Kepler’s First and Second Laws

Abstract

Our final reading selection from Kepler is taken from Part III of Book IV of his Epitome. It is subtitled On the real and true irregularity of the planets and its causes. By “irregularity,” Kepler means the fact that the planets, when observed carefully, do not exhibit regular, uniform, circular motion, as had been maintained by Aristotle. This irregular motion was, of course, recognized by Ptolemy. But he attempted to rationalize it as only apparently irregular motion. He invoked an equant—a hypothetical point about which the planet moves uniformly. The equant point of each planet was itself offset from the center of the deferent circle about which the planet orbits Earth—all of this to maintain a nominal allegiance to perfect circular motion. Kepler, on the other hand, describes this apparent irregular motion as a true irregular motion. As suggested by the subtitle of Part III, he not only defends a real and true irregularity of the planets’ motion, he also offers a causal explanation as to why the planets might exhibit such irregular motion. What is it?