(with the exception of the Moon, and she, fortunately, is an only child), the determination of the mass of the smaller by measurement of its motion about the larger,—as if only the pair of bodies under consideration existed, and the mass of both were concentrated in the greater of the two,—is very nearly exact. In consequence each planet discloses with some accuracy the mass of the Sun, but tells next to nothing about its own mass; and in the same way each satellite reveals the mass of its primary. The mass of a planet possessing a satellite is, therefore, easy of determination. Not so that of one which travels unattended. The only way to obtain its mass is from the perturbations or disturbing pulls it exerts upon the other planets, or upon stray comets from time to time, and these disturbances are, by the nature of the case, of a much smaller order of magnitude, to say nothing of the fact that all act coincidently to increased difficulty of disentanglement. The practical outcome of this in the case of Mars was that before his satellites were discovered the values obtained for his mass ranged all the way from 1/3700000 to 1/2500000 of the mass of the Sun, or, in other words, varied fifty per cent. His insignificant satellites, however, and just because they are insignificant, have made it possible to learn his mass with great exactness. It turns out to
