Page:Popular Astronomy - Airy - 1881.djvu/246

232 the amount of this effect is modified in some degree by the change which the earth's attraction undergoes in consequence of the change of the moon's distance, (the earth's attractive force varying inversely as the square of the moon's distance) but still the reasoning applies with perfect accuracy to the kind of alteration which is produced in the moon's orbit.

This particular inequality was discovered by Tycho Brahe before gravitation was known; and it underwent an examination and was explained by Newton as a result of gravitation. There are other perturbations even more important than this, (the Progression of the Apse, the Evection, the Annual Equation,) of which I only mention the names; they were discovered before gravitation was known, and they were most fully and accurately explained by gravitation.

The next point which I shall mention is this: that the planets disturb one another generally. For it is to be remarked, that the attraction of planets is not confined to the sun, although, in consequence of the sun's very great magnitude and very great attraction, accurate and long-continued observations may be necessary for discovering the comparatively small effect of the planets. But the law of gravitation asserts that every particle of matter attracts every other particle, and therefore every planet attracts every other planet; and therefore the motions of the planets are not exactly the same as if only the sun attracted them. The differences of the real movements from the movements computed on the supposition that only the sun attracts the planets are the perturbations or disturbances of the planets. These disturbances are exceedingly complicated. In fact there is nothing in science which presents the degree of complication that these perturbations of the