Page:Popular Astronomy - Airy - 1881.djvu/215

Rh spoken tends to pull it downwards. And as the rotation goes on, the forces still tend to pull it downwards and downwards, until at last it comes to meet the plane of the ecliptic at a point sooner than it otherwise would. This takes place with regard to the attraction of the sun on every part of this protuberance. The tendency of this force is to bring every part which happens to be above the ecliptic lower and lower towards the ecliptic; and to make its path intersect the circle a e b, Figure 48, sooner than it otherwise would. It amounts to this, that at the earth's equator the motion of each point is affected by such forces, that it tends constantly to come to its intersection with the plane of the ecliptic sooner than it otherwise would; or, to speak in other words, that intersection travels backwards to meet the rotatory motion of the earth. The same thing (as I fully explained) will happen if we consider the action of the sun on the distant parts of the earth, which I represented as being equivalent to a pushing force.

I then mentioned to you that the moon produces a larger part of precession than the sun does, although the moon is so very much smaller than the sun, (only $1⁄20,000,000$ part of the sun). She is, however, 400 times nearer than the sun; and this makes her whole attraction, in proportion to her mass, 160,000 times as great as the sun's; still her whole attraction is only $1⁄120$ of that of the sun. But the important thing to be remarked in the explanation above given is, that precession is not produced by the whole attraction of the sun or moon upon the earth, but by the difference between the attractions which they exert upon the earth's centre and upon the earth's nearest surface. For the moon, the proportion of the distance of these parts in nearly as 60 to 59, and then the Difference of