Page:L. Silberstein - The Theory of Relativity.djvu/20

4 much like, in fact, some primitive mental pictures of the universe. But the case becomes entirely different when we come to consider the far less numerous class of luminous points or little discs, the planets, and the comets, moving visibly among the 'fixed' shining points in a complicated way. Then, even before touching any dynamical part of the celestial problem, we are compelled to give up our earth as a system of reference and replace it by that of the 'fixed stars,' originally so inconspicuous, or—what turns out to be equally good—by a framework of axes pointing from an initial point fixed in the sun towards any given triad of fixed stars. It is needless to tell here again the long story of that admirable and ingenious system which was founded by Ptolemy (born about 140 B.C.), which held the field during fourteen centuries, to be replaced finally and definitely by the system of Copernicus (1473-1543), which transferred to the sun the previous dignity of the earth. The Copernican system of reference had the enormous advantage of simplicity, quite independently of any mechanical, i.e. (to put it more strictly) dynamical considerations. Its superiority to the geocentric system manifested itself already in the simplicity it gave to the paths of the solar family of bodies, the wonderfully simple shapes of the orbits of the planets. In the geocentric scheme we had the complicated system of 'excentrics and epicycles' of Ptolemy, whereas taking, in our drawing or model, the sun as fixed, the orbits of the planets became simple circles, which in the next step of approximation turned out to be slightly elliptic. Thus the Copernican system of reference had its enormous advantages before any properly mechanical point of the subject was entered upon. Historically, in fact, the mechanics of Galileo and Newton came a long time after Copernicus, so that the