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?
The routes or single circuits of the planets are not arranged exactly in a perfect circle but are ellipses.
—Johannes Kepler
The planet is really slower at its greater distance from the sun, and faster at its lesser.
—Johannes Kepler
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Notes
- 1.
One is tempted here to assume that Kepler is proposing gravitational attraction between the sun and the planets. This, however, is not the case. Kepler’s lines of force are neither attractive nor repulsive. Newton’s theory of gravitational attraction comes several decades later when his Principia is published. See Chap. 27 of volume II.
- 2.
See Chap. 4 of Book II of Gilbert, W., De Magnete, Dover Publications, New York, 1958.
- 3.
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Kuehn, K. (2015). Kepler’s First and Second Laws. In: A Student's Guide Through the Great Physics Texts. Undergraduate Lecture Notes in Physics. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-1360-2_17
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DOI: https://doi.org/10.1007/978-1-4939-1360-2_17
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