Le Gentil De La Galaisière, Guillaume-Joseph-Hyacinthe-Jean-Baptiste

(b. Coutances, France, 12 September 1725; d. Paris, France, 22 October 1792),

astronomy.

Following early schooling in his native city, Le Gentil, the only son of a none—too—wealthy Norman gentleman, went to Paris to study theology. While pursuing that course he also attended Delisle’s lectures at the Collège Royal on astronomy, to which he soon found himself more attracted. An introduction to Jacques Cassini at the observatory in 1748 definitely established this change in direction by bringing Le Gentil an offer of lodgings there and observational training under Cassini de Thury and G. D. Maraldi. He was soon engaged in a regular, well-rounded program of celestial observations.

Having obtained the support of important members of the Academy of Sciences and having demonstrated, as early as 1749, his acquired skill and promise through the discovery of a nebula, a drawing of which he presented to them, Le Gentil was elected to the Academy in 1753. During the succeeding six years his contributions to its Mémoires ranged from historical concerns with the saros, through observational and descriptive papers, to studies that combined his own observations with those of others to perfect existing theory and quantitative evaluation of orbit inclinations and the obliquity of the ecliptic.

The turning point in Le Gentil’s career was his commission to observe the 1761 transit of Venus at Pondicherry, India. Because the English had captured that settlement just as he arrived (in 1760), Le Gentil was obliged to witness the transit from shipboard, without any possibility for scientifically significant observations. Since another transit of Venus was to take place in 1769, Le Gentil resolved to remain in the East in order to complete his mission. He used the intervening years to collect vast amounts of material on Indian astronomy and to make numerous excursions, from Madagascar to Manila, during the course of which he amassed observations on a broad spectrum of phenomena. Although his own calculations showed that the latter site would be excellent for observing the transit of 3 June 1769, the Academy ordered him back to Pondicherry for that purpose. The decision was unfortunate, since Manila was very clear that day, while at Pondicherry a cloud obscured the sun precisely during the crucial period. It was thus an extremely disappointed Le Gentil who returned to Paris two years later after an absence of eleven and a half years.

Problems of a different sort now confronted him. Le Gentil’s relatives, believing him dead, had begun a division of his estate; and the Academy, some members of which apparently interpreted his absence as undertaken for personal enrichment, had relegated him to “veteran” status. Both problems were soon solved: the former by legal actions which, however, did not return stolen monies and left him responsible for court costs; and the latter by his reinstatement in 1772 as an associate and, a decade later, by his supernumerary promotion to the pensionary level. Le Gentil married and moved back to the observatory, and was soon dividing his time between attention to the only offspring of his marriage, a daughter, and to his writings, most of which were based on the materials that he had brought back from the East.

Le Gentil’s major work was the two-volume Voyage dans les mers d’Inde… (1779-1781). The first volume was devoted to India and, after a discussion of the customs and religion of its inhabitants, dealt at considerable length with the history of Brahman astronomy. Le Gentil’s largely conjectural contention of that science’s great antiquity was disputed by many contemporaries and was rejected by virtually all later scholars. On the other hand, his personal astronomical observations—as well as those dealing with geography, meteorology, and physics—were securely based and of more lasting importance. His instruments having been verified at the outset, Le Gentil’s latitude and longitude determinations inspired confidence; his newly calculated table of refractions for the torrid zone, even without barometer and thermometer readings, was a decided improvement over existing values and a remarkable anticipation of later ones. His solstitial observations confirmed his earlier conclusions about the diminution of the obliquity of the ecliptic.

The second volume of the Voyage was devoted to the Philippines, Madagascar, and the Mascarenes. Except for the difference in locale, the absence of a historical section, and greater emphasis on geography and navigation, it conveyed the same sort of useful information as did the first.

Some of these materials were abstracted and offered to the Academy as memoirs during the 1770’s, while the Voyage was in preparation. Even after its publication, however, Le Gentil continued to exploit his Indian data, sometimes combining them with observations made after his return. Paper of the 1780’s dealing with refraction and the obliquity of the ecliptic were representative of this type, and a group of historical offerings during the same period were largely restatements of his claims for Indian astronomy, occasionally buttressed by “evidence” from Gothic zodiacs in and near Paris. The most important of his totally new productions were a memoir on tides that Le Gentil had observed on the coasts of Normandy and a paper alleging certain advantages of binocular over monocular instruments. Neither work was responsible for any basic advance.

The reconstruction of the observatory displaced Le Gentil from his lodgings in 1787, and he never returned—nor did the Academy ever fill the vacancy created by his death in 1792.

BIBLIOGRAPHY

I. Original Works. Le Gentil’s major work was Voyage dans les mers de l’Inde (1760-1771), fait par orde du roi,à l’occasion di passage de Vénus, sur le disque du soleil, le 6 Juin 1761, & le 3 du même mois 1769, 2 vols. (Paris, 1779-1781). His only other non-Academic publication dealt with the transit of Venus and was written before his departure: “Mémoire de M. Le Gentil, au sujet de I’observation qu’il va faire, par ordre du roi, dans les Indes orientales, du prochain passage de Vénus pardevant le soleil,” in Journal des sçavans (Mar. 1760), 137-139. Except for his 1749 “Mémoire sur une étoile nébuleuse nouvellement découverte àcôté de celle qui est au-dessus de la ceinture d’Andromède,” in Vol. II (1755) of the socalled Savants étrangers series, all of his other publications were contributions to the Mémoires. These included a paper on the inequalities noticed in the movements of Jupiter and Saturn and tables of the oppositions of those planets with the sun that he had read to the Academy as a nonmember and that had been destined for the same Savants étrangers volume. Having attained membership before the latter was printed, however, he withdrew it from the printing office, made some changes in it, reread it to the Academy, and placed it in the Mémoires for 1754.

Inasmuch as Quérard provides a nearly complete chronological listing of forty papers in the Mémoires from 1752 through 1789 (with none between 1760 and 1770, because of Le Gentil’s absence), there is no need to repeat. long titles here. It has, however, been thought appropriate to indicate the years in which Le Gentil offered various types of memoirs. His historical studies began with two papers on the saros (1756), in which he rejected Halley’s explanation of that period and substituted a fanciful scheme of his own; continued with three memoirs on various aspects of Indian astronomy (1772, pt. 2)—the antiquity of which he specifically insisted upon in a later work (1784); and ended by treating the origin of the zodiac and related matters (1782, two in 1785, two in 1788, 1789), again with frequent claims for Indian primacy.

Le Gentil’s purely observational works of the 1750’s dealt with determinations of the apparent diameter of the sun (1752, 1755) and of the earth’s shadow during lunar eclipses (1755), and recorded such transient and permanent phenomena as nebulae (1759), a variable star (1759), rainbows (1757), lunar occultations of a star (1753)and Venus (1753), a lunar eclipse (1755), and inferior conjunctions of Venus (1753) and Mercury (1753). The latter, as with the observations of oppositions of Jupiter and Saturn (see above ), were combined with remarks of theoretical interest.

More important efforts in the realm of theory were Le Gentile’s researches into the obliquity of the ecliptic (1757) and his three-part study of the principal orbital elements of the superior planets (one part in 1757, two in 1758). In addition to the abstract items of the 1770’s—the most important of which were two papers on horizontal refraction in the torrid zone (1774)—during that period he offered remarks on the temperature of the cellars of the Paris observatory (1774), an observation of a lunar eclipse (1773, with Bailly), and a memoir on the disappearance of Saturn’s ring (1775). Although he thereafter reported both specifically astronomical and various general physical observations: for instance, on the intense cold of late 1783 and on the prevailing winds of Paris (both in 1784), the works noted in the text were far more important contributions: those on refraction (1789), the obliquity of the ecliptic (1783), tides (1782), and binocular instruments (1787).

II. Secondary Lliterature. In his series of annul histries of astronomy from 1781 to 1802, J. J. de Lalande almost always offered brief notices of astronomers who died during those years; his succinct treatment of Le Gentil may be most conveniently consulted in his Bibliographie astronomique… (Paris, an XI [1803]), p.722. During Condorcet’s absence from the Academy in 1792, the writing of an “official” éloge fell to the vice-secretary, J. D. Cassini (IV); the product was published in his Mémores pour servir à l’histoire des sciences et à celle de l’Observatoire royal de Paris (Paris, 1810), pp 358-372. That rather laudatory and uncritical account should be supplemented by J. B. J. Delambre’s trenchant analysis of Le Gentil’s most important works in Histoire de l’astronomie au dixhuitième Siècle (Paris, 1827), pp.688-709; Delambre had earlier criticized Le Gentil’s ideas about Indian astronomy in his Histoire de l’astronomie ancienne (Paris, 1817), I, 511-514.

As mentioned above, a nearly complete listing of Le Gentil’s offerings in the Mémoires, although virtually nothing else, is available in J. M. Quérard, La France littéraire… V(Paris, 1833), 95-96. The subsequent treatments in J. F. Michaud, ed., Biographie univarselle, XXIII, 618-619; and in Niels Nielsen, Géomètres Français du dixhuitième Siècle (Paris, 1935), pp.266-269, are flawed by minor errors. More specifically concerned with Le Gentil’s voyage and its outcome are Alfred Lacroix, Figures de savants, III (paris, 1938), 169-176; and, more important, Harry Woolf, The Transits of Venus (Princeton, 1959), esp. pp. 126-130; 151-156.

Seymour L. Chapin