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.[1] Milton's famous description (Par. Lost, VIII. 82-5) of
"The Sphere
With Centric and Eccentric scribbled o'er,
Cycle and Epicycle, Orb in Orb,"
applies therefore just as well to the astronomy of Coppernicus as to that of his predecessors; and it was Kepler (chapter vii.), 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 ii., § 43). As such a displacement had never been observed, Coppernicus explained the apparent contradiction by supposing the fixed stars so
- ↑ Recent biographers have called attention to a cancelled passage in the manuscript of the De Revolutionibus in which Coppernicus shews that an ellipse can be generated by a combination of circular motions. The proposition is, however, only a piece of pure mathematics, and has no relation to the motions of the planets round the sun. It cannot, therefore, fairly be regarded as in any way an anticipation of the ideas of Kepler (chapter vii.).
