f 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 projec