THE
WHYS AND WHEREFORES
OF
NAVIGATION
THE
WHYS AND WHEREFORES
OF
NAVIGATION
BY
GERSHOM BRADFORD 2d
_Navigating Officer and Senior Instructor New York State Nautical
Schoolship “Newport,” Late Nautical Expert,
Hydrographic Office, U. S. N._
_ILLUSTRATED_
[Illustration: colophon]
NEW YORK
D. VAN NOSTRAND COMPANY
25 PARK PLACE
1918
Copyright, 1918, by
D. VAN NOSTRAND COMPANY
PRESS OF
BRAUNWORTH & CO.
BOOK MANUFACTURERS
BROOKLYN, N. Y.
TO THE
AMERICAN SUB-PATROL
WHOSE WAR ZONE RECORD OF SKILL AND DARING
HAS THE AUTHOR’S PROFOUND RESPECT
AND ADMIRATION
PREFACE
When taking into consideration the number of excellent works now
published on navigation there would seem to be a small chance of
finding a different angle from which to discuss the subject. The
purpose of the majority of such books is to give instruction to
beginners. This book, however, is written particularly for the men who
do their work mostly by rote and wish to know more of the reasons; or,
perhaps, for schoolship graduates who may here find a chance to extend
their horizons.
I have not considered it desirable to avoid repetition and in order
to closely follow a line of thought have freely repeated many points
already taken up. This has in a number of cases avoided the distraction
of seeking a page of reference elsewhere.
I have had in mind that it serve as supplementary reading to the
American Practical Navigator, Bowditch, that great bulwark of
navigation which for over a hundred years has protected American ships
through every deed of valor and every commercial adventure. It is
placed beyond criticism by its venerable name and its remarkable record
and, as a reference book for the navigator, it stands without a peer,
but as a text book it founders the student.
If to such mariners as these a little insight is given to the “Whys and
Wherefores” of their work, I shall be well repaid for the work of many
watches below.
These discussions appeared several years ago, in a less extended form,
in the Master, Mate and Pilot, the magazine formerly published by the
American Association of Masters, Mates and Pilots.
I have freely consulted the following standard works: American
Practical Navigator, Bowditch; Wrinkles in Practical Navigation, Lecky;
The Theory and Practice of Navigation, Dunraven; General Astronomy,
Young; Navigation and Compass Adjustment, Muir; Guide to the Marine
Board’s Examinations, Reed.
I have received and am grateful for very valuable help and suggestions
from Mr. George W. Littlehales, Hydrographic Engineer, U.S.N., Mr.
Felix Riesenberg, C.E., Commander New York State Schoolship Newport,
and George A. Collie (deceased), Nautical Expert, Hydrographic Office,
U.S.N.
G. B.
NEW YORK, April 15, 1918.
CONTENTS
PAGE
CHAPTER I
INTRODUCTORY REMARKS 1
CHAPTER II
NAUTICAL ASTRONOMY 5
CHAPTER III
DECLINATION AND RIGHT ASCENSION, INCLUDING PRECESSION 15
CHAPTER IV
TIME 33
CHAPTER V
CORRECTIONS FOR OBSERVED ALTITUDES 68
CHAPTER VI
LATITUDE 78
CHAPTER VII
AZIMUTHS AND AMPLITUDES 93
CHAPTER VIII
LONGITUDE 101
CHAPTER IX
SUMNER METHOD, INCLUDING NEW NAVIGATION 107
CHAPTER X
THE MOON 139
CHAPTER XI
CHARTS 146
INDEX 161
THE WHYS AND WHEREFORES
OF NAVIGATION
CHAPTER I
Introductory Remarks
In embarking upon the study of navigation familiarity with the compass
is the first logical step: the quick mental conversion of a course or
direction given in points to the same direction in degrees expressed
in quadrants as S. 35° E., and again into a system by which direction
is indicated by degrees from 0° to 360°. A mariner will encounter all
three of these systems and will find constant conversion necessary
back and forth for various purposes. The 0° to 360° system is the most
up-to-date and the simplest form of handling direction.
Following the compass may be taken up the use and description of
other nautical instruments with which every mariner is supposed to be
familiar.
Dead reckoning is the first calculation to appear and this involves
the correction of the compass courses back and forth between true,
magnetic and compass directions. This is dealt with under Azimuths
and Amplitudes. In practical navigation a vessel commences her voyage
and attempts to sail in a certain direction, but the well-known
elements of compass error, variation and deviation, current, wind, seas
and poor steering all divert the vessel from the projected course.
In dead reckoning a navigator strives to keep track of his position
by keeping a record of actual courses steered and distances run. He
then is obliged to guess at the amount the vessel has diverted both
in direction and distance until an astronomical observation sets him
straight again. It is here particularly shown that navigation becomes
an art of estimating position and the better the navigator’s bump of
locality, the greater his success. This is a peculiar gift and usually
is born in the man, at least it cannot be learned from books. The
process of finding latitude and longitude by dead reckoning is supposed
to be already well known to the reader and will not be detailed at
length.
However, every course angle is laid off from a meridian (which is true
N. and S.) and terminates in a parallel of latitude. This meridian
and parallel intersect at right angles; hence these with the distance
run (which is the hypothenuse) form a plane right angle triangle,
plane because the curvature of the earth is not considered in short
distances. To solve this triangle, we have the course angle and one
side--the distance run. With these the other two sides are easily
found by computation, but more easily by tables No. 1 and 2, Bowditch.
The side along the meridian is represented by the column headed Lat.
(difference of latitude) and the side lying in the parallel is in the
column headed Dep. (departure). Thus the values of the sides of the
triangle are given in miles and tenths, showing the distance good
made N. or S. and E. or W. It will be noted that at the top of the
pages of these tables are four different courses and at the bottom are
likewise four courses making the same page serve for eight different
courses. This is accomplished by the fact that triangles formed by
these particular eight courses are the same in _shape_. Thus N. 30° E.,
for instance, makes a similar triangle to N. 30° W. (330°); S. 30° E.
(150°); or S. 30° W. (210°). They have identically the same difference
of latitude and departure. If this fact is not clear draw a diagram and
be convinced. In the cases of N. 60° E. (60°); N. 60° W. (300°); S.
60° E. (120°); S. 60° W. (240°), the same shaped triangle as above is
found, but reversed in that what was the difference of latitude side
now has become the departure side. The values of these sides are read
from the bottom of the page and are found in the reverse columns to fit
the reverse triangle. The latitude value read from the top of the page
as 30° becomes a departure value when read from the bottom with 60°.
The subject of Sailings is one of the early problems confronting the
student of navigation and will be considered briefly. The above remarks
on dead reckoning cover the principle of plane sailing, the simple
method where the spherical surface of the earth is ignored and a flat
ocean substituted. This method will not serve for anything but short
distances of a few hundred miles without sufficient error to render it
impracticable. Traverse sailing is a series of plane sailing courses
made, for instance, by a sailing vessel beating to windward.
In parallel sailing the vessel pursues a true E. or W. course and runs
along a parallel of latitude. Thus all her progress is in the terms
of departure with no difference of latitude. As all meridians converge
from the equator towards the poles the length in miles of a degree
of longitude keeps on diminishing as the poles are approached and,
conversely, miles of departure have an increasing value in degrees of
longitude. So in parallel sailing what we desire to know is what is
the value in the particular latitude of our course of our departure
(miles) in ° ´ ´´ of longitude. Having this and applying it to the
longitude left will give the longitude in. Middle latitude sailing is
very similar to parallel sailing in that it is assumed, for the purpose
of getting the difference of longitude, that the whole departure of the
course or courses sailed has been made in the mean or middle latitude,
because the greater value (in the northern hemisphere) in difference of
longitude of a mile northward of the middle latitude is counteracted by
the corresponding lesser value southward of the middle latitude.
Mercator sailing is perhaps the most extensively used, as the Mercator
principle is employed almost universally in the construction of
navigational charts. It is described under Charts in this book.
Also under Charts is a description of the Gnomonic Chart which is also
called the Great Circle Chart and used in Great Circle Sailing, also
referred to in those pages.
The young navigator is counselled never to _know_ where his vessel
is, lest through over confidence he be led into close and dangerous
quarters.