Guillaume Le Gentil

Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière (September 12 1725 – October 22 1792) was a French astronomer.

He was born in Coutances and first intended to enter the church before turning to astronomy. He discovered what are now known as the Messier objects M32, M36 and M38, as well as the nebulosity in M8, and he was the first to catalogue the dark nebula in the Cygnus constellation, sometimes known as Le Gentil 3 . However, he is chiefly remembered today for the unfortunate fate that befell him when he set out to observe the transit of Venus in 1761 at Pondicherry, a French colony in India.

He set out from Paris in March 1760, and reached Île de France (Mauritius) in July. But having learned that war had broken out between France and Britain, and deeming it dangerous to try and reach Pondicherry, he determined to go elsewhere; a frigate was bound for India's Coromandel Coast, and he sailed in March 1761. When they had nearly arrived they learned that the British had occupied Pondicherry, so the frigate was obliged to return to Île de France. June 6, the day of the transit, came, and the sky was clear, but he could not take astronomical observations with the vessel rolling about. He had gone abroad to see the transit of Venus, and there would be another in eight years, so he made up his mind to stay. (Transits of Venus occur in pairs 8 years apart, but each such pair is separated from the previous and next pairs by more than a century.)

After spending some time mapping the eastern coast of Madagascar, he decided to record the 1769 transit from Manila in the Philippines. Encountering hostility from the Spanish authorities there, he headed back to Pondicherry, which had been restored to France by peace treaty in 1763, where he arrived in March 1768. He built a small observatory and waited patiently. At last, the day in question (June 3, 1769) arrived, but although the mornings in the preceding month had all been lovely, on this day the sky became overcast, and Le Gentil saw nothing. For some time he was almost crazy, but at last he recovered enough strength to return to France.

The return trip was first delayed by illness, and further when his ship was caught in a storm and dropped him off at Île Bourbon (Réunion), where he had to wait until a Spanish ship took him home. He finally arrived in Paris in October 1771, only to find that he had been declared legally dead, he had been replaced in the Royal Academy of Sciences, his wife had remarried, and all his possessions had been distributed to his heirs. It required lengthy litigation to correct the latter problem, but intervention of the king soon gave him back a seat in the academy, and he remarried and lived apparently happily for another 21 years.

Reading

• The story of Le Gentil is memorably related in Timothy Ferris's Coming of Age in the Milky Way.

• Le Gentil's attempt to view the eclipse in India also figures in Bill Bryson's A Short History of Nearly Everything.

• Le Gentil himself was the author of Voyage dans les mers de l'Inde, fait par ordre du Roi, à l'occasion du passage de Vénus, sur le disque du Soleil, le 6 juin 1761 & le 3 du même mois 1769 par M. Le Gentil, de l'académie royale des sciences. Imprimé par ordre de sa Majesté, two volumes, Paris 1779 and 1781.