ted 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 deg. 28' 30". It will always be marked
+18 deg. 28' 30" and a south declination will be marked -18 deg. 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 deg. 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 deg. 28' 30".

The Polar Distance of any point is its distance in arc from either pole.
It must, therefore, equal 90 deg. minus the declination, if measured from
the pole of the same name as the declination or 90 deg. 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