Philosophiæ Naturalis Principia Mathematica
In 1687, Sir Isaac Newton published his "Philosophiæ Naturalis Principia Mathematica", in which he outlined his laws of gravity and motion. His work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were the same comet before and after passing behind the Sun (he was later found to be correct; see Newton's Comet), he was unable to completely reconcile comets into his model.