hălˈē, hôˈlē, 1656–1742, English astronomer and mathematician. He is particularly noted as the first astronomer to predict the return of a comet and the first to point out the use of a transit of Venus in determining the parallax of the sun. In 1676 he went to St. Helena to observe the southern skies and as a result made a catalog of 341 stars of the Southern Hemisphere. In 1677 he made the first complete observation of a transit of Mercury. He financed the publication of Isaac Newton's Principia and helped to prepare it for the press. On the basis of Newton's theory, Halley calculated the orbit of the great comet of 1682—since known as Halley's comet—and predicted its return in 1758. In 1698–1700 he made one of the first studies of compass variations in the North Atlantic. He was made astronomer royal in 1720. He observed the moon through the complete revolution of its nodes; this took 18 years. Other discoveries of Halley's are the proper motions of the stars and the acceleration of the moon's mean motion. His noted synopsis of known comets appeared in 1705; his Tabulae astronomicae (1749, tr. 1752) was published posthumously.

See his Correspondence and Papers (repr. 1975); biography by C. A. Ronan (1970); L. Baldwin, Edmond Halley and His Comet (1985).

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