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 deg. 25' 30". IE + 2' 30".
HE 16 ft. Required -(-)-.

2. April 15th, 1919, observed altitude of (_) 58 deg. 29' 40". IE - 2' 30".
HE 18 ft. Required -(-)-.

3. March 4th, 1919, observed altitude of (_) 44 deg. 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 deg. N. You are
practically certain, then, that you are somewhere in latitude 50 deg. 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 a