Page:Popular Astronomy - Airy - 1881.djvu/104

90 the centre of the sphere. Now, our use of the celestial globe or sphere is founded on the assumption that it is a representation of the heavens, on the supposition that the eye of the observer is at the centre of the sphere. This circle, then, in which the sun appears to move, being one whose plane passes through the centre of the sphere, or through the eye of the observer; it comes to this, that the sun appears to move around the earth in a plane, or that the earth moves around the sun in a plane.

If the sun moves round the earth, we have only to suppose that the earth stands with its centre stationary, but that it is whirling round its axis, and that the sun travels round and round. If you suppose that the earth travels round the sun, it is necessary to suppose that the earth's axis retains its parallelism to itself without any respect to the sun. Now, is it likely that the axis of the earth would remain parallel to itself without respect to the sun? In my former lecture, I called your attention to the motion of a quoit and a top, in order to show you the strong tendency which rotatory motion has to maintain the position of its axis unaltered. It is the quoit whose motion has the most striking analogy to the motion of the earth. The quoit is not impeded by contact with the floor as the top is. This has been made the subject of mathematical investigation, as well as of experiment, and the result of both is, that the earth, if revolving round the sun, would carry its axis of rotation always parallel to one line, as we see it does.

To this I may add one remark. Geologists have observed that important changes have taken place in the climates of different parts of the earth. Some have supposed that the axis of the earth must have changed its position; but there is no greater impossibility