Galileo’s discovery of the four satellites of Jupiter was followed in time by the discovery that every planet was attended by satellites, except the two whose orbits lay inside the earth’s. In 1655 Huyghens discovered Titan, the largest of Saturn’s satellites, and by 1684 Cassini had discovered four more. Then, after the lapse of a full century, Sir William Herschel discovered two satellites of Uranus in 1787 and two more satellites of Saturn in 1789. We shall discuss the full system of planetary satellites and also the smaller bodies of the solar system—comets, meteors and shooting-stars—in a later chapter, when we come to deal with the way they came into being. THE GALACTIC SYSTEM Our next landmark is the survey of the stars by the two Herschels, Sir William Herschel, the father (1738-1822) and Sir John Herschel, the son (1792-1871). What Galileo had done for the solar system, the two Herschels set out to do for the huge family of stars—the “galactic” system, bounded by the Milky Way—of which our sun is a member. On a clear moonless night the Milky Way is seen to stretch, like a great arch of faint light, from horizon to horizon. It is found to be only part of a full circle of light—the galactic circle—which stretches completely round the earth and divides the sky into two equal halves, forming a sort of celestial “equator,” with reference to which astronomers are accustomed to measure latitude and longitude in the sky. Galileo’s telescope had shewn that it consists of a crowd of faint stars, each too dim to be seen individually without telescopic aid (see Plate I). And, as might be expected, the proper interpretation of this great belt of faint stars has proved to be fundamental in understanding the architecture of the universe. If stars were scattered uniformly through infinite space, we should at last come to a star in whatever direction we looked, so that the sky would appear as a uniform blaze of intolerable light. It is true that this would not be the case if light were dimmed or blotted out after travelling a certain distance, but even then, the sky would appear the same in all directions, for there would be no reason why one part of the sky should be more lavishly spangled with stars than another. Thus the existence of the Milky Way shews that the system of the stars does not extend uniformly to infinity. It must have a definite structure, and it was the architecture of this that Sir William Herschel set himself to unravel. The work he did for the northern half of the sky was subsequently extended to the southern hemisphere by his son, Sir John Herschel.