Page:Popular Astronomy - Airy - 1881.djvu/218

204 of the earth's equator to the ecliptic, and for an equal time it tends to increase the inclination of the earth's equator to the ecliptic; and thus a considerable inequality is produced in the inclination of the earth's equator to the ecliptic.

The general effects of precession will be more easily conceived if, instead of considering the intersection of the equator with the ecliptic, we consider the motion of the earth's axis; as the change in the intersection of the equator with the ecliptic must produce a change in the position of the earth's axis as directed towards the stars. In consequence of this, the real Celestial Pole does change among the stars. The bright star of the Little Bear, for instance, now our Polar Star, is not at the same distance from the Pole now at which it was one hundred years ago. In the time of Dr. Bradley, the Polar Star was more than two degrees from the Pole; now it is one and a half. In the course of a great many centuries the Celestial Pole describes a circle among the stars, and different stars successively take the position of the Polar star. About 4500 years ago, the Polar Star was the bright star in the constellation Draco.

The third subject which I mentioned was the aberration of light; an effect produced by the combination of the earth's motion with the motion of light. I endeavoured to illustrate this in several ways. One was this: if in a summer shower you stand still and watch the rain, you will see it falling in its proper direction; but if you walk forward, you will see the drops falling in an inclined direction, as if they were meeting you; and if you step backwards you will see immediately that the drops of rain appear to be falling as if they were coming from behind you. As another illustration, I supposed that a ship is sailing past a