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 deg. 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 deg..
Hence, having secured the true meridian altitude of the sun, you have
only to subtract it from 90 deg. 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 deg.
|/ |
+------v---------------------- C

A = Zenith, B = Sun, C = Horizon.]

The arc ABC measures 90 deg.. That is the distance from your zenith to the
horizon. Now if BC is the true meridian altitude of the sun at noon,
90 deg.-BC or AB is your zenith distance. If BC measures by sextant 60 deg., AB
measures 90 deg.-60 deg. or 30 deg.. This 30 deg. is your Zenith Distance. Now suppose
that from the Nautical Almanac we fin