In 1679, Robert Hooke wrote to Isaac Newton of his hypothesis concerning orbital motion, which partly depends on an inverse-square force. In 1684, both Hooke and Newton told Edmond Halley that they had proven the inverse-square law of planetary motion, in January and August, respectively. While Hooke refused to produce his proofs, Newton was prompted to compose De motu corporum in gyrum ('On the motion of bodies in an orbit'), in which he mathematically derives Kepler's laws of planetary motion. In 1687, with Halley's support (and to Hooke's dismay), Newton published Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), which hypothesizes the inverse-square law of universal gravitation. In his own words: