Page:Popular Astronomy - Airy - 1881.djvu/244

230 may be able to give you some notion of it. It is that which is called the Moon's Variation.

In Figure 59, suppose E to be the earth, M'M"M'"M"" the moon's orbit, and C the sun. The sun, by the law of gravitation, attracts bodies which are near with greater force than those which are far distant from it. Therefore, when the moon is at M' the sun attracts the moon more than the earth, and tends to pull the moon away from the earth. When the moon is at M'" the sun attracts the earth more than the moon, and therefore tends to pull the earth from the moon, producing the same effect as at M' or tending to separate them. When the moon is at M" the force of the sun on the moon is nearly the same as the force of the sun upon the earth, but it is in a different direction. If the sun pulls the earth through the space Ee, and if it also pulls the moon through the space M"m, these attractions tend to bring the earth and the moon nearer together, because the two bodies are moved as it were along the sides of a wedge which grows narrower and narrower. Thus, at M' and M'" the action of the sun tends to separate the earth and the moon, and at M" and M"" the action tends to bring the earth and the moon together.

You might perhaps infer from this that the moon's orbit is elongated in the direction M' M'″. No such thing: the effect is exactly the opposite. The fact really is, that the moon's orbit is elongated in the direction M" M"". And if you consider what has been