The ancients assumed that the planets moved in exact circles, and, in fact, that all motion in the heavens was circular, the circle being the simplest and most perfect curve. To account for the loops described by the planets, they imagined that each planet revolved in a circle around a centre, which, in turn, revolved in a circle around the earth. The circle described by this centre around the earth they called the _cycle_, and the circle described by the planet around this centre they called the _epicycle_. 38. _The Eccentric._--The ancients assumed that the planets moved at a uniform rate in describing the epicycle, and also the centre in describing the cycle. They had, however, discovered that the planets advance eastward more rapidly in some parts of their orbits than in others. To account for this they assumed that the cycles described by the centre, around which the planets revolved, were _eccentric_; that is to say, that the earth was not at the centre of the cycle, but some distance away from it, as shown in Fig. 51. _E_ is the position of the earth, and _C_ is the centre of the cycle. The lines from _E_ are drawn so as to intercept equal arcs of the cycle. It will be seen at once that the angle between any pair of lines is greatest at _P_, and least at _A_; so that, were a planet moving at the same rate at _P_ and _A_, it would seem to be moving much faster at _P_. The point _P_ of the planet's cycle was called its _perigee_, and the point A its _apogee_. [Illustration: Fig. 51.] As the apparent motion of the planets became more accurately known, it was found necessary to make the system of cycles, epicycles, and eccentrics exceedingly complicated to represent that motion. The Copernican System. 39. _Copernicus._--Copernicus simplified the Ptolemaic system greatly by assuming that the earth and all the planets revolved about the sun as a centre. He, however, still maintained that all motion in the heavens was circular, and hence he could not rid his system entirely of cycles and epicycles. Tycho Brahe's System. 40. _Tycho Brahe._--Tycho Brahe was the greatest of the early astronomical observers. He, however, rejected the system of Copernicus, and adopted one of his own, which was much more complicated. He held that all the planets but the earth revolved around the sun, while the sun and moon revolved around the earth. This system is shown in Fig. 52. [Illustration: Fig. 52.] Kepler's System. 41. _Kepler._--While Tycho Brahe devoted his life to the observation of the planets. Kepler gave his to the study of Tycho's observations, for the purpose of discovering the true laws of planetary motion. He banished the complicated system of cycles, epicycles, and eccentrics forever from the heavens, and discovered the three laws of planetary motion which have rendered his name immortal.