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Guillaume Le Gentil | |
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| Born | Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière 11 or(1725-09-12)12 September 1725 |
| Died | 22 October 1792(1792-10-22) (aged 67) Paris, France |
| Known for | M32 |
| Scientific career | |
| Fields | Astronomy |
Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière (French pronunciation:[ɡijomləʒɑ̃ti], 11 or 12 September 1725 – 22 October 1792) was a Frenchastronomer who discovered severalnebulae and was appointed to theRoyal Academy of Sciences. He wrote on the estimation of the distance from the Earth to the Sun usingsolar transits, made unsuccessful attempts to observe the 1761 and 1769transits of Venus from India during an 11-year journey from France, and wrote a popular account of his adventures and misadventures during the journey.
Guillaume Le Gentil was born on 11[1] or 12 September 1725[2][a] inCoutances and first intended to enter the church before turning to astronomy when inspired by a lecture byJoseph-Nicolas Delisle.[3] By 1753 he was a professional astronomer and he wrote influentially on the subject of estimating the distance from the Earth to the Sun, theastronomical unit (AU), usingtransits of Mercury.[3] He argued, contraEdmond Halley, that these transits could not be used for an accurate measurement of the AU.[3] He discovered what are now known as theMessier objectsM32,M36 andM38, as well as the nebulosity inM8, and he was the first to catalogue thedark nebula sometimes known asLe Gentil 3 (in the constellationCygnus).
He was part of the international collaborative project organized byMikhail Lomonosov to measure the distance to the Sun, by observing thetransit of Venus at different points on the earth.Edmond Halley had suggested the idea, but it required careful measurements from different places on earth, and the project was launched with more than a hundred observers dispatched to different parts of the globe to observe the transit coming up in 1761.
As a part of the French expedition, Le Gentil set out forPondicherry, a French possession inIndia.[3][4] He set out from Paris in March 1760, and reachedIsle de France (nowMauritius) in July. However, theSeven Years' War had broken out betweenFrance andBritain in the meantime, hindering further passage east.[3] He finally managed to gain passage on a frigate that was bound for India'sCoromandel Coast, and he sailed in March 1761 with the intention of observing the transit from Pondicherry. Even though the transit was only a few months away, he was assured that they would make it in time. The ship was blown off-course by unfavorable winds and spent five weeks at sea. By the time it finally got close to Pondicherry, the captain learned that the British had occupied the city,[3] so the frigate was obliged to return to Isle de France. When 6 June came the sky was clear, but the ship was still at sea, and he could not take astronomical observations with the vessel rolling about.[5]
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).[6] After spending some time mapping the eastern coast ofMadagascar, he decided to record the 1769 transit fromManila in thePhilippines.[3] Encountering hostility from the Spanish authorities there, he headed back to Pondicherry,[3] which had been restored to France by peace treaty in 1763, where he arrived in March 1768. He built a small observatory to view the transit. On the day of the event, 4 June 1769, the sky became overcast, and Le Gentil saw nothing.[3]
The return trip was first delayed bydysentery, 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, having been away for eleven years, only to find that he had been declared legally dead and been replaced in theRoyal Academy of Sciences.[3] His wife had remarried,[dubious –discuss] and all his relatives had "enthusiastically plundered his estate".[7] 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.[8][dubious –discuss] 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.[9][10] He reported that the duration of thelunar eclipse of 30 August 1765 was predicted by aTamil astronomer, based on the computation of the size and extent of the earth-shadow, and was found short by 41 seconds, whereas the charts ofTobias Mayer were wrong by 68 seconds.[11]
He died in Paris on 22 October 1792.[3][12]
Le Gentil memoir from 1749 contains:[13]
Aside from M41 previously found byJohn Flamsteed, the others are considered to have been first found or discovered independently by Le Gentil.[13]
Le Gentil is the subject of a play by Canadian playwrightMaureen Hunter.Transit of Venus was first produced at theManitoba Theatre Centre in 1992. It was subsequently made into an operaof the same name with music byVictor Davies, presented byManitoba Opera in 2007, andOpera Carolina in 2010.
Le Gentil is the subject of a novel byAntoine LaurainLes Caprices d'un astre, published in 2022, and translated into English in 2023 as An Astronomer in Love.
Boulevard Legentil-de-la-Galaisière, one of the main streets in his home town Coutances, was named after him in 1866.[14]Le Gentil crater on the Moon was named in his honour in 1935,[b] and asteroid12718 Le Gentil in 2004.[15]
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