Page:Popular Astronomy - Airy - 1881.djvu/195

Rh the same. The sun's greater attraction upon any part of the protuberance when nearest to the sun, at which time that part is, in the case of Figure 47, below the ecliptic, tends to raise it towards the plane of the ecliptic, and therefore it cuts the ecliptic sooner than otherwise it would. And the sun's smaller attraction upon it when furthest from the sun, producing the effect of a pushing force upon it when above the ecliptic, tends to made it descend to cut the plane of the ecliptic sooner than it otherwise would, and therefore, in both halves of the diurnal rotation (as at the winter solstice) the place of intersection of the earth's equator with the ecliptic will move in the direction opposite to the earth's rotation.

At the equinoxes, the plane of the earth's equator passes through the sun, and then the sun's action does not tend to tilt the earth at all, and consequently does not tend to alter the position of its equator at all; but at all other times the sun's action produces a motion, greater or less, of the intersection of the earth's equator with the plane of the ecliptic, in a direction opposite to the direction of the earth's rotation. And this is the motion called the Precession of the Equinoxes.

It is to be observed, that the principal part of precession is not produced by the sun, but by the moon. The moon's mass is not a twenty-millionth part of the sun's; but she is four hundred times as near as the sun. Still she does not pull the earth, as a mass, with more than a hundred-and-twentieth part of the sun's force. But, because the difference of the distances of the different parts of the earth from the moon bears a greater proportion to the whole distance than for the sun, the differential effects of the moon in pulling the near parts of the earth from the earth's