PIERRE CHARLES LEMONNIER 0[715-1799), French astronomer, was born on the 23rd of November 1715 in Paris, where his father was professor of philosophy at the college d'Harcourt. His first recorded observation was made before he was sixteen, and the presentation of an elaborate lunar map procured for him admission to the Academy, on the 21st of April 1736, at the early age of twenty. He was chosen in the same year to accompany P. L. Maupertuis and Alexis Clairault on their geodetical expedition to Lapland. In 1738, shortly after his return, he explained, in a memoir read before the Academy, the advantages of J. Flamsteed's mode of determining right ascensions. His persistent recommendation, in fact, of English methods and instruments contributed effectively to the reform of French practical astronomy, and constituted the most eminent of his services to science. He corresponded with J. Bradley, was the first to represent the effects of nutation in the solar tables, and introduced, in 1741, the use of the transitinstrument at the Paris observatory. He visited England in 1748, and, in company with the earl of Morton and James Short the optician, continued his journey to Scotland, where he observed the annular eclipse of July 25. The liberality of Louis XV., in whose favour he stood high, furnished him with the means of procuring the best instruments, many of them by English makers. Amongst the fruits of his industry may be mentioned a laborious investigation of the disturbances of Jupiter by Saturn, the results of which were employed and confirmed by L. Euler in his prize essay of 1748; a series of lunar observations extending over fifty years; some interesting researches in terrestrial magnetism and atmospheric electricity, in the latter of which he detected a regular diurnal period; and the determination of the places of a great number of stars, including twelve separate observations of Uranus, between 1765 and its discovery as a planet. In his lectures at the college de France he first publicly expounded the analytical theory of gravitation, and his timely patronage secured the services of J. J. Lalande for astronomy. His temper was irritable, and his hasty utterances exposed him to retorts which he did not readily forgive. Against Lalande, owing to some trifling pique, he closed his doors "during an entire revolution of the moon's nodes." His career was arrested by paralysis late in 1791, and a repetition of the stroke terminated his life. He died at Heril near Bayeux on the 31st of May 1799. By his marriage with Mademoiselle de Cussy he left three daughters, one of whom became the wife of J. L. Lagrange. He was admitted in 1739 to the Royal Society, and was one of the one hundred and forty-four original members of the Institute.
He wrote Histoire celeste (1741); Theorie des cometes (1743), a translation, with additions of Halley's Synopsis; Institutions astronomiques (1746), an improved translation of J. Keill's text book; Nouveau zodiaque (1755); Observations de la lune, du soleil, et des etoiles fixes (1751-1775); Lois du magnetisme (1776-1778), &c. See J. J. Lalande, Bibl. astr., p. 819 (also in the Journal des savants for 1801); F. X. von Zach, Allgemeine geog. Ephemeriden, iii. 625; J. S. Bailly, Hist. de l'astr. moderne, iii.; J. B. J. Delambre, Hist. de l'astr. au XVIII e. siecle, p. 179; J. Madler, Geschichte der Himmelskunde, ii. 6; R. Wolf, Geschichte der Astronomic, p. 480.
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