French mathematician and astronomer whose theory of lunar motion advanced the development of planetary-motion theories.

Delaunay was educated as an engineer at the École des Mines from 1836, becoming an engineer in 1843 and chief engineer in 1858. He studied mathematics and astronomy with Jean-Baptiste Biot at the Sorbonne (1841–48). He taught mechanics at the École Polytechnique from 1850 and also later taught at the École des Mines. He was made a member of the Académie des Sciences in 1855; and in 1870 he succeeded U.-J.-J. Le Verrier as director of the Paris Observatory.

He wrote Cours élémentaire de mécanique (1850; 8th ed., 1874; “Elementary Course of Mechanics”), Cours élémentaire d'astronomie (1853; 5th ed., 1870; “Elementary Course of Astronomy”), La Théorie du mouvement de la lune, 2 vol. (1860–67; “The Theory of Lunar Motion”), Traité de mécanique rationnelle (1856; 4th ed., 1873; “Treatise of Theoretical Mechanics”), Ralentissement de la rotation de la terre (1866; “Slowing of the Rotation of the Earth”), and Rapport sur les progrès de l'astronomie (1867; “Report on the Progress of Astronomy”).