Page:Hector Macpherson - Herschel (1919).djvu/63

Rh before Herschel began his observations, that many double stars were certainly binaries. But there was no direct proof until Herschel, after twenty years of patient study, announced his results in 1802. The discovery gave scientific proof of the unity of the Cosmos and of the universal validity of the Newtonian law.

Another brilliant discovery, made almost casually, was announced in 1783 in a paper entitled "On the proper motion of the Sun and Solar System; with an account of several changes that have happened among the fixed stars since the time of Mr. Flamsteed". Halley had in 1718 announced that a number of the brighter stars, so far from being "fixed," in reality were moving through space with considerable velocity. Herschel gave it as his view that "there is not, in strictness of speaking, one fixed star in the heavens". "Now, if the proper motion of the stars in general be once admitted, who can refuse to allow that our Sun, with all its planets and comets, that is, the Solar System, is no less liable to such a general agitation as we find to obtain among all the rest of the celestial bodies?" The idea had occurred to several contemporary astronomers, Wilson, Lalande, and Mayer, but no one had succeeded in attacking the problem practically. It was plainly obvious that the motion of the Sun could only be detected through the resulting apparent motion of the stars, just as the orbital motion of the Earth is reflected in the planetary motions. If the Sun is moving in a certain direction, the stars in front will appear to disperse, while those behind will seem to draw closer together. But the problem is complicated by the individual motions of the "stars; and these motions—themselves very minute—have to be decomposed into two parts, the real motions of the stars themselves and the apparent motion resulting from the translation of the Solar System. "We ought, therefore," said Herschel, "to resolve that which is common to all the stars, which are bound to have what is called a proper motion,