Had the planet Mars not existed: Kepler's equant model and its physical consequences
Résumé
We examine the equant model for the motion of planets, which was the starting point of Kepler's investigations before he modified it because of Mars observations. We show that, up to first order in eccentricity, this model implies for each orbit a velocity, which satisfies Kepler's second law and Hamilton's hodograph, and a centripetal acceleration with an r-2 dependence on the distance to the Sun. If this dependence is assumed to be universal, Kepler's third law follows immediately. This elementary exercise in kinematics for undergraduates emphasizes the proximity of the equant model coming from ancient Greece with our present knowledge. It adds to its historical interest a didactical relevance concerning, in particular, the discussion of the Aristotelian or Newtonian conception of motion.