ABSTRACT
The effort made by Kepler in developing and checking his hypothesis vicaria was not entirely wasted, even though it failed to provide the results he hoped to obtain. The hypothesis in question, whilst revealing its own inadequacy, was nevertheless a very valuable working tool; and calculation of the eccentricity starting from the latitudes, whilst showing that bisection of the eccentricity was not completely in agreement with fact, did show, however, that it was much nearer the truth than Kepler had at first thought. Consequently, it was not surprising that, in his subsequent investigations, he should use the hypothesis vicaria (for calculating the heliocentric longitude of Mars), and once again accept the Ptolemaic hypothesis of bisection, even for the Earth.
