Page:Popular Astronomy - Airy - 1881.djvu/243

Rh called the focus of the parabola. This investigation tends most powerfully to confirm the law of gravitation; showing that the same moving object, which at one time is very near to the sun and at another time is inconceivably distant from it, is subject to an attraction of the sun varying inversely as the square of the distance.

But if it is true that every particle of matter attracts every other particle of matter, with a force varying inversely as the square of the distance, the effects of these attractions will be shown in other ways besides influencing the periodic revolutions of planets round the sun, or of satellites round their primaries. For instance, the sun attracts both the earth and the moon, and as they are always either at different distances from the sun or lie in different directions from the sun, they will be differently attracted by the sun; and hence their relative motions will be disturbed. Thus arise the perturbations of the moon's apparent motion. These perturbations naturally divide themselves into several classes; and they had been discovered from observation and divided into these classes, long before the theory of gravitation was invented. One of the first triumphs of the theory was the complete explanation of these classes of perturbation of the moon; and the suggestion of many others, which have been verified by the observations made since that time with more accurate instruments.

Of these applications of the theory of gravitation to explain the different perturbations of the moon a great deal might be said. It is a subject involved in mathematical perplexity beyond anything else that I know. But there is one perturbation of the moon which is of so singular a character that probably I