Page:Popular Astronomy - Airy - 1881.djvu/254

240 planets motion will be altered in that proportion. We therefore find by trial what must be the proportion to make the calculated place of the disturbed planet agree most exactly with its observed place; and then, having settled to our satisfaction the measure of the disturbing power of Venus, or Mars, &c., we can try in all subsequent observations whether it makes the computed places agree equally well. It is found that they do agree perfectly well.

I shall only add to this that the motions of our moon are sensibly disturbed by the planet Venus. An irregularity which has been discovered by observation, and has puzzled all astronomers for fifty years, was explained about two years ago by Professor Hansen, of Gotha, on the theory of gravitation, as a very curious effect of the attraction of Venus.

We have thus a mass of irresistible evidence to prove that the attraction of the sun upon the planets and upon our moon, of the planets upon their satellites, and of the planets one upon another, do follow the law of gravitation. But now comes another question: how do we know that these attractions are produced by every particle of matter in each of these different bodies, as is asserted by the law of gravitation? To prove this I must refer you to a totally different set of computations and observations. I must make a comparison. of the results of theory with the facts of observation, in some of the cases in which it is necessary to consider one body as attracting separately every particle of another, or to consider every particle of one body as separately attracting another body, or to consider every particle as separately attracting every particle.

The first subject to which I shall allude, is the precession of the equinoxes and nutation, which are