Jean-Sylvain Bailly (1736-1793), French astronomer and orator. Bailly was one of the leaders of the early part of the French Revolution (1789-1799), and was guillotined during the Reign of Terror. His astronomical achievements include calculating an orbit for Halley's comet, writing an essay on the satellites of Jupiter, and writing several volumes on the history of science. He was elected a member of the French Academy of Sciences in 1763. 19th-century engraving.