Page:Popular Astronomy - Airy - 1881.djvu/194

180 its path, (just as in the former half of its rotation) will not be altered. Thus the effect produced by this action of the sun, in both halves of the rotation of this mountain, is, that it comes to the place of intersection with the plane of the circle a b, or with the plane of the ecliptic, sooner than it otherwise would. And whatever number of points or mountains in the protuberant part of the earth we consider, we shall find the same effect for every one; and therefore, the effect of the sun's action upon the entire protuberance will be the same; that is, its inclination to the circle a b or the plane of the ecliptic will not be altered, but the places in which it crosses that plane will be perpetually altering, in such a direction as to meet the direction of rotation of the earth.

I have spoken of this as if the protuberance were the only part to be considered. But in reality, this protuberance is attached to the remaining spherical part of the earth; and the action of the sun on the different masses of that spherical part balances exactly; so that, as regards it, we need not consider the sun's action at all The effect therefore of that spherical part will be, to impede the motion which the protuberant part would otherwise have; not to destroy it, but to diminish it.

On the whole, therefore, the effect of the sun's action on the spheroidal earth will be, that the points at which the earth's equator intersects the plane of the ecliptic move very slowly in the direction opposite to that in which the earth revolves; but the inclination is not altered.

All that I have said here applies to the position of the earth at the winter solstice. But if we consider the earth at the summer solstice, as in Figure 47, we shall find that the effect of the sun's action is exactly