Jean Sylvain Bailly (ʒɑ̃ silvɛ̃ baji; 15 September 1736 – 12 November 1793) was a French astronomer, mathematician, freemason, and political leader of the early part of the French Revolution. He presided over the Tennis Court Oath, served as the mayor of Paris from 1789 to 1791, and was ultimately guillotined during the Reign of Terror.
Born in Paris, Bailly was the son of Jacques Bailly, an artist and supervisor of the Louvre, and the grandson of Nicholas Bailly, also an artist and court painter. As a child he originally intended to follow in his family's footsteps and pursue a career in the arts. He became deeply attracted to science, however, particularly astronomy, by the influence of Nicolas de Lacaille. An excellent student with a "particularly retentive memory and inexhaustible patience", he calculated an orbit for the next appearance of Halley's Comet (in 1759), and correctly reduced Lacaille's observations of 515 stars. He participated in the constr