Planetary Theory of Latitude

Abstract

“Copernicus, ignorant of his own riches, took it upon himself for the most part to represent Ptolemy, not nature, to which he had nevertheless come the closest of all.” In this famous, and just, assessment of Copernicus, Kepler was referring to the latitude theory of Book VI, specifically to the “librations” of the inclinations of the planes of the eccentrics, not in accordance with the motion of the planet, but (quod monstri simile est) by the unrelated motion of the earth. This improbable connection between the inclinations of the orbital planes and the motion of the earth was the result of Copernicus’s attempt to duplicate the apparent latitudes of Ptolemy’s models in which the inclinations of the epicyclic planes were variable. In a way this is nothing new since Copernicus was also forced to make the equation of center of the inferior planets depend upon the motion of the earth rather than the planet. But the image of the orbital planes of all the planets dutifully inclining one way and another in synchronization with the motion of the earth was to Kepler an incongruous combining of unrelated motions that he found incredible even before he was able to show, from Tycho’s observations, that it was false.