Page:Astronomy for Everybody.djvu/49

In this chapter I have to use and explain some technical terms. The ideas conveyed by them are necessary to a complete understanding of the celestial motions, and of the positions of the stars at any hour when we may wish to observe them. To the reader who only desires a general idea of celestial phenomena, this chapter will not be necessary. I must invite one who wants a knowledge more thorough than this to make a close study of the celestial sphere as it was described in our second chapter. Turning back to our first figure, we see ourselves concerned with the relation of two spheres. One of these is the real globe of the earth, on the surface of which we dwell, and which is continually carrying us around by its daily rotation. The other is the apparent sphere of the heavens, which surrounds our globe on all sides at an enormous distance, and which, although it has no reality, we are obliged to imagine in order to know where to look for the heavenly bodies. Notice that we see this sphere from its centre, so that everything we see upon it appears upon its inside surface, while we see the surface of the earth from the outside.

There is a correspondence between points and circles on these two spheres. We have already shown how the axis of the earth, which marks our north and south poles,