Laplace, Pierre Simon, marquis de (pyĕr sēmôNˈ märkēˈ də läpläsˈ) [key], 1749–1827, French astronomer and mathematician. At 18 he went to Paris, proved his gift for mathematical analysis to Jean le Rond d'Alembert, and was made professor of mathematics in the École militaire of Paris. He had a seat in the senate (1799) and became its vice president and (1803) chancellor. He was elected to the French Academy in 1816. He investigated the variations of the moon's motions, especially as affected by the eccentricity of the earth's orbit; the inequalities in the motions of Jupiter and Saturn; the motion of the satellites of Jupiter; the aberration in the movements of comets; and the theory of the tides. With J. L. Lagrange he established beyond a doubt Newton's theory of gravitation. The results of his researches were published in his famous *Mécanique céleste* (1799–1825, tr. by Nathaniel Bowditch, 1829–39). In the more popular work, *Exposition du système du monde* (1796), a summary of the history of astronomy is included. This work contains also a statement of the nebular hypothesis of the origin of the solar system. His *Théorie des attractions des sphéroides et de la figure des planètes* (1785) introduced "Laplace's coefficients" and the potential function, two means of applying analysis to physical problems. The *Théorie analytique des probabilités* (1812), a mathematical classic, was followed by *Essai philosophique sur les probabilités* (1814).

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