The Ptolemaic system persisted unshaken for about fourteen hundred years after the death of its author. Clearly men were flattered by the notion that their earth was the most important body in nature, that it stood still at the centre of the universe, and was the pivot upon which all things revolved. CHAPTER II THE MODERN VIEW It is still well under four hundred years since the modern, or Copernican, theory of the universe supplanted the Ptolemaic, which had held sway during so many centuries. In this new theory, propounded towards the middle of the sixteenth century by Nicholas Copernicus (1473-1543), a Prussian astronomer, the earth was dethroned from its central position and considered merely as one of a number of planetary bodies which revolve around the sun. As it is not a part of our purpose to follow in detail the history of the science, it seems advisable to begin by stating in a broad fashion the conception of the universe as accepted and believed in to-day. The Sun, the most important of the celestial bodies so far as we are concerned, occupies the central position; not, however, in the whole universe, but only in that limited portion which is known as the Solar System. Around it, in the following order outwards, circle the planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (see Fig. 2, p. 21). At an immense distance beyond the solar system, and scattered irregularly through the depth of space, lie the stars. The two first-mentioned members of the solar system, Mercury and Venus, are known as the Inferior Planets; and in their courses about the sun, they always keep well inside the path along which our earth moves. The remaining members (exclusive of the earth) are called Superior Planets, and their paths lie all outside that of the earth. [Illustration: FIG. 2.--The Copernican theory of the Solar System.] The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have been known from all antiquity. Nothing then can bring home to us more strongly the immense advance which has taken place in astronomy during modern times than the fact that it is only 127 years since observation of the skies first added a planet to that time-honoured number. It was indeed on the 13th of March 1781, while engaged in observing the constellation of the Twins, that the justly famous Sir William Herschel caught sight of an object which he did not recognise as having met with before. He at first took it for a comet, but observations of its movements during a few days showed it to be a planet. This body, which the power of the telescope alone had thus shown to belong to the solar family, has since become known to science under the name of Uranus. By its discovery the hitherto accepted limits of the solar system were at once pushed out to twice their former extent, and the hope naturally arose that other planets would quickly reveal themselves in the immensities beyond.