 Over a century later precession was explained in Isaac Newton's "Philosophiae Naturalis Principia Mathematica" (1687), to be a consequence of gravitation (Evans 1998, p. 246).Newton's original precession equations did not work, however, and were revised considerably by Jean le Rond d'Alembert and subsequent scientists.Hipparchus gave an account of his discovery in "On the Displacement of the Solsticial and Equinoctial Points" (described in "Almagest" III.1 and VII.2).