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§§ 91, 92] which he played in the overthrow of the Ptolemaic system is so conspicuous, that we are sometimes liable to forget that, so far from rejecting the epicycles and eccentrics of the Greeks, he used no other geometrical devices, and was even a more orthodox "epicyclist" than Ptolemy himself, as he rejected the equants of the latter. Milton's famous description (Par. Lost, VIII. 82-5) of

applies therefore just as well to the astronomy of Coppernicus as to that of his predecessors; and it was Kepler (chapter .), writing more than half a century later, not Coppernicus, to whom the rejection of the epicycle and eccentric is due.

92. One point which was of importance in later controversies deserves special mention here. The basis of the Coppernican system was that a motion of the earth carrying the observer with it produced an apparent motion of other bodies. The apparent motions of the sun and planets were thus shewn to be in great part explicable as the result of the motion of the earth round the sun. Similar reasoning ought apparently to lead to the conclusion that the fixed stars would also appear to have an annual motion. There would, in fact, be a displacement of the apparent position of a star due to the alteration of the earth's position in its orbit, closely resembling the alteration in the apparent position of the moon due to the alteration of the observer's position on the earth which had long been studied under the name of parallax (chapter, § 43). As such a displacement had never been observed, Coppernicus explained the apparent contradiction by supposing the fixed stars so