed as per
formula in N.A.
* * * * *
Note to Instructor:
Spend rest of time in solving examples similar to the following:
1. At sea, Feb. 14th, 1919. CT 13d 21h 52m 33s. CC 1m 14s fast. In
Lo. 72° 49' 00" W. IE + 1' 10". HE 15 ft. Observed altitude Polaris
42° 21' 30" N. Required latitude in.
2. At sea, March 31st, 1919. In Lo. 160° 15' E. CT 7h 15m 19s. Observed
altitude Polaris 38° 18' N. IE + 3' 00". HE 17 ft. Required latitude in.
Etc.
THURSDAY LECTURE
MARC ST. HILAIRE METHOD BY A STAR SIGHT
You have already been given instructions for finding a Line of Position
by the Marc St. Hilaire Method, using a sight of the sun. Today we will
work out the same method by using a sight of a star. Put this in your
Note-Book here and also under I(b) of the formula given you in Week
IV--Friday's Lecture:
Get G.M.T. from corrected chronometer time. With your G.M.T. find the
corresponding G.S.T. according to the formula already given you. With
your G.S.T. apply the D.R. longitude
(- W. Lo.)
----------
(+ L. Lo.)
to get the L.S.T. With the L.S.T. and the star's R.A. subtract the less
from the greater and the result is the star's H.A. at the ship or "t."
In using Sun Azimuth tables always take "t" from the P.M. column. Mark
Azimuth N or S according to the lat. in and E or W, according as to
whether the Star is East or West of your meridian. Then proceed as in
the case of a sun sight. Formula:
(-W. Lo.)
G.M.T. + (.).R.A. + (+)CP = G.S.T. --------- = L.S.T.--Star's R.A.
(+E. Lo.)
(or vice versa if Star's R.A. is greater) = Star's H.A. at ship (t).
Then proceed as in case of sun sight.
Example:
On May 31st, 1919, in D.R. Lat. 50° N, Lo. 45° W, G.M.T. 31d 14h 33m
30s. What was Star's H.A. at ship?
G.M.T. 14h -- 33m -- 30s
(.).R.A. 4 -- 31 -- 44.2
(+).C.P. 2 -- 23
--------------------
G.S.T. 19h -- 07m -- 37.2s
W Lo.-- 3 -- 00 -- 00
--------------------
L.S.T. 16h -- 07m -- 37.2s
Star's R.A.(Spica) 13 -- 20 -- 59
--------------------
Star's H.A. (t) 2h -- 46m -- 38.2s
Now let us work out some examples by this method:
1. Nov. 29th, 1919. CT 30d 2h 14m 39s A.M. CC 3m 14s fast. D.R. position
Lat. 41° 14' N, Lo. 68° 46' W. Observed altitude Star Aldebaran East of
meridian 50° 29' 40". HE 29 ft. Required Line of Position by Marc St.
Hilaire Method and most probable position of ship.
2. Jan. 23rd, 1919. P.M. at ship. CT 3h 45m 40s. Lat. by D.R. 38° 44'
19" N. Lo. 121° 16' 14" E. Observed altitude Star Rigel 28° 59' 20" West
of meridian. IE + 4' 30". HE 42 ft. Required Line of Position by Marc
St. Hilaire Method and most probable position of ship.
Assign for Night Work one or two examples similar to the above.
FRIDAY LECTURE
EXAMPLES: LATITUDE BY MERIDIAN ALTITUDE OF A STAR, LATITUDE BY POLARIS,
MARC ST. HILAIRE METHOD BY A STAR SIGHT
1. At sea, Dec. 5th, 1919. Observed meridian altitude Star Aldebaran 69°
28' 40" S. No IE. HE 26 ft. Required latitude in.
2. At sea, Jan. 20th, 1919. CT 21d 2h 16m 48s A.M. In longitude 56° 29'
46" W. Observed altitude of Star Polaris 48° 44' 30" N. IE + 10' 20". HE
37 ft. Required latitude in.
3. At sea, June 4th, 1919. A.M. at ship. CT 10h 16m 32s. CC 5m 45s fast.
Lat. by D.R. 42° 44' N, Longitude 53° 13' 44" E. Observed altitude of
Star Altair East of meridian, 52° 19' 30". IE--14' 00". HE 56 ft.
Required line of position by Marc St. Hilaire Method and most probable
position of ship.
Etc.
* * * * *
Assign for Night Work the following Articles in Bowditch: 336 through
341, disregarding the formulas.
SATURDAY LECTURE
LONGITUDE BY CHRONOMETER SIGHT OF THE SUN (TIME SIGHT)
You have now learned, first, how to get your latitude by a meridian
altitude of the sun or a star and second, how to get your Line of
Position and most probable fix, including both latitude and longitude,
by the Marc St. Hilaire Method, using for your calculations either the
sun or a star. We are now going to take up a method of getting your
longitude only. This method requires as much, if not more, calculation
than the Marc St. Hilaire Method. Its results, on the other hand, are
far less complete, for while the Marc St. Hilaire Method will give you a
fairly accurate idea of both your latitude and longitude, this method
will, at best, only give you your longitude. Moreover, you can use it
for accurate results only when the sun bears almost due East or West of
you, for that is the best time, as you have already learned, to get a
line of position running due North and South, which is nothing more than
a meridian of longitude. The only reason we explain this method at all
is because it is in common practice among merchantmen and may,
therefore, be of assistance to you, if you go on a merchant ship.
Remember, however, that it belongs to Old Navigation as distinguished
from New Navigation, exemplified by the Marc St. Hilaire Method. It is
undoubtedly being used less and less among progressive, up-to-date
navigators, and will continue to be used less as time goes on. The fact
remains, however, that at present many merchantmen practice it, and so
it will do you no harm to become familiar with the method, too.
This method is based on securing your longitude by a time sight or
longitude by chronometer sight, meaning that at the time the sun bears
as near due East or West as possible, you take a sight of it by sextant
and at the same instant note the time by chronometer. With this
information you proceed to work out your problem and secure your
longitude according to the following formula. Put in your Note-Book:
To find your longitude by chronometer (or time) sight.
1. Take sight by sextant only when the sun bears as near as possible due
East or West. At exact time of taking sight, note chronometer time.
2. Get G.M.T. from corrected chronometer time. Apply Equation of Time to
get the corresponding G.A.T.
3. Correct observed altitude to get T.C.A. Also have at hand Lat. by D.
R. and Polar Distance. (Note: Secure P. D. by subtracting Dec. from 90°,
if Lat. and Dec. are of same name. If Lat. and Dec. are of opposite
name, secure P. D. by adding Dec. to 90°.)
4. Add together the T.C.A. the Lat. by D.R. and the P.D. Divide the sum
by 2 and call the quotient Half Sum. From the Half Sum subtract the
T.C.A. and call the answer the Difference.
5. Add together the secant of the Latitude, the cosecant of the P.D.,
the cosine of the Half Sum and the sine of the Difference (Table 44).
The result will be the log haversine of the S.H.A. or L.A.T. It must
always be less than 10. If greater than 10, subtract 10 or 20 to bring
it less than 10.
6. From Table 45, take out the corresponding S.H.A. (L.A.T.), reading
from the top of the page if P.M. at ship, or from bottom of page if A.M.
at the ship.
7. Find the difference between L.A.T. and G.A.T. This difference is Lo.
in Time which turns into degrees, minutes and seconds by Table 7. If
G.A.T. is greater than L.A.T. longitude is West; if G.A.T. is less than
L.A.T. longitude is East. Example:
August 26th, 1919, A.M. CT 26d 2h 29m 03s A.M. CC 16m 08s slow. (_) 44°
57' 00". IE--1' 30". HE 32 ft. D.R. Lat. 4° 55' 32" N. Required
longitude in at time of observation.
26d--2h--29m--03s A.M.
- 12
------------------
CT 25d--14h--29m--03s --90° 00' 00"
CC+ +16 --08 Dec. 10 49 48
------------------ --------------
G.M.T. 25d 14h 45m--11s P.D. 79° 10' 12"
Eq. T. -2 --05
------------------
G.A.T. 25d 14h--43m--06s - 1' 30"
+ 9 27
(_) 44° 57' 00" ---------
Corr. + 7 57 Corr. + 7' 57"
-----------
-(-)- 45° 04' 57"
Lat. 4 55 32 N sec. .00160
P.D. 79 10 12 cosec. .00781
------------
2) 129° 10' 41"
------------
1/2 S 64° 35' 20" cos. 9.63266 - 9
-(-)- 45 04 57
------------
Diff. 19° 30' 23" sin. 9.5235O--14
-------------
9.16557 + 5
+ 5
-------------
log. hav. S.H.A. (L.A.T.) 9.16562
S.H.A. (L.A.T.) 25d--21h--00m--01s
G.A.T. 25 --14 --43 --06
-----------------
Lo. in T. 6h--16m--55s E
Lo. (Table 7) 94° 13' 45" E
I wish to caution you about confusing this method with the one Bowditch
uses, and still another which Henderson uses in his book "Elements of
Navigation." It is not exactly like either one. It requires one
operation less than either, however, and it also requires the use of
fewer parts of the various tables involved. For that reason it is given
you.
Assign for work in class room and also for work at night examples
similar to the following:
1. Oct. 1st, 1919. A.M. (_) 17° 15' 00". G.M.T. 1d 11h 30m 00s A.M.
D.R. Lat. 40° 30' N. IE--2' 20". HE 25 ft. Required longitude in.
2. Oct. 10th, 1919. P.M. (_) 25° 14' 30". CT 1h 15m 20s. CC 4m 39s
slow. IE--3' 10". HE 26 ft. D.R. Lat. 41° 29' 00" S. Required longitude in.
3. May 27, 1919. P.M. Lat. by D.R. 40° 55' N. (_) 34° 4' 00". IE +
1' 10". HE 10 ft. CT 8h 55m 42s. CC 2m 02s fast. Required longitude in.
4. May 18th, 1919. A.M. (_) 29° 41' 15". WT 7h 20m 45s. C-W 2h
17m 06s. CC 4m 59s slow. Latitude by D.R. 41° 33' N. IE--1' 30". HE
23 ft. Required longitude in.
5. August 24th, 1919. A.M. (_) 23° 32' 10". IE--2'00". HE 16 ft.
In latitude 39° 04' N. CT 24d 2h 47m 28s A.M. CC + 4m 28s. Required
longitude in.
6. June 26th, 1919. P.M. (_) 44° 08' 20". IE--2' 20". HE 37 ft.
CT 8h 18m 45s. CC 3m 20s fast. Latitude by D.R. 6° 43' S. Required
longitude in.
7. July 29th, 1919. A.M. CT 29d 11h 14m 39s A.M. CC 2m 18s slow.
(_) 28° 08' 30". IE + 0' 30". HE 38 ft. Latitude by D.R. 39° 48' N.
Required longitude in.
8. May 22nd, 1919. P.M. CT 9h 14m 38s. CC 5m 28s slow. (_) 21°
07' 40". In latitude 41° 26' N. IE + 3' 10". HE 40 ft. Required longitude
in.
WEEK VI--NAVIGATION
TUESDAY LECTURE
LONGITUDE BY CHRONOMETER SIGHT OF A STAR
In getting your longitude by a time sight of a star, you proceed
somewhat differently from the method used when observing the sun. What
you wish to get first is G.S.T., i.e., the distance in time Greenwich is
from the First Point of Aries. If you can then get the distance the ship
is from the First Point of Aries, the difference between the two will be
the longitude in, marked East or West according as to which is greater.
By looking at the diagram furnished you when we were talking of Sidereal
Time, all this becomes perfectly clear. The full rule for finding
longitude by a star is as follows, which put in your Note-Book:
Correct your CT to get your G.M.T. From the G.M.T. get the G.S.T. From
the observed altitude of the star, obtain the star's H.A. at the ship in
the same way L.A.T. is secured in case of the sun. To or from the R.A.
of the star add, if West of your meridian, subtract if East of your
meridian, the star's H.A. at the ship, just obtained. The result is the
R.A. of the ship's meridian or L.S.T.
Find the difference between G.S.T. and L.S.T. and the result is the
longitude, marked East or West according as to whether G.S.T. is less or
greater than L.S.T. Note: Always take the star's H.A. from the top of
the page of Table 45.
Dec. 2, 1919. A.M. Observed altitude Star Sirius 2O° 05' 20", West of
meridian. CT 11h--45m--29s P.M. CC 1m--28s slow. IE--1' 20". HE 21 ft.
Latitude by D. R. 38° 57' N. Required longitude in.
CT 11h--45m--29s
CC + 1 --28
-------------------
G.M.T. 11h--46m--57s
(.)RA 16 --37 --10.3
(+)CP 1 --56.1
-------------------
G.S.T. 28h--26m--03.4s IE -1' 20"
--24 HE -7 08
------------------- -------
G.S.T. 4h--26m--03.4s Corr. -8' 28"
Obs. Alt. 20° 05' 20
Corr. -8 28
----------
T.C.A. 19° 56' 52"
Lat. 38 57 sec. .10919
P.D. 106 36 24 cosec. .01849 + 1
2 ) 165° 30' 16"
-------------
1/2 S 82° 45' 08" cos. 9.10106 - 13
T.C.A. 19 56 52
-------------
Diff. 62° 48' 16" sin. 9.94911 + 2
---------
9.17785
- 11
---------
log. hav. Star's H.A. at ship 9.17774
Star's H.A. 3h--02m--40s
Star's R.A. 6 --41 --39
--------------
L.S.T. 9h--44m--19s
G.S.T. 4 --27 --01
--------------
Lo. in T. 5h--17m--18s E
Longitude in 79° 19' 30" E
Assign for Night Work or work in the class room examples similar to the
following:
1. April 16, 1919, in Latitude 11° 47' S. Observed altitude of the Star
Aldebaran, West of the meridian 23° 13' 20". CT 6h 58m 29s. CC 2m 27s
fast. IE--2' 00". HE 26 ft. Required longitude in.
2. Dec. 10th, 1919. Observed altitude of Star Sirius 20° 05' 40" West of
meridian. CT 11h 45m 29s. CC 1m 28s slow. IE--1' 20". HE 21 ft. D.R.
latitude 38° 57' N. Required longitude in.
Note to Instructor: If any time in the period is left or for Night Work
assign examples to be worked by Marc St. Hilaire Method, changing
slightly the D.R. Lat. and Longitude just obtained by the Time Sight
Method.
WEDNESDAY LECTURE
EXAMPLES ON LONGITUDE BY CHRONOMETER SIGHT OF A STAR
1. Dec. 9th, 1919. In latitude 36° 48' N. Observed altitude Star
Capella, East of meridian 46° 18' 30". IE 2' 50" off arc. HE 33 ft. CT
10d 3h 05m 05s A.M. CC 1m 18s slow. Declination of star is 45" 55' N.
Required longitude in.
2. October 26th, 1919. In latitude 39° 54' S. Observed altitude Star
Rigel, West of meridian 42° 18' 40". CT 27d 10h 32m 55s A.M. CC 2m 18s
fast. IE 4' 20" off arc. HE 42 ft. Required longitude in.
3. April 11th, 1919. P.M. at ship. In latitude 43° 16' 48" S. Observed
altitude Star Spica 33° 18' 20", East of meridian. CT 11h 08m 44s P.M.
IE 3' 20" on arc. CC 4m 18s slow. HE 39 ft. Required longitude in.
4. September 15th, 1919. P.M. at ship. In latitude 49° 38'N. Observed
altitude Star Deneb, East of meridian, 36° 16' 50". IE 3' 40" off arc.
HE 40 ft. CC 6m 18s slow. CT 10h 00m 13s P.M. Declination of star is 44°
59' 36" N. Required longitude in.
If any time is left, work same examples by Marc St. Hilaire Method
assuming a position near the one found by Time Sight.
Assign for Night Work any of the above examples, to be worked either as
Time Sights or by the Marc St. Hilaire Method, and also the following
Arts. in Bowditch: 326-327-328-329.
THURSDAY LECTURE
LATITUDE BY EX-MERIDIAN ALTITUDE OF THE SUN
You have learned that when you calculate your latitude from a meridian
altitude of the sun, one of the necessary requisites is to have the sun
exactly on your meridian. In fact, that is just another way of
expressing meridian altitude, i.e., an altitude taken when the sun is on
your meridian. Now suppose that 10 or 15 minutes _before_ noon you fear
that the sun will be clouded over _at_ noon so that a meridian altitude
cannot be secured. There is a way to calculate your latitude, even
though the altitude you secure is taken by sextant some minutes before
or after noon. This is called latitude by an ex-meridian altitude. It
must be kept in mind that this method can be used accurately only within
26 minutes of noon, either before or after, and only then when you know
your longitude accurately. Put in your Note-Book:
1. Get your L.A.T. (S.H.A.).
2. Subtract it from 24h 00m 00s, or vice versa, according as to whether
L.A.T. is just before or just after local apparent noon. Call the result
"Time Interval from Meridian Passage."
3. With your D.R. latitude, declination and Time Interval from Meridian
Passage, enter Table 26 to get the proper amount of Variation of
Altitude in one minute from meridian passage.
4. With the Time Interval from Meridian Passage and the Variation, enter
Table 27 to get the total amount of Variation of Altitude.
5. Add this total amount of Variation to the true observed altitude
taken before or after noon, and the result is the corrected altitude.
6. Then proceed to get your latitude according to the rules already
given you for latitude by meridian altitude.
Example: At sea, Jan. 23rd, 1919. CT 4h 22m 14s. CC 1m 10s fast.
Longitude 66° 04' W. Latitude by D.R. 19° 16' 00" N. (_) 50° 51' 00" S.
HE 49 ft. IE--1' 30". Required latitude in.
CT 4h - 22m - 14s
CC - 1 - 10
---------------------
G.M.T. 4h - 21m - 04s
Eq. T. - 11 - 50
---------------------
G.A.T. 4h - 09m - 14s
Lo. in T 4 - 24 - 16 (W-)
---------------------
L.A.T. 22d - 23h - 44m - 58s
24h - 00m - 00s
- 23 - 44 - 58
-----------------
15m - 02s = Time Interval from Meridian Passage.
Dec. 19° 34' 48" S Table 26 = 2.8 Variation
Lat. 19° 16' 00" N For 1 min. 0 altitude.
* * * * *
Time Interval from Meridian Passage 15m 02s - 2.8" Variation for 1
minute (Table 27) 2" = 7' 30"
.8 = 3 00
-------------
10' 30" +
IE - 1' 30" (_) 50° 51' 00"
HE + 8 42 + 7 12
--------- -----------
Corr. + 7' 12" -(-)- 50° 58' 12"
+ 10 30
-----------
51° 08' 42"
- 90 00 00
-----------
ZD 38° 51' 18" N
Dec. 19 34 48 S
-----------
Lat. in 19° 16' 30" N
Assign for work in class room and Night Work, examples similar to the
following:
1. At sea, July 11th, 1919. Latitude by D.R. 50° 01' 00" N. Longitude
40° 05' 16" W. Observed ex-meridian altitude (_) 61° 45' 30" S. HE
15 ft. IE--4' 10". CT (corrected) 2h 38m 00s. Required latitude in.
2. At sea, June 6th, 1919. Latitude by D. R. 49° 21' N, Longitude 18°
18' W. Observed ex-meridian altitude (_) 61° 30' 22" S. HE 42 ft. CT 1h 06m
18s. CC--1m 14s. IE 0' 30" off the arc. Required latitude in of ship.
If any time is left, work similar examples by Marc St. Hilaire Method.
FRIDAY LECTURE
EXAMPLES: LATITUDE BY EX-MERIDIAN ALTITUDE OF THE SUN
1. Jan. 1st, 1919. WT 11h 53m 18s A.M. C-W 5h 56m 16s. Latitude
by D. R. 58° 05' S. Longitude 89° 00' 48" W. (_) ex-meridian 55° 16' 30" N.
IE 2' 00" off the arc. CC 1m 28s fast. HE 36 ft. Required latitude in.
2. March 11th, 1919. CT 11d 9h 14m 39s A.M. Latitude by D. R.
39° 20' N, Longitude 39° 48' 16" E. (_) ex-meridian 46° 17' 30" S.
IE 2' 00" on the arc. CC 1m 16s slow. HE 29 ft. Required latitude in.
3. April 26th, 1919. CT 26d 4h 46m 38s A.M. Latitude by D. R.
24° 25' S, Longitude 107° 16' 56" E. (_) ex-meridian 52° 18' 50" N. IE--2'
40". CC 3m 56s slow. HE 33 ft. Required latitude in.
4. May 10, 1919. CT 2h 18m 46s A.M. Latitude by D. R. 23° 54' S,
Longitude 143° 20' 18" E. (_) ex-meridian 48° 26' 20" N. IE 3' 20" on the
arc. CC 4m 18s fast. HE 41 ft. Required latitude in.
5. June 21st, 1919. CT 4h. 56m 18s. Latitude by D. R. 42° 01' N,
Longitude 75° 00' 18" W. (_) ex-meridian 71° 29' 40" S, IE--2' 30". CC
3m 04s slow. HE 28 ft. Required latitude in.
6. Dec. 18th, 1919. WT 11h 50m 18s A.M. C-W 3h 14m 18s. Latitude
by D. R. 11° 55' S. Longitude 48° 02' 29" W. (_) ex-meridian 78° 32' 30" S.
IE 3' 30" on the arc. CC 2m 44s slow. HE 35 ft. Required latitude in.
If there is any time left, give examples of latitude by meridian altitude,
Marc St. Hilaire Method by sun or star sight, etc.
SATURDAY LECTURE
FINDING THE WATCH TIME OF LOCAL APPARENT NOON
Noon at the ship is the pivotal point of the day's work at sea. It is
then that the navigator must report to the commanding officer the
latitude and longitude by dead reckoning, the latitude and longitude by
observation, the course and distance made good, the deviation of the
compass and the course and distance to destination. Apparent noon, then,
is a most important time to calculate accurately, and to do so when the
ship is under way, is not so easy at it first appears.
If the ship is stationary, and you know the longitude you are in, the
problem is simple. Then it is merely a question of starting with L.A.T.
of 00h-00m-00s, adding or subtracting the longitude, according as to
whether it is West or East, to get G.A.T.; applying the equation of time
with sign reversed to get G.M.T.; applying the C. Cor. with sign
reversed to get the C.T.; and applying the C-W to get the WT. If, for
instance, this WT happens to be 11h-42m-31s, when the watch reads that
number of hours, minutes and seconds, the sun will be on the meridian
and it will be apparent noon.
When the ship is moving, the problem is more difficult. At first thought
you might imagine that all you would have to do would be to take the
difference between the L.A.T. of the morning sight and 24 hours,
calculate the distance the ship would run in this time and from that
determine the longitude you would be in at noon. Then proceed as in the
case of the ship being stationary. But such a calculation does not take
into consideration the easting or westing of the ship itself. Suppose
that at the morning sight the L.A.T. is found to be 20h-10m-30s. If the
ship does not move, it will be 3h-49m-30s to noon. But suppose the ship
is moving eastward. Then, in addition to the speed at which the sun is
approaching the ship, there must be added the speed at which the ship is
moving toward the sun--i.e. the change in longitude per hour which the
ship is making, expressed in minutes and seconds of time. Likewise, if
the ship is moving westward, an allowance must be made for the westing
of the ship. And this change of longitude in minutes and seconds of time
must be subtracted from the speed of the sun's approach since the ship,
in going west, is traveling away from the sun.
There are various ways to calculate this allowance for the ship's speed,
among the best of which is given in Bowditch, Art. 403, p. 179. Another,
and even easier way, is the following, which was explained to the writer
by Lieutenant Commander R.P. Strough, formerly head of the Seamanship
Department of this School:--
1. Take the morning sight for longitude when the sun is on or as near as
possible to the prime vertical.
2. Subtract the L.A.T. of the morning sight from 24 hours. This will
give the total time from the morning sight to noon if the ship were
stationary.
3. From the course to noon and speed of the ship, figure the change in
longitude per hour in terms of seconds of time. For instance, suppose a
ship were steaming a course of 275° at the rate of 11 knots per hour in
approximately 38° North latitude. The change of longitude per hour for
this speed would be 14' of arc or 56s of time.
4. Now the sun travels at the rate of 60 minutes or 3600 seconds per
hour. To this hourly speed of the sun must be added or subtracted the
hourly speed of the ship according as to whether the ship is going in an
easterly or westerly direction. If, as mentioned above, the ship is
steaming a course of 275° (W 1/2 N) and hence changing its longitude at
the rate of 56s per hour, then the net rate of approach of the sun per
hour would be 3600s - 56s, or 3544s per hour.
5. Divide the total time to noon from the L.A.T. of the morning sight
(expressed in seconds of time) by the net rate of approach of the sun
per hour. The result will be the corrected time to noon--i.e. the time
at which the sun will be on the ship's meridian when the ship is
changing its longitude to the westward at the rate of 56s per hour.
6. One more step is necessary. To the watch time of the morning sight,
add the corrected time to noon. The result will be the watch time of
Local Apparent Noon. Thirty minutes before will be the watch time of
11:30 A.M. and at 11:30 A.M. all deck clocks should be set to the local
apparent time of the place the ship will be at local apparent noon.
The following example illustrates the explanation just given and should
be put in your Note Book:--
Example:--At sea, August 7th, 1919. About 7:30 A.M. by ship's time,
position by observation just found to be Latitude 30° 05' N, Longitude
58° 08' W. WT of morning sight 6h-53m-13s A.M. C-W 4h-37m-21s. CC +
3m-38s. Course 275°. Speed 11 knots. TZ N 90° E. What will be the Watch
Time of Local Apparent Noon?
WT 6h -- 53m -- 13s A.M.
+ 12
-------------------
18 -- 53 -- 13
C-W 4 -- 37 -- 21
-------------------
CT 23 -- 30 -- 34
CC + 3 -- 38
-------------------
G.M.T. 23 -- 34 -- 12
Eq. T. -- 5 -- 42
-------------------
G.A.T. 23 -- 28 -- 30
Lo. in T. 3 -- 52 -- 32
-------------------
L.A.T. 19 -- 35 -- 58
24 -- 00 -- 00
-------------------
Total time to Noon 4h 24m 02s
Course -- 275°
Change in Lo. per hr.-- 14', 56s.
3600s
-56
-----
3544s, Net rate of approach of sun
4h
60
----
240m
+ 24
-----
264m x 60 = 15840s
15840s
+ 02
-----
15842s, Total time to noon.
3544) 15842 (4.47 hours
14176
-----
16660
14176
-----
24840
24808
-----
Corrected time to Noon 4h -- 28m -- 12s
WT of A.M. sight 6h -- 53m -- 13s
-----------------
WT of L.A.N. 11h -- 21m -- 25s
WT of 11:30 A.M. 10h -- 51m -- 25s
When, therefore, the watch reads 10h--51m--25s, the deck clocks should
be set to 11.30 A.M. and thirty minutes later it will be apparent noon
at the ship.
In all these calculations it is taken for granted that the speed of the
ship and hence the change in longitude can be gauged accurately. A check
on this can be made by comparing the longitude of the A.M. sight with
the D.R. longitude of the same time. Any appreciable difference between
the two can be ascribed to current. Now, if a proportionate amount of
current is allowed for in reckoning the speed of the ship from the time
of the A.M. sight to noon, then a proper correction can be made in the
net rate of approach of the sun and the corrected time to noon will be
very close to the exact time of noon. Of course there will be an error
in this calculation but it will be small and the result gained will be
accurate enough for ordinary work.
So much for finding the watch time of Local Apparent Noon. Careful
navigators carry the process further and get the watch times of 15, 10
and 5 minutes before noon, so that by the use of constants for each one
of these times, an accurate check on the noon latitude can be quickly
and easily secured. We have not time in this course to explain how these
constants are worked out but it is well worth knowing. The information
regarding it is in Bowditch Art. 325, p. 128, and Art. 405, p. 181.
A word about the watch used by the navigator should be included here.
This watch should be a good one and receive as much care, in its way, as
the chronometer. It should be wound at the same time every day,
carefully handled and, in other respects, treated like the fine
time-piece that it is.
While authorities differ on this point, the best practice seems to be
not to change the navigator's watch to correspond with the apparent time
of each day's noon position. The reason for this is two-fold. First,
because constant moving of the hands will have an injurious effect on
the works of the watch, and second, because, by not changing the watch,
the C-W remains approximately the same, and thus a good check can be
kept on both the watch and the chronometer as well as on the navigator's
figures in reckoning the times of his various sights.
Assign for night reading the following Arts. in Bowditch: 323, 324, 333.
Also problems similar to the following:
1. At sea, July 28, 1919. Position by observation just found to be Latitude
44° 58' N, Longitude 22° 06' W. WT of morning sight 6h-02m-20s. CC
3m 34s slow. Course S 24° W. TZ N 90° E. Speed 9 knots. What will be
the watch time of Local Apparent Noon?
2. At sea, August 9th, 1919. Position by observation just found to be
Latitude 38° 48' N, Longitude 70° 46' W. WT of morning sight 8h-15m-01s A.M.
C-W 3h-56m-32s. CC 3m-43s slow. Course 272°. Speed 12 knots. TZ N
90° E. What will be the watch time of Local Apparent Noon?
WEEK VII--NAVIGATION
TUESDAY LECTURE
COMPASS ERROR BY AN AZIMUTH
The easiest and most accurate way to find the error of your compass is,
first, to find the bearing of the sun by your pelorus. If you set your
pelorus, so that it will exactly coincide with the course you are
steaming as shown by the compass in your chart house and then get a
bearing of the sun by noting where the shadow from the pelorus vane cuts
the circumference, this bearing will be the bearing of the sun by
compass. At the same time, get your true bearing of the sun from the
Azimuth Tables. The difference between the two will be the compass
error, marked East or West according to the following rule which put in
your Note-Book:
1. Express your Compass Bearing and your True Bearing by NEW compass
reading.
2. If TZ is to the right of CZ, C.E. is East. Formula:
True--Right--East.
3. If TZ is to the left of CZ, C.E. is West. Formula: True--Left--West.
You must now remember that what you have is a Compass Error, consisting
of both Variation and Deviation. To find the Deviation, the Variation
and C.E. being given, is merely to apply the rules already given you
under Dead Reckoning. For instance, if you had a C.E. of 10° W and a
Variation of 4° E, the Deviation would be 14° W.
Put this example in your Note-Book:
LAT 20h 59m 57s
Lat. 4° 55' N
Dec. 10° 39' 30" N
Ship heading N 11° W. CB of (.) S 88° E. Variation 10° W. What was the
ship's true course and Deviation of Compass on direction ship was
heading?
[Illustration]
[Illustration]
CZ 92° (New compass reading)
TZ 80° (New compass reading)
---
CE 12°
CE = 12° W
Variation 10° W
-----
Deviation 2° W
True course being sailed N 23° W or 337°.
Let us now work out some of the following examples:
1. L.A.T. 22h--14m--18s
Lat. 30° 29' S
Dec. 17° 28' 44" N
Ship heading S 84° W
Compass Bearing 44°
Variation 10° W.
Required T.C. and Deviation on ship's loading.
2. August 29th, 1919. CT 2h 29m 18s A.M. Longitude 120° 19' 46" E.
Latitude 44° 14' N. Ship heading 98°.
Compass Bearing S 42° E.
Variation 4° E.
Required T.C. and Deviation on ship's heading.
3. June 17th, 1919. CT 4h 18m 44s A.M. Longitude 60° 14' 59" E.
Latitude 38° 48' 00" S. Ship heading SW x S.
Compass Bearing 40°
Variation 12° W.
Required T.C. and Deviation on ship's heading.
Etc.
WEDNESDAY LECTURE
CORRECTING LONGITUDE BY A FACTOR
We are now almost ready to begin the discussion of a day's work at sea.
The only method we have not taken up is the one which is the subject of
today's lecture. It is a method to correct your longitude to correspond
with the difference between your latitude by Dead Reckoning and your
latitude by observation.
Suppose you take a sight in the morning for longitude. The only latitude
you can use is a D. R. latitude, advanced from your last known position.
Now suppose you run until noon and at that time take a sight for
latitude. In comparing your D. R. latitude, advanced the true course and
distance steamed to noon, and your latitude by observation taken at
noon, suppose there is a difference of several minutes. The question
is--How can we correct our longitude to correspond with this error
discovered in the latitude? This is the method which put in your
Note-Book:
Find the difference between the latitude by D. R. and the corresponding
latitude by observation (in most cases secured from a sight at noon or
from the Star Polaris). Call this the Error in Latitude. With the D. R.
Latitude of the preceding sight and the azimuth or bearing of the
preceding sight (always expressed as a bearing of less than 90°, old
compass reading) enter Table 47 for the correct Longitude Factor.
Multiply this Factor by the Error in Latitude. The result is the
correction to apply to the Longitude. It is applied East or West
according as to whether the Latitude by Observation is to the East or
West of the D. R. Latitude on the Line of Position (the line at right
angles to the Azimuth) of the preceding sight.
Example:
Position about 7:30 A.M. Latitude by D. R. 25° 40' S, Longitude (just
secured by observation) 104° 05' 38" E. L.A.T. 7h 32m 30s A.M.,
Declination 4° 59' N. Thence ship ran to noon 109°, true course, 46
miles, when the latitude by meridian altitude of the sun was found to be
25° 52' S. Required corrected longitude at noon.
7:30 A.M. D.R. Lat. 25° 40' S Lo. 104° 05' 38" E
109°--46 k. 15 S 48 18 E
---------- --------------
Noon--Lat. by D.R. 25° 55' S Lo. 104° 53' 56" E
Noon--Lat. by obs. 25° 52' S
----------
Error in Lat. 3'
Enter Table 47 with azimuth (S 105° E) N 75° E as bearing and Latitude
25° 40' or 26°, Factor is found to be .3.
3' (Error in Latitude) times .3 (Factor) = .9' or 54", Correction in
Longitude. Is it East or West? Since azimuth is N 74° E, Line of
Position is N 16° W. The D. R. Latitude and Latitude by Observation are
plotted on this line as follows:
\
* Lat. by obs. (25° 52' S)
\
* Lat. by D.R. (25° 55' S)
\
Latitude by observation is West of Latitude by D.R. Hence correction in
longitude of 54" is applied West. Position by observation, therefore, is
as follows:
Lo. 104° 53' 56" E
Corr. in Lo. 54 W
--------------
Lo. by obs. 104° 53' 02" E
Lat. by obs. 25° 52' S
Note to Instructor:
Assign the following examples for work in the class room:
1. April 20th, 1919 A.M. at the ship. G.M.T. 20d 10h 28m 24s A.M.
(_) 31° 55' 40". HE 30 ft. No IE, CC. Latitude by D. R. 26° 30' N.
Longitude 36° 55' West.
Ship then sailed a true course of S 36° E--40 knots until noon when
observed altitude (_) 75° 40' 50" S. What was the position at noon
corrected for Longitude? (Note: Work the A.M. sight by both Time Sight and
Marc St. Hilaire Method.)
2. June 25th, 1919, A.M. Latitude by D. R. 36° 20' S. Longitude
96° 30' E. CT 1h 37m 16s A.M. CC 1m 30s fast. IE 2' 30" off arc.
HE 36 ft. (_) 7° 34' 20". Log registered 114.
True course to noon S 76° E. Log registered 174. Same IE, HE, CC.
Observed altitude (_) 29° 44' 40" N. Required position at noon by Longitude
factor. (Note: Work A.M. Sight by Marc St. Hilaire Method.)
3. At sea, May 30th, 1919. In D. R. Latitude 38° 14' 29" N. Longitude
15° 38' 49" W. Observed altitude (_) 39° 05' 40" and bearing by compass
259°. IE 1' 00" on arc. HE 27 ft. WT 3h 04m 49s. C-W 1h 39m 55s. CC
1m 52s fast.
Changed course to 94° p.s.c. and steamed 75 knots to about 8 o'clock. WT
8h 06m 18s. C-W 1h 39m 58s. At this time observed altitude of Star Arcturus
68° 30' 40", East of meridian. Same IE, HE, CC.
Changed course to 95° (true). Steamed 60 knots until midnight when ran
into heavy fog. Slowed down to 7 knots per hour until 8 A.M. when
observed altitude (_) 48° 45' 10". CT 9h 45m 18s A.M. Same HE, IE, CC.
Required fix at 8 A.M. by Marc St. Hilaire Method, laid down on chart.
Note to Instructor:
Spend rest of period in familiarizing pupils with laying down runs and
intersecting lines of position on Mercator plotting charts.
THURSDAY LECTURE
THE NAVIGATOR'S ROUTINE--A DAY'S WORK AT SEA
You are now familiar with the principal kinds of sights and the methods
used in working them as explained in the foregoing pages. This
information, however, relates only to each individual kind of sight.
Today I will explain briefly how those sights are made use of in your
daily work at sea. Such an explanation necessarily cannot include the
navigator's work under all conditions and on all classes of ships. It
merely gives a brief outline of and a few suggestions relating to
navigating conditions on board a medium-sized transport, in time of war.
I say "in time of war" because navigating then is different, to some
extent, from the ordinary routine in time of peace.
Suppose you are ordered to a ship as navigator. What are your duties (a)
before leaving port, (b) while at sea, and (c) on entering pilot waters?
_(a) Before Leaving Port_
Ascertain the height of the eye of the bridge and any other place on the
ship where you would be likely to take sights.
Have posted in the chart room and on the bridge the deviation of the
compass on each 15° heading, so that it can be easily referred to.
Keep in each chronometer case or in a book nearby the error and daily
rate of all chronometers on board.
Test each sextant for index error and record the result where you can
refer to it easily.
See that all charts of the harbor out of which you are to steam are
corrected to date and are familiar to you, both as to sailing directions
and buoys, and also as to lights and other aids to navigation.
Examine, in detail, the steering engine and steering apparatus. In case
of its disarrangement your intimate knowledge of it may be most
valuable.
See that the patent log and sounding machine are in good order. See that
the lead lines are well soaked in water, stretched, and properly marked.
See that the lighting system in the chart room and the navigator's room
is such that when any door is not tightly closed the lights in the room
are extinguished. Likewise, when the doors are closed, see that the
lights will light and without repeated slamming of the doors.
If possible, provide yourself with a flashlight set back in a metal tube
so that the rays of the light are not diffused but can be focussed only
on one spot at a time.
See that your charts are arranged neatly in the drawers provided for
them in the chart room. If, as is usual, the charts must be folded to
get them in the drawers, mark them legibly on the outside and in the
same place on each chart. Put in the top drawers those charts you know
you will use most frequently. This will save endless time and confusion.
Be sure you have a full complement of necessary instruments, including
sextants, a stadimeter, binoculars, watches, stop watch, dividers,
parallel rulers, pencils, work books; also all necessary books, such as
smooth and deck log books, several volumes of Bowditch, Nautical
Almanacs, Azimuth Tables, Pilot books, Light and Buoy lists, Star
Identification Tables, etc. You will be repaid a thousand times for
whatever effort you expend to have your navigational equipment complete
to the smallest detail. The shortage, for instance, of a pair of
dividers would be an unending annoyance to you. This is also true of
almost any other item mentioned above. Prepare yourself, then, while you
are in port and have plenty of opportunity to secure the equipment you
desire.
_(b) While at Sea_
The least amount of work required of a navigator in time of peace would
include (1) a morning sight for longitude, (2) a noon sight for
latitude, (3) an afternoon sight for longitude, (4) an A.M. azimuth to
check the deviation of the compass, and (5) the dead reckoning for the
day's run from noon to noon.
Navigating in war time requires more work than this. If possible, the
ship's position must be known accurately at any time of day or night
for, in case of an emergency, the lives of all on board may be
imperilled by inaccurate knowledge of your whereabouts. This means that
more sights must be taken and more celestial bodies observed. While
every navigator has his own idea as to the proper amount of work to do
in a day, it would seem as though the following would cover the minimum
amount of work necessary under present conditions:
1. An A.M. sight of the sun for longitude.
2. An azimuth of the sun for checking the deviation of the compass,
taken right after the A.M. sun sight.
3. The watch time of Local Apparent Noon.
4. Ex-meridian and meridian altitudes of the sun for latitude.
5. A P.M. sight of the sun for longitude.
6. An evening twilight sight of three or four stars, preferably one in
each quadrant. If these altitudes are taken correctly your position can
be found to the dot.
7. A morning twilight sight for a fix or, at least, for latitude by
Polaris.
8. The dead reckoning from noon to noon.
9. Distance run during the last 24 hours, from noon to noon
10. Distance to destination.
11. Set and drift of the current.
_1. The A.M. Sun Sight_
In order to make this a valuable sight for longitude it should be taken
when the sun is on or as near as possible to the prime vertical. As the
sun, in North latitudes, passes the prime vertical before sunrise in the
winter, the following remarks do not hold for that season. In winter the
only rule to follow is to observe the sun as soon as it is 10° or more
above the horizon. In summer find out from the Azimuth Tables the local
apparent time when the sun will bear 90°. Estimate, as closely as
possible, the longitude you will be in the next morning when the local
apparent time is as just found in the Azimuth Tables. This can be done
by calculating the dead reckoning from the previous sight, or, what is
even simpler, laying the distance off on the plotting chart. With this
information find the W.T. corresponding to the L.A.T. mentioned above by
some such formula as this: L.A.T. ± Lo. = G.A.T. ± Eq. T. (sign
reversed) = G.M.T. ± C.C.(sign reversed) = C.T. - (C-W) = W.T. This will
not be absolutely accurate, for the longitude you are in is only
approximate, but it will be close enough for good results. This
resulting W.T. will be the time to take the A.M. sight. About fifteen
minutes before that time compare your watch with your chronometer to get
the C-W. Also bring up the C.C. to date and make a note of it so that as
much as possible of this detail work is accomplished before the sight is
taken. Next, take your sextant and test it for index error. This should
be done regularly before each series of sights as it is impossible to
tell what may have happened when the sextant is lying idle, except by
the above test. Now, with your sextant, watch and notebook, go to the
place from which you have decided to take your observations and, at the
proper watch time, start taking your altitudes. It is always advisable
to take a number of sights, closely following each other, so that an
error in one may be corrected somewhat by the others. Take at least
three sights in close succession. At the same time have the log read and
enter it in your notebook. An equally good method in fair weather is to
secure the distance run from the revolutions of the propeller.
Having taken your sights, go to the standard compass and get a bearing
of the sun, at the same time noting in your book the W.T. of the bearing
and the compass heading of the ship. You are now ready to go below into
the chart room and work out your position. What method shall you use?
That depends upon your preference. You have missed the point of the
previous lectures, however, if you forget that the New Navigation is
based upon the Marc St. Hilaire Method, and this is undoubtedly the
method your captain will prefer you to use if he is an Annapolis
graduate. In this connection let me remind you again of the one fact,
the oversight of which discourages so many beginners with the Marc St.
Hilaire Method. The most probable fix, which you get by one sight only,
is not actually a fix at all. Nor does any other method give you an
accurate fix under like conditions. What the most probable fix is, and
all it claims to be, is a point through which the required Sumner line
is to be drawn. If your D.R. position happens to be only one mile away
from the most probable fix, that is no assurance that the most probable
fix is near the actual position of the ship. You may be 25 miles away
from it. But the important information gained is that, though you may be
25 miles away, you know on what line you are, and when this line is
later crossed with another line of position that fix will be accurate.
"Two sights make a fix" is the whole matter in a nutshell.
_2. The Compass Error_
Having secured your morning sight, the next duty is to get the compass
error. From your morning sight computation you know the watch time
corresponding to the L.A.T. of the same sight. Find the difference
between the two and apply this difference to the watch time of the
compass azimuth. That will give you the L.A.T. with which to enter the
Azimuth Tables to get the true bearing corresponding to the compass
bearing recently observed. Apply the variation from the chart to get the
magnetic bearing. The difference between this magnetic bearing and the
compass bearing will be the required deviation, which you should compare
with your Deviation Table. If there is a marked difference, and you are
sure of your figures, use the new deviation in computing courses on this
heading of the ship.
_3. The Watch Time of Local Apparent Noon_
You are now ready to figure the watch time of local apparent noon.
Unless you have a decided preference to the contrary, do this by the
method explained in the Saturday Lecture, Week VI. Do not forget that in
subtracting the L.A.T. of the morning sight from 24 hours to get the
total time to noon, in case the ship were stationary, you do _not_ use
the L.A.T. of the D.R. position, but the L.A.T. found by subtracting
from G.A.T. the longitude of the most probable fix. This will give you
the L.A.T., based on the longitude of the most probable fix, which will
be slightly different from the L.A.T. based on the D.R. longitude. When
you have secured the watch time of local apparent noon, subtract 30
minutes from it and notify the quartermaster that at that time by your
watch the deck clocks are to be set to 11.30 A.M. If this change of time
is very great (providing you are on an almost easterly or westerly
course), it is wise to have the clocks set back in the night watches to
allow for most of the time you figure you will lose. This will not work
such a hardship or such an advantage to the officers and men who have
the forenoon watch and will also be easier for the cooks. The clocks can
then be slightly but accurately changed at 11.30 A.M., as mentioned
above.
_4. Ex-Meridian and Meridian Altitudes_
You know the principles and methods governing sights of this character.
To know your latitude exactly at noon is usually required when you are
steaming in convoy, for at that time your position signals are hoisted,
and it is a matter of pride with the navigator not only to have his
position exact but promptly. If your A.M. sight was taken when the sun
was on or near the prime vertical, a change in latitude at noon will
make no change in longitude. Hence you can figure your longitude at noon
just as soon as you have secured the corrected time from the A.M. sight
to noon (which you have done right after working the A.M. sight). You
will have your longitude, then, before you go on the bridge to observe
for ex-meridian and meridian altitudes.
Sharply at noon you take your meridian altitude and tell a messenger to
notify the captain that it is noon at the ship. The captain then orders
eight bells struck, and you are ready to hand in your noon report,
consisting of latitude and longitude by observation, latitude and
longitude by dead reckoning, deviation of the compass on the ship's head
at 8 A.M., distance made good since the preceding noon, distance to
destination, set and drift of current (Note:--When steaming in convoy
this is unnecessary and usually omitted), and any other pertinent
remarks. If the sun was not taken on or near the prime vertical at the
time of the A.M. sight, take out your longitude factor for the coming
noon position and calculate your D.R. latitude at noon. By correcting
the longitude of the A.M. sight, run to noon, with the difference of
longitude, readily found at noon with the longitude factor and the error
in latitude, you will have the correct noon longitude to hand in, with
only a moment's delay. It will be very hard, however, to get all this
information in on time without the use of latitude constants. There is
no room for a discussion of these constants here, but they are easy to
work and you should learn how to use them. The information is in
Bowditch Art. 325, p. 128, and Art. 405, p. 181.
_5. The P.M. Sun Sight_
This is another longitude sight and so any previous remarks about sights
of this character are applicable here. If the day is fine you need not
work out this sight until after evening twilight, for a fix then by
stars will give both latitude and longitude, whereas your afternoon sun
sight will only give you a longitude. This P.M. sun sight is a good
check sight, to be used or not, according as to whether other earlier or
later sights have been obtained.
_6. The Evening Twilight Sight_
The beauty of using stars is that by almost simultaneous altitudes of
different ones you can ascertain your position, both as to latitude and
longitude. In the North Atlantic during the summer months Vega, Deneb or
Altair in the East, Antares or Deneb Kaitos in the South, Arcturus in
the West, and Polaris, Mizar, or Kochab in the North form an ideal
combination which includes every quadrant of the compass. In the winter
months, Capella, Castor or Pollux in the East, Sirius or any star in
Orion's belt in the South, Deneb in the West, and Polaris in the North
are equally as good.
_7. The Morning Twilight Sight_
In clear weather this should be primarily a sight for latitude, since
the A.M. sun sight for longitude will follow it. A latitude by Polaris,
and at the same time some star in one of the southern quadrants, as a
check, will give admirable results.
_8. The Dead Reckoning from Noon to Noon_
If there is no change of course in the late forenoon, as is usually the
case, the dead reckoning for the day's run can be figured any time
before noon so that it will be all ready to hand in to the captain with
the other noon data. It is much easier to lay this off on the chart than
to go to the trouble of calculating it by Table 2, Bowditch. On the
other hand, such a calculation checks the chart work and should be
worked out if you wish to make "assurance doubly sure."
_9. Distance Run During the Last 24 Hours_
Here, again, an answer by chart and an answer by figures is a good thing
to secure. As you become accustomed to your work you will find the
answer by chart infinitely easier and quicker to get. It is just as
accurate, too, if you lay the distances off carefully with the dividers
and parallel rulers.
_10. Distance to Destination_
The same remarks as are made under (9) hold true here.
_11. Set and Drift of Current_
Find the difference between your D.R. position and your position by
observation at noon, i.e., the course and distance from your D.R.
position to your position by observation. The course is the set of the
current, the distance the amount of drift, all of which is easily
calculated by Table 2, Bowditch. This difference between the two
positions is seldom due to current. It is due to all errors of steering
and the like. But these are all ascribed to current, for the sake of
convenience. This calculation of the current is seldom used now,
particularly when steaming in convoy.
It is obvious that a schedule, such as outlined above, cannot be adhered
to in all kinds of weather or under all conditions. It is merely an
outline of what might properly be included in a 24 hour day, the weather
conditions of which will lend themselves at any time to taking the
observations mentioned. The weather of each succeeding day may force you
to adopt a different routine. Nevertheless, the closer you can keep to
the above schedule the more exact will your various positions be.
_(c) On Entering Pilot Waters_
See that all charts of the locality you intend to enter are corrected to
date.
Study these charts carefully, making notes, in detail, of the aids to
navigation that you intend to pick up.
In noting lights give their distinctive appearance, range of visibility,
approximate time of sighting them, and any other information that you
think you may need. If you have this information with you when on the
bridge it will save much time and trouble that you would otherwise have
to spend, at possibly a critical time, in the chart room.
See that log lines, sounding machine, etc., are in order for instant
use.
Remember that in entering pilot waters the safest landmarks are
permanent ones. Buoys, cans, etc., may drag from their positions or be
lost altogether. This can also happen to lightships.
Become familiar with soundings, rise and fall of the tides, and the
like, in the neighborhood in which you intend to anchor. If possible
choose an anchorage that will enable you to get bearings from two or
three fixed points on shore. As soon as possible after anchoring secure
your bearings by pelorus and have them checked up by the quartermaster
at regular intervals. This will determine how much, if any, dragging has
taken place.
Lastly, always remember that no amount of advice can make up for your
own carelessness. Hold yourself ready for any emergency, keep cool, keep
patient and keep pleasant. Common sense is the best antidote in the
world for strange situations. If you have that, and the knowledge you
should have secured from these lectures, you cannot go far wrong.
While the day's work which follows does not include every sight in
regular sequence as given in the above discussion, it will give a fair
idea of the navigator's work during a day's run. Put it in your
notebook. (Note to Instructor:--Spend the rest of the period in
explaining carefully each step of this example.)
A DAY'S WORK AT SEA
Departure taken from noon position Jan. 25th, 1919, in Latitude 30° 01'
N, Longitude 73° 47' 20" W. Course p.s.c. NE 3/4 N. Deviation 2° E.
Variation 4° W. Log registered 20. Ship continued on this course until
about 6:30 P.M. when log registered 98 and observed altitude of Star
Rigel, East of meridian, 39° 36' 20". WT 6h 33m 19s P.M. C-W 4h 55m 04s.
CC 2m 16s slow. HE 37 ft. IE 0' 20" off arc.
Changed course to 40° (true) and steamed until 3 A.M. when log
registered 198. At this time ship ran into heavy NE gale. Slowed down to
7 knots per hour until about 8:30 A.M. when observed (_) 18° 25' 10"
and bearing by compass S 46° E. Variation 7° W. WT 8h 31m 16s A.M. C-W
4h 55m 04s. IE + 0' 10". Same HE, CC.
Ship then steamed on true course of 39° at 7 knots per hour until noon
when log registered 261 and observed meridian altitude (_) 37° 59' S.
Same IE (+ 0' 10"), HE, CC. Required:--
1. Position by D. R. at noon.
2. Position by observation at noon (corrected for Longitude by a
factor).
3. Deviation of compass at 8:30 A.M.
4. Watch Time of Local Apparent Noon.
(See Next Page)
At Sea, Jan. 25, 1919. Lat. in 30° 01' N Lo. in 73° 47' 20" W
Steamed until 6:30 P.M.
==========================================
Course | Dist. | D. Lat. | Dep. | D. Lo.
------------------------------------------
N 35° E | 78 | 63.9 | 44.7 | 51.5
==========================================
Lat. Left 30° 01' N Lo. Left 73° 47' 20" W
D. Lat. 1 03 54 N D. Lo. 51 30 E
------------ ------------
Lat. in 31° 04' 54" N Lo. in 72° 55' 50" W
Mid Lat. 30° 30' N
----------------------------------------------------------------
At 6:30 P.M. Obs. * Rigel.
Obs. Alt. * 39° 36' 20" W.T. 6h 33m 19s Log hav "t" 8.92502
E + 00 20 C.W. 4 55 04 Log cos Lat. 9.93269
------------- C.C. 2 16 Log cos. Dec. 9.99544
39° 36' 40" ----------- --------
HE - 7 09 G.M.T. 11h 30m 39s Log hav S 8.85315 Nat hav S .07130
------------- (.).R.A. 20 14 58 Nat hav L--D .11349
T.C.A. * 39° 29' 31" (+).C.P. 1 53.3 ------
------------- ----------- Nat hav ZD .18479
Dec. of * 8° 17' 42"S G.S.T. 31h 47m 30.3s 50° 55' 00" ZD
Lat. 31 04 54 N --W.Lo. 4 51 43 -90 00 00
------------- ----------- ------------
L.~D. 39° 22' 36" L.S.T. 2h 55m 47.3s 39° 05' 00" C.Alt.
------------- *R.A. 5 10 41 39 29 31 T.Alt
----------- ------------
"t" 2h 14m 54s 24' 31" Alt. Diff. Toward.
[Illustration: TZ N 135° E, S 45° E]
================================================
Course | Dist | D. Lat. | Dep. | D. Lo.
------------------------------------------------
S45° E | 24.5 | 17.3 | 17.3 | 20
================================================
Lat. Left 31° 04' 54" N Lo. Left 72° 55' 50" W
17 18 S 20 E
------------- -------------
Lat. in 30° 47' 36" N Lo. in 72° 35' 50" W
Changed course to 40° (true)
===============================================
Course | Dist | D. Lat. | Dep. | D. Lo.
-----------------------------------------------
N40° E | 138.5 | 106.1 | 89 | 105
===============================================
Lat. Left 30° 47' 36" N Lo. Left 72° 35' 50" W
1 46 06 N 1 45 E
------------- -------------
Lat. in 32° 33' 42" N Lo. in 70° 50' 50" W
8.30 A.M.
(_) 18° 25' 10" W.T 20h 31m 16s Log hav "t" 9.28284
IE + 10 C-W 4 55 04 Log cos Lat. 9.92573
---------- C.C. 2 16 Log cos Dec. 9.97595
18° 25' 20" ----------- -------
HE + 7 34 G.M.T. 1h 28m 36s Log hav S 9.18452 Nat hav S .15294
---------- Eq.T - 12 32 Nat hav L~D .18841
-()- 18° 32' 54" ----------- ------
G.A.T. 1h 16m 04s 71° 30' 00" ZD Nat hav ZD .34135
Dec. 18° 53' 24" S -W.Lo. 4 43 51 -90 00 00
Lat. 32° 33 42 N ----------- -----------
------------ L.A.T.(t) 20h 32m 13s 18° 30' 00" C.Alt.
L~D 51° 27 06" 18 32 54 T.Alt.
-----------
2' 54" Toward
[Illustration: TZ N 128° E S 52°E]
===========================================
Course | Dist. | D.Lat. | Dep. | D.Lo.
---------|-------|--------|--------|-------
S 52° E | 3 | 1.8 | 2.4 | 3
===========================================
Lat. Left 32° 33' 42" N Lo. 70° 50' 50" W
1 48 S 3 E
----------- -----------
Lat. in 32° 31' 54" N Lo. in 70° 47' 50" W
Bearing of Sun by Compass S 46° E
True Bearing of Sun S 52° E
-------
Total Error 6° W
Variation 7° W
-----
Deviation 1° E
==========================================
Course | Dist. | D. Lat | Dep. | D. Lo.
---------+-------+--------+------+--------
N 39° E | 24.5 | 19 | 15.4 | 18.4
==========================================
Lat. Left 32° 31' 54" N Lo. Left 70° 47' 50" W
19 N 18 24 E
------------ ------------
Lat. in 32° 50' 54" N Lo. in 70° 29' 26" W
_At Noon._ L.A.T. 00h 00m 00s
(_) 37° 59' S + W. Lo. 4 41 58
-----------
IE + 10 G.A.T. 4h 41m 58s
------------ E.T. (sign reversed) + 12 34
37° 59' 10" -----------
HE 8 55 G.M.T. 4h 54m 32s
17
----------
-(-)- 38° 08' 22" S
51° 51' 38" N (ZD)
18 51 12 S (Dec.)
----------
33° 00' 26" N. Lat. in at Noon.
Lo. Factor = .93 Lat. by Obs. 33° 00' 26" N
Lat. Diff. 9.5 Lat. by D.R. 32° 50' 54" N
------ ------------
Diff. Lo. 8.83 Lat. Diff. 9' 32"
Since Obs. Lat. is East of D.R. Lat. on Line of Pos.
Lo. Diff. is applied East.
/ Lo. by D.R. 70° 29' 26" W
/ Lo. Diff. 8 50 E
Obs. Lat. * ------------
/ Lo. in at Noon 70° 20' 36" W
D.R. Lat. *
/
/
_By Dead Reckoning from Noon to Noon_
===========================================================
Course | Dist. | D. Lat. | Dep. | D. Lo.
----------+---------+-----------+--------+-----------------
N 35° E | 78 | 63.9 | 44.7 | 176' or 2° 56'
N 40° E | 138.5 | 106.1 | 89. |
N 39° E | 24.5 | 19. | 15.4 |
| | ----- | ---- |
| | 189. | 149.1 |
===========================================================
Lat. left. 30° 01' N Lo. left 73° 47' 20" W
3 09 N 2 56 E
--------- -------------
Lat. in 33° 10' N Lo. in 70° 51' 20" W
Min. Lat. 31° 35'
The Watch Time of Local Apparent Noon
Date--Jan. 26, 1919
G.A.T. of A.M. Sight 1h 16m 04s
Lo. in T. of A.M. sight 4 43 12
-----------
L.A.T. 20h 32m 52s
Total Time to Noon 3h 27m 08s
Course to Noon -- 39°
Change in Lo. per hr. -- 5.2', 21s
3600s + 21s = 3621s
3 X 60 = 180
27
-----
207
60
------
12,420
08
--------
3621 ) 12,428 (3.43 hrs
10,863
------
15,650
14,484
------
11,660
10,863
------
3.43 hrs = 3h 25m 48s
W.T. of A.M. sight 8 31 16 A.M.
----------
W.T. of L.A.N. 11h 57m 04s A.M.
ANSWER.
_By D.R._
Lat. 33° 10' N Lo. 70° 51' 20" W
_By Observation_
Lat. 33° 00' 26" N Lo. 70° 20' 36" W
_Dev. at 8:30 A.M._
1° E
_W.T. of L.A.N._ 11h 57m 04s A.M.
FRIDAY LECTURE
DAY'S WORK
At sea, November 28th, 1918. Departure taken at noon in Latitude 20° 50'
N, Longitude 73° 15' 20" W. Log at noon registered 34. Sailed on course
p.s.c. 73°, Deviation 3° E, Variation 1° W until twilight when log
registered 152.
Changed course to E 3/4 N p.s.c. and observed altitude of Star
Aldebaran, East of meridian 37° 10' 10" and bearing by compass N 89° E,
Variation 2° W. WT 8h 10m 16s. C-W 4h. 51m 30s. CC 4m 08s slow. IE 1'
10" off the arc. HE 38 ft.
Ship steamed on this course, in heavy fog and rain, until 2:30 A.M. when
log registered 200. Ship changed course to E 1/2 N (true) and steamed at
8 knots per hour until 6:30 A.M. when weather cleared and observed
altitude Star Polaris 21° 04' 20" N. WT 6h 35m 47s A.M. C-W 4h 51m 30s.
Same IE, HE and CC.
Ship continued on same course and speed until about 9:30 A.M. when
observed altitude of (_) 38° 45' 20". WT 9h 39m 10s A.M. C-W 4h 51m 30s.
Same IE, HE and CC.
Ship then steamed a true course of 93° at a rate of 10 knots per hour
until noon when log registered 294 and observed meridian altitude (_)
46° 49' 30". Same IE, HE and CC.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a
factor).
3. Deviation at 8:10 P.M.
4. Watch Time of Local Apparent Noon.
SATURDAY LECTURE
DAY'S WORK
At sea, April 21st, 1918. Departure taken from noon position in Latitude
31° 50' N, Longitude 76° 30' 31" W. Log registered 128 at noon.
Course p.s.c. until about 4:30 P.M. was N 10° E. Deviation 1° W.
Variation 3° W. At about 4:30 P.M. observed altitude of sun's lower limb
25° 13' 10" and bearing by compass N 87° W. WT 4h 26m 46s. C-W 5h 04m
52s. CC 1m 03s slow. IE 0' 10" off arc. HE 29 ft. Log registered at this
time 188.
Course was then changed to NE x N (true). Weather cloudy. At about
twilight clouds broke away and observed altitude of Star Procyon West of
meridian 40° 01' 00". CT 1h 37m 28s A.M. CC 1m 03s slow. IE 0' 10" off
arc. HE 29 ft. Log registered 236. Continued on same course until
midnight, at which time log registered 290. At midnight ship ran into
dense fog and slowed down to 8 knots until about 6:30 A.M., when fog
blew away and observed altitude of Star Polaris 35° 29' 10" N. WT 6h 32m
14s A.M. C-W 4h 59m 02s. IE 0' 10" off arc. CC 1m 03s slow. HE 29 ft.
From 6:30 A.M. ship steamed a true course of N 32° E until noon at a
rate of 15 knots per hour, at which time a meridian altitude of the (_)
was observed 65° 37' 10" S. Log registered 424. HE 29 ft. IE 0' 10" off
arc. CC 1m 03s slow.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a
factor).
3. Deviation of Compass at 4:30 P. M. sight.
4. Watch Time of Local Apparent Noon.
WEEK VIII--NAVIGATION
MONDAY LECTURE
DAY'S WORK
At sea, Nov. 12th, 1918. Departure taken from noon position in Latitude
39° 40' N, Longitude 33° 20' 04" W. Log registered at noon 1. Course
p.s.c. was 294° until about 3:30 P.M., Deviation 1° W, Variation 24° W,
at which time observed altitude (_) 13° 55' 10" and bearing by pelorus S
79° W. WT 3h 22m 18s. C-W 2h 13m 20S. CC + 1m 10s. IE 1' 10" off arc. HE
32 ft. Log registered 46.
Course was then changed to 290° p.s.c. until about 6:30 P.M. when
observed altitude Star Polaris 40° 15' 40" N. WT 6h 32m 18s. C-W 2h 13m
20s. Same HE, IE, CC. Log registered 90.
Ship steamed on same course until 1:30 A.M. when log registered 196. At
1:30 A.M. sighted sub. on port bow. Ordered full speed ahead and made 17
knots per hour until 8 A.M. when observed altitude (_) 8° 40' 00". WT 8h
01m 30s A.M. C-W 2h 13m 20s. Same HE, IE, CC.
Ship then steamed a true course of 272° at a rate of 15 knots per hour
until noon, at which time observed meridian altitude (_) 32° 35' 40" S.
Same HE, IE, CC. Log registered 362.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a
factor).
3. Deviation of Compass at 3:30 P.M.
4. Watch Time of Local Apparent Noon.
TUESDAY LECTURE
DAY'S WORK
At sea, Dec. 10th, 1918. Departure taken from Latitude 19° 50' N,
Longitude 20° 01' 20" W. Noon position. Log registered 20. Course p.s.c.
N 16° E. Deviation 2° E, Variation 18° W. Ship steamed on this course
until 8 P.M. when changed course to N 18° E p.s.c. and observed altitude
Star Polaris 22° 33' 14" N. WT 8h 09m 10s. C-W 1h 20m 05s. CC 2m 00s
fast. IE none. HE 39 ft. Log registered 104. Ship then steamed at 14
knots per hour until midnight. At midnight changed course to N 14° E
p.s.c. and steamed at 12 knots per hour until 4 A.M. At 4 A.M. slowed
down to 9 knots per hour and steamed at that rate until 8:30 A.M. when
course p.s.c. was changed to N 17° E and observed altitude (_) 22° 40'
30". WT 8h 34m 16s A.M. C-W 1h 20m 05s. Same IE, HE, CC. Sun bore by
compass S 65° E, Variation 18° W. Continued on this course p.s.c. for
two hours, speed 12 knots. Thence steamed a true course of 4° at same
speed to noon when observed meridian altitude (_) 42° 36' 50" S. Same
IE, HE, CC.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude
by a factor).
3. Deviation of Compass at 8:30 A.M.
4. Log reading at noon.
WEDNESDAY LECTURE
DAY'S WORK
At sea, July 19th, 1918. Departure taken from Latitude 40° 30' N,
Longitude 45° 00' 10" W. Noon position. Log registered at noon 68.
Steamed until 2:30 P.M. on a course p.s.c. 115°. Deviation 1° W.
Variation 25° W. Log registered 125. Changed course to 118° p.s.c. (Same
Variation and Deviation) and steamed until about 5 P.M. At about 5 P.M.
observed altitude (.) 22° 40' 20" and bearing by pelorus N 55° W. WT 5h
01m 16s. C-W 3h 00m 02s. IE 2' 20" on arc. CC 3m 32s slow. HE 32 ft. Log
registered 168. Course p.s.c. was then changed to 113° until about 8
P.M. when observed meridian altitude of Star Vega 88° 15' 10" S. WT 8h
02m 26s. C-W 3h 00m 02s. Same HE, IE, CC. Log registered 210. Continued
on same course p.s.c. until about 2 A.M. when observed altitude of Star
Vega 47° 19' 20" West of meridian. WT 2h 04m 24s A.M. C-W 3h 00m 02s.
Same IE, HE, CC. Log registered 299. TZ of Star N 86° W. Continued on
same course until 4 A.M. when log registered 329.
At 4 A.M. heavy fog and rain forced ship to slow down to 5 knots per
hour until about 9 A.M. when weather cleared and observed altitude (_)
51° 52' 40". WT 9h 03m 18s A.M. C-W 3h 00m 02s. Same HE, IE, CC. Thence
ship steamed a true course of 88° at a rate of 13 knots per hour to
noon, when log registered 394 and observed meridian altitude (_) 69° 52'
20" S. Same IE, HE, CC.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by
a factor).
3. Deviation of Compass at 5 P.M.
4. Watch Time of Local Apparent Noon.
THURSDAY LECTURE
DAY'S WORK
At sea, Nov. 25th, 1918. Departure taken at noon from Latitude 25° 05'
N, Longitude 37° 10' 40" W. Log registered at noon 32. Course p.s.c. was
119°, Deviation 2° E, Variation 19° W until twilight when log registered
110 and observed altitude Star Polaris 25° 30' 40" N. IE 1' 10" off arc.
HE 28 ft. WT 5h 40m 18s. C-W 2h 28m 11s. CC 4m 15s slow. Changed course
to SE x E 3/4 E, same Variation and Deviation, and steamed on this
course until about 8:30 P.M. when observed altitude of Star Markab,
West of meridian, 59° 48' 10". Log registered 157. WT 8h 34m 48s. C-W 2h
28m 11s. Same HE, IE, CC. Steamed on same course until midnight when log
registered 210. Changed course to 110° p.s.c. (same Variation and
Deviation), and steamed at 12 knots speed until about 8 A.M. when
observed altitude (_) 23° 05' 10" and bearing by compass S 33° E.
Variation 19° W. WT 8h 04m 10s, A.M. C-W 2h 28m 11s. Same HE, IE, CC.
Continued on same course p.s.c. at a speed of 15 knots per hour until
noon when observed meridian altitude (_) 44" 30' 50" S. Same IE, HE, CC.
Log registered 366.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a
factor).
3. Deviation of compass at 8:04 A.M.
4. Watch Time of Local Apparent Noon.
ADDITIONAL LECTURE
COMPASS ADJUSTMENT
The aim of this lecture is to give you a very few facts about magnetism
in general and compass adjustment in particular. The reason for
including the lecture in this book is because of repeated requests on
the part of graduates who have been consulted about the adjustment of
the compass on their ships and who have realized that their advice might
have been more helpful if they had learned more about the matter while
at this school.
The earth is a huge magnet. It is the effect of the magnetism in the
earth upon the compass needle which causes the compass error and makes
it necessary to correct it. How can it be corrected? To know that we
must first know the fundamental law of magnetism, namely, that opposite
poles of two magnets attract each other and similar poles repel each
other. From which it follows that if we decide to color red, for
instance, that end of a magnetic needle which points to North, the
magnetism of that part of the earth must be considered blue, i.e., of
opposite magnetism to the north-seeking end of the red magnetic needle.
Now, there are various kinds of magnetism which affect a ship's compass.
One is from the earth, another from the iron in the ship, etc. To
discuss them and, the theoretical cause of them in detail is beyond the
scope of this lecture. To correct them, four sets of magnets are
necessary, two of which are usually found in the binnacle of the compass
itself. One is a fore and aft magnet or set of magnets, the other an
athwartship magnet or set of magnets. The third set consists of the two
globes of cast iron placed on either side of the compass bowl (called
Quadrantal Correctors). The fourth magnet, or set of magnets, is to
correct the compass in case of severe heeling by the ship.
If you are ordered to adjust the compass the first thing to do is to
choose a fine day with smooth water. Take your ship to a certain spot,
the exact location of which you have found from the chart, and where
you are certain you will have plenty of sea-way in which to swing. Set
your watch to local apparent time (which you have calculated before
coming out). Take from the Azimuth Tables the sun's true bearing for
every four minutes of the time during which you will be occupied
adjusting, and convert it into the magnetic bearing by applying the
variation at the place selected (secured from the chart). Write down in
a small book these times and corresponding magnetic bearings.
Now go to your compass and see that its lubber line is exactly fore and
aft and in the keel line of the ship. Have another officer who is
thoroughly familiar with the pelorus stand by it as the ship is swung.
All being ready, secure the lubber's point of the pelorus at North and
clamp the sight vane to the sun's magnetic bearing at the time you have
figured to take the first heading. Starboard or port your helm until at
the time calculated the reflection of the sight vane on the pelorus dial
cuts on the proper magnetic bearing. The vessel's head will then be
pointing to magnetic North. If, now, the compass were correct it would
agree with the pelorus in showing the ship's head to be North. If it
does not do so, there is Deviation in the compass and its amount is the
amount of Deviation on that particular course. Suppose the Deviation
were to starboard, i.e., Easterly, and were due to magnetism in the
ship's starboard side. Then, if the magnetism in the North end of the
needle be considered red, the magnetism in the starboard side of the
vessel, in order to attract the red end of the needle, would be
considered blue and the ship's magnetism, with the compass needle
included, would look like this:
[Illustration]
To counteract this blue attractive force on the starboard side, screw up
the athwartship magnet in the binnacle toward the compass dial. Its
magnetism, if it were laid on the deck, would look like this:
[Illustration]
In other words, as this magnet is moved nearer the compass needle, by
the law of magnetism just given, the red end of the magnet repels the
red end of the compass needle from the starboard side and the blue end
of the magnet attracts the red end of the compass needle toward the port
side. When the compass needle points to North, as shown by the correct
pelorus bearing, the Deviation on this heading (i.e., North) is
corrected.
Now turn the lubber line of the pelorus to East. Steady the ship on this
heading until the shadow from the pelorus vane at the proper L.A.T. cuts
the circumference of the pelorus dial at the proper magnetic bearing.
The ship's compass should then show the ship's head pointing to East.
Suppose that it does not (as will usually be the case) but points to the
right of East. Then the ship's magnetism and compass would look like
this:
[Illustration]
To bring the compass needle back to North it would be necessary to move
up nearer the compass dial the fore-and-aft magnet (shown below), whose
magnetism would act on the compass needle on this heading of the ship
exactly as the athwartship magnet acted on the compass needle when the
ship was headed North:
[Illustration]
Now the ship's compass has been corrected for the North and East
headings respectively. The next correction is for the heading half way
between, i.e., North-east. If there is any Deviation on this heading,
adjust the cast iron cylinders (called Quadrantal Correctors), which are
on each side of the compass bowl, by moving them toward or away from the
compass until the ship's head by compass is North-east at the proper
time and bearing by pelorus.
The ship's compass has now been corrected for one whole quadrant,
namely, from North to East, and this will suffice for all four quadrants
since the relationships of the magnets themselves and the magnetism of
the compass needle is the same for any of the other three quadrants as
for the first. Compass adjustment, however, can never be absolutely
accurate. For that reason, it is wise to steam the ship completely
around, steadying on every fifteen degrees by pelorus to determine and
keep a record of remaining errors.
There is one more correction to make, i.e., for the heeling error. This
correction is necessary in case the ship is yawing in a sea-way so much
that the relationship of the ship's magnetism to the compass needle is
decidedly different from what it is when the ship is on a comparatively
even keel. It is compensated by a vertical magnet directly underneath
(or over) the binnacle, details in regard to which can be secured from
Bowditch Art. 125, p. 53.
It must be borne in mind that compass adjustment is not an exact
science, that an adjustment for one latitude is not correct for another,
that anyone of a hundred different causes can affect the magnetism of
the ship or of the compass needle, which in turn directly affects the
Deviation. In this connection, it would be well to read Bowditch Art.
129, p. 55. You should also read Arts. 119-130 in which are given, more
fully and in more scientific language, the contents of this lecture.
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