Jean Sylvain Bailly (1736-1793), French astronomer and orator, was born at Paris on the 15th of September 1736. Originally intended for the profession of a painter, he preferred writing tragedies until attracted to science by the influence of Nicolas de Lacaille. He calculated an orbit for the comet of 1759 (Halley's), reduced Lacaille's observations of 515 zodiacal stars, and was, in 1763, elected a member of the Academy of Sciences. His Essai sur la théorie des satellites de Jupiter (1766), an expansion of a memoir presented to the Academy in 1763, showed much original power; and it was followed up in 1771 by a noteworthy dissertation Sur les inégalités de la lumière des satellites de Jupiter. Meantime, he had gained a high literary reputation by his éloges of Charles V., Lacaille, Molière, Corneille and Leibnitz, which were issued in a collected form in 1770 and 1790; he was admitted to the French Academy (February 26, 1784), and to the Académie des Inscriptions in 1785, when Fontenelle's simultaneous membership of all three Academies was renewed in him.

Thenceforth, he devoted himself to the history of science, publishing successively: - Histoire de l'astronomie ancienne (1775); Histoire de l'astronomie moderne (3 vols. 1779-1782); Lettres sur l'origine des sciences (1777); Lettres sur l'Atlantide de Platon (1779); and Traité de l'astronomie indienne et orientale (1787). Their erudition was, however, marred by speculative extravagances.

The cataclysm of the French Revolution interrupted his studies. Elected deputy from Paris to the states-general, he was chosen president of the Third Estate (May 5, 1789), led the famous proceedings in the Tennis Court (June 20), and acted as mayor of Paris (July 15, 1789, to November 16, 1791). The dispersal by the National Guard, under his orders, of the riotous assembly in the Champ de Mars (July 17, 1791) rendered him obnoxious to the infuriated populace, and he retired to Nantes, where he composed his Mémoires d'un témoin (published in 3 vols. by MM. Berville and Barrière, 1821-1822), an incomplete narrative of the extraordinary events of his public life. Late in 1793, Bailly quitted Nantes to join his friend Pierre Simon Laplace at Melun; but was there recognized, arrested and brought (November 10) before the Revolutionary Tribunal at Paris. On the 12th of November he was guillotined amid the insults of a howling mob. He met his death with patient dignity, having, indeed, disastrously shared the enthusiasms of his age, but taken no share in its crimes.

Notices of his life are contained in the éloges by Mérard de Saint Just, Delisle de Salles, Lalande and Lacretelle; in a memoir by Arago, read the 26th of February 1844 before the Académie des Sciences, and published in Notices biographiques, t. ii. (1852). See also Delambre, Histoire de l'astronomie au 18me siècle, p. 735, and Lalande, Bibliographie astronomique, p. 730.