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Jean-Sylvain Bailly (September 15, 1736–November 12, 1793) was a French astronomer and orator, one of the leaders of the early part of the French Revolution. He was ultimately guillotined during the Reign of Terror.

Jean Sylvain Bailly (from: Dr. Nuno Carvalho de Sousa Private Collections - Lisbon.)

Born in Paris, he was originally intended for the profession of a painter, but preferred writing tragedies, until attracted to science by the influence of Nicolas de Lacaille. He calculated an orbit for Halley's Comet when it appeared in 1759, reduced Lacaille's observations of 515 zodiacal stars, and was, in 1763, elected a member of the French Academy of Sciences. His Essai sur la theorie des satellites de Jupiter (Essay on the theory of the satellites of 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 inegalites de la lumiere des satellites de Jupiter (On the inequalities of light of the satellites of 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 (A history of ancient astronomy, 1775); Histoire de l'astronomie moderne (A history of modern astronomy, 3 vols., 1779-1782); Lettres sur l'origine des sciences (Letters on the origin of the sciences, 1777); Lettres sur l' Atlantide de Platon (Letters on Plato's Atlantide , 1779); and Traite de l'astronomie indienne et orientale (A treatise on Indian and Oriental astronomy, 1787). The 1911 Encyclopædia Britannica remarks that "Their erudition was… 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 - immediately after the storming of the Bastille - became the first mayor of Paris under the newly adopted system of the Commune (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 left 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. In the words of the 1911 Encyclopædia Britannica, "He met his death with patient dignity; having, indeed, disastrously shared the enthusiasms of his age, but taken no share in its crimes."

The lunar crater Bailly was named in his honour.

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

The 1911 Encyclopædia Britannica, in turn, gives the following references:

• Eloges by Merard de Saint Just, Delisle de Salles, Jerome de Lalande and Lacretelle
• A memoir by François Arago, read on February 26, 1844 before the Academie des Sciences, and published in Notices biographiques, t. ii. (1852)
• Delambre, Histoire de l'astronomie au 18^me siecle, p. 735
• Jerome de Lalande, Bibliographie astronomique, p. 730.

Additionally, see:

• Chapin, Seymour L. (1970). "Bailly, Jean-Sylvain". Dictionary of Scientific Biography 1. New York: Charles Scribner's Sons. 400-402. ISBN 0684101149.