and the Astronomia nova
THE ASTRONOMIA NOVA, published in 1609, is probably Kepler's greatest contribution to astronomy, containing the first articulation of what are known today as Kepler's first two planetary laws. Having access to Brahe's data at last, Kepler advanced his “war on Mars.” But, as Stephenson observes, even Brahe's data was insufficient to decide between competing theories: “Kepler's mathematical ingenuity was such that he could invent alternative theories, each good enough to “save” even the excellent observations of Tycho Brahe. These observations, therefore, did not provide enough guidance for him to reach his final solution to the problem of planetary motion” (Stephenson 1987, 2). To resolve this problem, Kepler applied the method recommended in the Apologia of grounding astronomy in physics. As its full title announces, the Astronomia is an investigation of the motion of Mars through considerations of its physical causes.1 I argue here that Kepler was also guided by archetypal considerations, and that when he spoke of grounding astronomy in physics, he meant this in the broad sense used in the Apologia, where physics includes cosmology.
Kepler's archetypes seldom make an explicit appearance in the Astronomia. In Kepler's arguments to the ellipse they rarely play the roles assigned to them by the Mysterium and Apologia, as part of an a priori inference to the same phenomena, or as adjudicating between competing astronomical hypotheses.2 Kepler's belief in an archetypally structured universe, I believe, nonetheless guided his physical exploration of the Martian orbit by narrowing the scope of hypotheses under consideration. Given the historical context, this approach is of great significance, for Kepler was the first to combine physics and astronomy in the manner he did, and he faced an open field of possible theory types. Kepler also needed to develop a new physics, because traditional Aristotelian physics was inappropriate to the task. His archetypes allowed him to rule out as implausible a number of physical investigations. Moreover, when Kepler failed to provide an account of the efficient causes of planetary motion, his archetypal commitments preserved his confidence that he was, nevertheless, on the right track.
After discussing Kepler's epistemological attitude toward hypotheses in physical astronomy, I trace the use of archetypal reasoning in his