At a discussion at the Royal Society, Robert Hooke claims to have found proof that gravity is a force emanating from the Sun, but refuses to reveal it to Edmond Halley. Halley, furious, rushes to Isaac Newton and poses the problem: “What would be the trajectory of a planet if the Sun emanated a force that attracted it with the inverse square of the distance?” Newton promptly replies, “An ellipse,” and says he had already performed the necessary calculations four years earlier: this is the birth certificate of the Philosophiae Naturalis Principia Mathematica.
