Page:The Solar System - Six Lectures - Lowell.djvu/106



What this force may be, we shall now proceed to ascertain.

In order to make the action in the case, complicated at best, as understandable as possible, I shall begin by considering what causes the precession of the equinoxes, or that slow rotation of the pole of the Earth round the pole of the ecliptic.

Were the Earth a sphere, its axis would maintain an invariable position in the heavens, since any other body would act upon it as if all its matter were collected in its centre; but with a spheroid the case is different. We may consider the equatorial protuberance as a ring of matter fastened after the manner of a life-preserver around the Earth's waist. Now suppose the Earth tilted up from the line joining its centre and the centre of the attracting body. That body would tend to pull the nearer part of the ring down into its plane and the more distant portion of the same up into the same plane, and the result would be, if the Earth were not rotating, a swing round an axis at right angles to the line joining the centres of the two bodies, which would, after a few oscillations, bring the equatorial bulge to rest in the orbital plane of the outside body.

Now suppose the Earth to be rotating at the time the pull is applied; then the simultaneous