Page:Popular Astronomy - Airy - 1881.djvu/282

268 both to the same distance; and we shall therefore first say, if the earth draw the moon through 10·963 miles in an hour when at the distance of 238,800 miles, how far would it draw the moon in an hour if it were at the distance of 95,000,000 miles? Diminishing 10·963 in the proportion of the inverse squares of the distances, we find that the earth would draw the moon through 0·00006927 mile or 4·389 inches in an hour, if it were at the distance of 95,000,000 miles. Comparing this with 24·402 miles through which the sun draws the earth or moon when at the same distance, we find that the sun's attraction is 352,280 times as great as the earth's, and therefore, that the sun's mass is 352,280 times as great as the earth's. You can, if you please, combine this with the numbers which I gave before, to express the weight of the sun in pounds.

The angular diameter of the sun, as viewed from the earth, is 32 minutes of a degree. Computing from this the sun's diameter, we find that the sun's bulk is 1,400,070 times as great as the earth's bulk. Therefore the sun's mean density is only about $1⁄4$ of the earth's mean density, or about 1·4 times the density of water.

The principle which has been used above for comparing the mass of the earth with that of the sun, is used without the smallest alteration for comparing the mass of Jupiter, Saturn, Uranus, or Neptune, with that of the sun; and in all cases where the satellites can be easily observed, it can be applied with very great accuracy. For those planets which have no satellites there is considerable uncertainty. The only way in which they are determined is by the perturbation of other planets. For instance, we see that in certain positions, the earth is disturbed by Mars a few