Page:Popular Astronomy - Airy - 1881.djvu/113

Rh planet Venus puts on all the phases of the moon. When the planet Yenus is at that part of her orbit at which, in conformity with our theory, she is beyond the sun, as at V', Figure 28, we then see her as round as the full moon; and when the planet Venus is at those parts at which, in conformity with our theory, she is almost between us and the sun, as at V", it is found, by observation with the telescope, that she then puts on the phases of a young moon. These are precisely the appearances that would be seen if the theory is true. This is a most important confirmation, which was wanting in the time of Copernicus, and which with us is so convincing, that any one who has seen Venus will not doubt the truth of the theory.

The great step made by Copernicus was the assumption that the sun is the centre of the motion of all the planets (including the earth). But he could not get rid of the epicycles. As in Figure 26, where Mars is carried at N, at the extremity of a small arm, jointed on to a longer arm and revolving round the joint; so it was still necessary to suppose that each of the planets, as well as the earth, was carried by a similar apparatus; and even this did not represent the movements with perfect accuracy. This was reserved for Kepler—to explain, who not so much from his own observations as by examining accurately the observations which Tycho Brahé had made of the planets, and especially the planet Mars, and comparing them with his own—ascertained that the whole would be represented to the utmost accuracy by supposing, that Mars moves in an ellipse. It is impossible now to explain in a few words how Kepler came to that conclusion; generally speaking, it was by the method of trial and error. The number of 2