How to use Kepler’s first and second laws in a geo-heliocentric system? Ask G.B. Riccioli

Abstract

Kepler’s laws provided sufficient geometry and kinematics to strengthen astronomers’ preference for heliocentrism. While Kepler outlined some dynamic arguments, they were not rigorous enough to turn his laws into kinematic tools. As a result, some astronomers found ways to reconcile Kepler’s findings with geo-heliocentrism. One of these was the Jesuit astronomer Giovanni Battista Riccioli, who proposed a method known as the “epic-epicycle” (Riccioli, Almagestum novum, 1651). This paper will explore how Riccioli received and interpreted Kepler’s first and second laws within his own astronomical framework. This analysis will include a discussion of how Riccioli understood the concept of “physics” in his work, beginning with a study of the Sun’s motion (Riccioli, Astronomia reformata, 1665)