Le Monnier, Pierre-Charles

(b. Paris, France, 20 November 1715; d, Herils, Calvados, France 3 April 1799)

astronomy.

Le Monnier’s father, for whom he was named, was professor of philosophy at the Collège d’Harcourt and a member of the Académie des Sciences. His brother was Louis-Guillaume Le Monnier, also a member of the Academy, professor of botany at the Jardin du Roi and premier médecin ordinaire to Louis XV. In 1763 he married a Mlle. Cussy from a prominent Norman family and had three daughters, one of whom married the mathematician Lagrange. A second daughter married his brother, Louis-Guillaume Le Monnier.

Le Monnier was presented to Louis XV by the due de Noailles and remained a great favorite of the king, before whom he made important observations of the transits of Venus (1761) and Mercury (1753) across the face of the sun. The king in turn procured for him some of the best astronomical instruments in France. At that time the English instruments were superior to the French ones. He obtained a telescope by Short and two excellent quadrants by Bird and Sisson. He also constructed a great meridian at the church of Saint Sulpice in 1743.

Le Monnier began his astronomical career early with the assistance of his father. In 1732 J. P. Grandjean de Fouchy allowed him to observe at the Rue des Postes in Paris, and his first Le Monnier presented an elaborate lunar map to the Academy of Sciences and was admitted as adjoint géomètre on 23 April 1736, at the age of twenty. He rose to pensionnaire by 1746, became professor at the Collège Royal, and was admitted to the Royal Society, the Berlin Academy, and the Academie de la Marine.

Le Monnier’s career was launched when he accompanied Clairaut and Maupertuis on their 1736 expedition to Lapland to measure a degree of an arc of meridian. Le Monnier had indicated his skill by observations made in 1731 on the opposition of Saturn, and his youth and vigor were an asset on such a hazardous and demanding voyage. In addition to surveying the degree of meridian, he made observations to calculate atmospheric refraction at various latitudes and in different seasons.

Of all his diverse interests, Le Monnier’s work on lunar motion was the most extensive and the most important. In the first edition of the Principia, Newton had shown that the principal inequalities of the moon could be calculated from his law of universal gravitation; and in the second edition he applied these calculations to the observations of John Flamsteed. His methods, however, added little to the theory that Jeremiah Horrocks had suggested long before. Flamsteed calculated new tables based on Horrocks’ theory incorporating Newton;s corrections, but he did not publish them. They appeared for the first time in Le Monnier’s Institutions astronomiques(1746), his most famous work. It was basically a translation of John Keill’s Introductio ad veram astronomiam (London, 1721) but with important additions and with new tables of the sun and moon. The book, the first important general manual of astronomy in France, later was largely replaced by the textbooks of Lalande and Lacaille.

Le Monnier supported the view of Edmond Halley that the irregularities of the moon’s motion could be discovered by observing the moon regularly through an entire cycle of 223 lunations (the saros cycle of approximately eighteen years and eleven days), with the assumption that the irregularities would repeat themselves throughout each cycle. Le Moinnier and James Bradley began such a series of observations, Le Monnier continuing his work for fifty years. During the 1740’s Clairaut, d’Alembert, and Euler were working in competition to create a satisfactory mathematical lunar theory, which demanded an approximate solution to the three-body problem. In the ensuing controversy d’Alembert was seconded by Le Monnier, and he used Le Monnier’s Institutions astronomiques as the basis for his tables. In 1746 Le Monnier presented a memoir to the inequalities in the motion of Saturn caused by the gravitational attraction of Jupiter, and he persuaded the Academy to choose as its prize question for 1748 the problem of providing a satisfactory theory to explain the observed inequalities. Euler entered the contest, used Le Monnier’s data, and won the prize. His results were important for lunar theory, because an explanation of the perturbations of Saturn also required a treatment of three-body problem.

The best lunar tables of the eighteenth century were those of Tobias Mayer, which were based on Euler’s theory but with the magnitude of the predicted perturbations taken from observations. Le Monnier, however, adhered stubbornly to the task of correcting Flamsteed’s tables even while confessing the superiorinty of Mayer’s. Le Monnier’s stubbornness is explained in part by his admiration for English methods in astronomy. In addition to his study of lunar motion he published La théorie des comètes (1743), which was largely a translation of Halley’s Cometography but with several additions, constructed the first transit instrument in France at the Paris observatory, corresponded with James Bradley, and was the first to apply Bradley’s discovery of the earth’s nutation to correcting solar tables. He traveled to England in 1748 and observed an annular eclipse of the sun in Scotland. An indefatigable observer, Le Monnier undertook a star catalog and in 1755 produced a map of the zodiacal stars. His records show no fewer than twelve observations of Uranus, but he never identified it as a planet. He also wrote on navigation, magnetism and electricity.

His most prominent pupil was Jérôme Lalande. Lalande attended Le Monnier’s lectures in mathematical physics at the Collège Royale and with Le Monnier’s strong recommendation and assistance went to Berlin in 1751 to make measurements of the moon simultaneous with those by Lacaille at the Cape of Good Hope, the purpose being to determine the lunar parallax. Le Monnier allowed Lalande to take his best quadrant on the journey. Soon afterward, however, the two men quarreled, partly because of Lalande’s growing attachment to Lacaille, whom Le Monnier saw as a bitter foe. Le Monnier’s irascibility led him into frequent controversy. He worked continually until 10 November 1791, when an attack of paralysis ended his career as a practicing astronomer.

BIBLIOGRAPHY

I. Original Works. The most important works by Le Monnier are Historie Céleste ou recueil de toutes les observations astronomiques faites par ordre du roy, avec un discours préliminaire sur le progrés de l’astronomie (Paris, 1741); La théorie des comètes, où l’on trite du progrès de cette partie de l’astronomie avec des tables pour calculer les mouvements des comètes, du soleil et des principales étoiles fixes (Paris, 1743); Institutions astronomiques ou leçons élémentaires d’astronomie … précédées d’un essai sur l’histoire de l’astronomie (Paris, 1746); Observations de la lune, du soleil et des étoiles fixes pour servir à la physique céleste et aux usages de la navigation …, 4 vols. (Paris, 1751-1773); Nouveau zodiaque réduit à l’année 1755 avec les autres étoiles dont la latitude s’étend jusqu’à 10 degrés au nord et au sud de l’écliptique … (Paris, 1755); Astronomique nautique lunaire, oç l’on traite de la latitude et de la longitude en mer de la période saros (Paris, 1771); Mémories concernant diverse question d’astronomie, de navigation, et de physique lus et communiqués à l’Académie royale des sciences, 3 vols. (Paris, 1781—1788); and many contributions to the Mémoires of the Académie des Sciences.

II. Secondary Literature. Along and highly critical analysis of Le Monnier’s works appears in J. B. J. Delambre, Histoire de l’astronomie au dix-huitiéme siècle (Paris, 1827), pp. 179-238. A more favorable account is in Jérôme Lalande, Bibliographie astronomique avec l’histoire de l’astronomie depuis 1781 jusqu’à 1802 (Paris, 1803), pp. 819-826. Biographical information is in Michel Robida, Ces bourgeois de Paris. Trois siècles de chronique familiale, de 1675 à nos jours (Paris, 1955). An account of the expedition to Lapland is in Pierre Brunet, Maupertuis, étude biographique, I (Paris, 1929), pp. 33-64; Le Monnier’s involvement in the controversy over lunar theory is described in an unpublished paper by Craig B. Waff, “The Introduction of the Theory of Universal Gravitation Into Lunar Theory: Après le system sysème du monde” ; and in Thomas L. Hankins, Jean d’Alembert: Science and the Enlightenment (oxford, 1970), pp. 28-42.

Thomes L. Hankins