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180 the earth at opposition than the sun was, and this decided him to reject the Ptolemean system. He adds in his letter to Rothmann, that the comets when in opposition did not move in a retrograde direction like the planets, for which reason he had to reject the Copernican system also. It did not strike him that comets might move in orbits greatly differing from those of the planets. Having rejected the two existing systems, there was nothing to do but to design a new one.

The Tychonic system could explain the apparent motions of the planets (including their various latitudes), and it might have been completed in detail by being furnished with excentrics and epicycles like its rival. Copernicus had referred the planetary motions, not to the sun, but to the centre of the earth's orbit, from which the excentricities were counted, and through which the lines of nodes passed, so that the earth still seemed to hold an exceptional position. The Copernican system, so long as it was not purged of the artificial appendage of epicycles by the laws of Kepler, was not very much simpler than the Tychonic, and, mathematically speaking, the only difference between them was, that the one placed the origin of co-ordinates in the sun (or rather in the centre of the earth's orbit), the other in the earth. Tycho's early death prevented the further development of the theory of the planets by his system, which he intended to do in a work to be called Theatrum astronomicum. He only gives a sketch of the theory of Saturn in the first volume of his book, in which the planet moves in a small epicycle in retrograde direction,