Guillaume Le Gentil

Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière (, 11 or 12 September 1725 – 22 October 1792) was a French astronomer who discovered several nebulae and was appointed to the Royal Academy of Sciences. He wrote on the estimation of the distance from the Earth to the Sun using solar transits, made unsuccessful attempts to observe the 1761 and 1769 transits of Venus from India during an 11-year journey from France, and wrote a popular account of his adventures and misadventures during the journey.

Biography Guillaume Le Gentil was born on 11 or 12 September 1725 in Coutances and first intended to enter the church before turning to astronomy when inspired by a lecture by Joseph-Nicolas Delisle. By 1753 he was a professional astronomer and he wrote influentially on the subject of estimating the distance from the Earth to the Sun, the astronomical unit (AU), using transits of Mercury. He set out from Paris in March 1760, and reached Isle de France (now Mauritius) in July. However, the Seven Years' War had broken out between France and Britain in the meantime, hindering further passage east.

Having already completed the trip from Paris, he stayed for the next transit of Venus, which would come in another eight years (they occur in pairs 8 years apart, but each pair is separated from the next by 121 or 105 years). After spending some time mapping the eastern coast of Madagascar, he decided to record the 1769 transit from Manila in the Philippines. Due to shipwrecks and wartime attacks on ships, none of the letters he had sent to the academy or to his relatives had reached their destinations. Lengthy litigation and the intervention of the king were ultimately required before he recovered his seat in the academy and remarried.

During the time he spent in India, Le Gentil examined local astronomical traditions and wrote several notes on the topic. He reported that the duration of the lunar eclipse of 30 August 1765 was predicted by a Tamil astronomer, based on the computation of the size and extent of the earth-shadow, and was found short by 41 seconds, whereas the charts of Tobias Mayer were wrong by 68 seconds.

He died in Paris on 22 October 1792.

Catalogue Le Gentil memoir from 1749 contains:

Messier 32 Lagoon Nebula Le Gentil 3 Messier 41 Messier 36 Messier 38 Aside from M41 previously found by John Flamsteed, the others are considered to have been first found or discovered independently by Le Gentil. Le Gentil crater on the Moon was named in his honour in 1935, and asteroid 12718 Le Gentil in 2004.