much like, in fact, some primitive mental pictures of the universe.[1] 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.[2] 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
- ↑ The earth as the centre of the universe, with the 'crystal spheres,' with the stars stuck to them, spinning round the earth, still formed part of the teachings of the Ionian school of philosophers founded by Thales (born about 640 B.C.). The first to suggest the rotation of the earth round its axis and its motion round the sun seems to have been Pythagoras, one of Thales' disciples, though it has been later unjustly attributed to Philolaus, one of Pythagoras' disciples (born about 450 B.C.).
- ↑ Although I do not claim to give here anything like a history of astronomy, it may be worth mentioning that the Pythagoreans already taught that the planets and comets were circling round the sun. But at any rate the Ptolemaean geocentric system reigned universally from the second till the fifteenth century, the only serious objection against its complexity having been raised in the thirteenth century by Alphonso X., king of Castile, the author of the astronomical 'Tables' associated with his name (published during 1248-1252).
