Kepler’s Third Law

Abstract

In the previous reading selection from Part I of Book IV of the Epitome of Copernican Astronomy, Kepler argued that it would be most fitting for the sun to be located at the center of the World, from which point it can readily illuminate, warm, regulate and (perhaps) even move the planets which orbit around it at divinely ordained distances. Their motion, however, cannot be caused by the motion of celestial spheres. Why? Because there are no such spheres, as Tycho Brahe has demonstrated. Then what is the cause of their motion? In Part II of Book IV, he begins to address this question. After summarizing the observed motions of the planets, he carefully recounts the planetary models of Aristotle and Ptolemy, which he rejects. He then proceeds to reconsider the ancient idea that an intelligence, or mind, is required to govern the complex yet orderly motion of the planets. Such a solution might seem particularly tempting for Kepler since he has already explicitly rejected matter—in the form of vast rotating solid celestial spheres—as the cause of the orbital motion the planets. To what cause does Kepler finally attribute the motion of the planets? In the course of this text, he casually mentions the true trajectory of the planetary orbits and he also introduces what is now known as Kepler’s third law of planetary motion. What does this law state? He also asserts that the planets themselves must must have inertia. Why is this significant? And in what way is this contradictory to Aristotelian physics?