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Rh any one else before Tycho had proposed the Tychonic system. Tycho's death made this memoir superfluous, and Kepler laid it aside, so that it has only recently been published in the complete edition of his works. The same was the case with an unfinished reply to the attack of Craig on Tycho's book on the comet of 1577.

In 1601 Kepler also occupied himself with the theories of Mercury, Venus, and Mars, and noticed that it was not possible to represent the apparent motion of the planets by assuming for the orbit of the sun (or the earth) a simple excentric circle with uniform motion, as had always hitherto been done, but that it would be necessary to have recourse to an equant as in the planetary theories of Ptolemy. When Kepler asked Tycho if he would not mention this in the Progymnasmata, he declined to do so, as it would take time to investigate the equal motion, and he wanted the book published at once. The subject was therefore merely alluded to by Kepler in the Appendix with which he wound up the book after Tycho's death.

While Kepler was thus reconnoitring the ground for his future work on the planets, Longomontanus had before his departure finished the lunar theory and tables, the incomplete state of which had so long delayed the publication of the Progymnasmata. It is much to be regretted that the