Page:Popular Astronomy - Airy - 1881.djvu/22

 8 own eyes that the. stars really do partake of these motions; that the Polar Star does nearly stand still; that the stars at various distances from the Polar Star, do move round in the way I say, one in a circle of one size, and another in a circle of another size: that others do move round in circles still larger, so that at their lowest points they just touch the north horizon; that others move round in circles so large that the lower part of these circles is lost, whilst the higher part rises above the horizon. It is of importance that anybody, who wishes to understand Astronomy thoroughly, should look out, and see for himself, that these things do happen in the way I have attempted to describe; by the observations so made, he will acquire a conviction of the truth far deeper and more lasting than from anything that can be pointed out in a course of Lectures.

From observing the way in which these motions take place, that we may assume one point of the sky as a centre, and that the movements of the stars are of such a nature that they will appear to turn round that one centre; the first idea that naturally occurs is, that the starry heavens, as we see them (I do not affix any precise meaning to that term), or a shell in which the stars seem to be fixed, do turn round an axis. It is necessary to show that this is supported by accurate means of observation. Now there is one instrument in use in the best Astronomical Observatories, which is specially intended for the elucidation of this phenomenon—it is the instrument called the Equatoreal. I should be glad if some of the wealthy manufacturers in this town would set up an Equatoreal instrument. The Equatoreal is an instrument, which, in one form, is represented in Figure 1. It turns round an axis