When Kepler concluded that the orbit of Mars was not a circle, he was led to the belief that the orbit was an oval touching the circle at the apsides and lying within the circle at other points. In the definition of the oval, physical hypotheses played a primary role. Two forces were involved; a tractive force arising from the effect of the solar rays rotating with the sun, and a directing force arising from a natural instinct of the planet (...) itself. The former pushed the planet along the orbit while the latter enabled the planet to steer itself across the stream of the solar vortex in a small epicycle. In adopting this physical theory to determine the oval, Kepler was led into what he himself described as ‘a new labyrinth’. After several attempts to construct the oval, and by progressively eliminating the sources of error from his calculating procedures in order to arrive at an accurate mathematical formulation of the physical hypotheses, he was able to conclude that the oval was inconsistent with the empirical data and the physical theory in need of modification. (shrink)

Here, Rhonda Martens offers the first extended study of Kepler's philosophical views and shows how those views helped him construct and justify the new astronomy.