Tychonic Worries on Newton: Comments from Book III of the 1822 Geneva Edition of Principia Mathematica

The third book of Isaac Newton’s Principia Mathematica Philosophia Naturalis (Principia, hereafter) rigorously established the case for heliocentrism by integrating observations and Kepler’s laws. It is structured around two main hypotheses: first, that the cosmos has a static center located at the center of planetary motion, and second, that if Earth itself moved, this would concord with observational data. The first hypothesis directly mentions the possibility of having the Earth as a center, even if all the propositions claim that the correct center is the Sun. The Minimal Friars Thomas Le Seur (1703–1770) and François Jacquier (1711–1788), along with their editor Jean-Louis Calandrini, chose to collaborate and publish their influential commentary in Geneva (Newton 1822)—likely to avoid the Catholic prohibition on promoting heliocentrism. This article examines key contributions by these Commentators, highlighting their strong scientific focus and commitment to strengthening heliocentric principles in astronomy. It also discusses the educational aim evident in many extensive notes, especially on propositions that stress the transition from geo-heliocentric to heliocentric perspectives (notably propositions 4, 13, 14, and 16).