Jean Sylvain Bailly (September 15, 1736 - November 12, 1793) was a French astronomer and orator.
He was born at Paris. Originally intended to be a painter, he preferred writing tragedy until attracted to science by the influence of Nicolas Louis de Lacaille. He calculated an orbit for the comet of 1759 (Edmund Halley's), reduced Lacaille's observations of 515 zodiacal stars, and was, in 1763, elected a member of the Académie des 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 inégalites de la lumière des satellites de Jupiter.
Meantime, he had gained a high literary reputation by his Éloges of King Charles V of France, Lacaille, Molière, Pierre Corneille and Gottfried Leibniz, which were issued in collected form in 1770 and 1790; he was admitted to the Académie française on (February 26, 1784), and to the Académie des Inscriptions in 1785, when Bernard le Bovier de Fontenelle's simultaneous membership of all three Academies was renewed in him. From then on, 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 French Revolution interrupted his studies. Elected deputy from Paris to the Estates-General, he was elected 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) made him unpopular, 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 November 12 he was guillotined amid the insults of a howling mob.
Notices of his life are contained in the Éloges by Mérard de Saint-Just, Delisle de Salles, Joseph Jerôme Lefrançais de Lalande and Lacretelle; in a memoir by François Arago, read on February 26, 1844, before the Académie des Sciences, and published in Notices biographiques, t. ii. (1852).