Guillaume Le Gentil Biography

Guillaume Le Gentil was born on September 12, 1725, in Coutances, Lower Normandy, France.

He wanted to enter the church and attended Collége de France. While studying there, he was influenced by astronomy lectures of French astronomer and cartographer, Joseph-Nicolas Delisle, and resolved to turn to astronomy.

Guillaume Le Gentil started working as assistant of French astronomer, Jacques Cassini, at the Paris Observatory and took part in the geodesic surveys of Cassini. Le Gentil also began observing deep sky objects.

In 1749, he discovered a satellite galaxy of the Andromeda Galaxy (M31) called Messier 32, also referred as M32 and NGC 221. It is a dwarf "early-type" galaxy that appears in the constellation Andromeda and is situated about 2.65 million light-years from Earth.

In 1749, he also independently re-discovered two open clusters of stars in the Auriga constellation namely the Messier 36, also referred as M36 and NGC 1960; and Messier 38, also referred as M38 and NGC 1912.

Such observations were then presented to the Paris Royal Academy of Sciences on the same year. While investigating the Milky Way star cloud in Aquila, Le Gentil probably discovered the globular cluster NGC 6712 on July 9, 1749.

He discovered the nebulosity in M8 and became the first one to catalogue the dark nebula in the constellation Cygnus that is sometimes referred as Le Gentil 3.

He was appointed to the Royal Academy of Sciences in 1753.

The observations and knowledge of Le Gentil on the Northern "nebulae" were summarised by him in a memoir on July 26, 1758, and presented to the French Royal Academy. This was just about a month prior to discovery of first nebula by French astronomer Charles Messier. Le Gentil’s work was released in the volume of the Royal Academy for 1759 and was ultimately printed in 1765.

Meanwhile, he became part of the French expedition of an international collaborative project that was organized by Russian polymath, scientist and writer, Mikhail Lomonosov, for measuring the distance to the Sun, by observing the transit of Venus from different parts of the world. As the project launched, 120 observers from nine nations travelled to different parts of the world to observe the transit of Venus coming up in 1761.

Le Gentil was commissioned to observe the 1761 transit of Venus at Pondicherry, a French possession in India. He departed from Paris in such pursuit in March 1760. In July that year, he reached Isle de France (presently Mauritius). Meanwhile the Seven Years' War between Britain and France broke out on May 17, 1756, which impeded Le Gentil’s further travel towards the east.

In March 1761, Le Gentil managed to board a frigate that was sailing to the Coromandel Coast in India, and hoped to reach Pondicherry on time for observing the transit. Although he was assured that he would reach his destination on time, which was before the transit date of June 6, the ship had to spent five weeks at sea due to unfavourable winds.

When the ship neared Pondicherry, the captain was informed that the British captured Pondicherry which led the ship to commence a return journey to Isle de France. On June 6, although the sky was clear, the ship was still rolling about at sea and Le Gentil could not take astronomical observations.

Instead of returning to Paris, Le Gentil resolved to stay back in the East and observe the 1769 transit of Venus. He mapped the eastern coast of Madagascar for some time. He then resolved to record the forthcoming transit from Manila in the Philippines, however hostile behaviour of Spanish authorities there led him to travel back to Pondicherry, which by such time was restored to France following a 1763 peace treaty. He reached Pondicherry in March 1768 and built a small observatory to observe the June 4, 1769, transit of Venus, however when the day arrived, the sky remained cloudy during the crucial period thus hindering Le Gentil’s effort in observing the transit.

While in India, he examined local astronomical traditions and penned down many notes on the subject. He mentioned that prediction of a Tamil astronomer on duration of the August 30, 1765, lunar eclipse was found short by 41 seconds while the Tobias Mayer charts were long by 68 seconds. The Tamil astronomer made the prediction based on computation of the size and extent of the earth shadow.

Following two unsuccessful attempts to observe the transits of Venus from India, Guillaume Le Gentil decided to return to Paris. His return trip was initially delayed due to dysentery and after he boarded the ship, it was caught in a storm. He was dropped off at Île Bourbon (Réunion). He had to wait there for some time till he managed to board a Spanish ship.

He reached Paris in October 1771 and was shocked to find that he was declared legally dead, his wife had remarried, his relatives took possession of his estate and his seat at the Royal Academy of Sciences was taken by someone else. Although Le Gentil wrote letters to the Academy and to his relatives while he was away from Paris, none of those reached their destinations amidst shipwrecks and wartime attacks on ships.

Such uncalled for developments led Le Gentil to take legal actions and following lengthy litigation and the intervention of the king, Le Gentil got back his seat in the Academy and remarried.

Guillaume Le Gentil had a daughter from his remarriage and divided his time between taking care of his daughter and his writings, particularly the writings that were based on materials he gathered from the East. A Voyage in the Indian Ocean, an account of his travel, was published in two volumes in 1779 and 1781.

Guillaume Le Gentil was displaced from his lodgings in 1787 due to re-construction of the observatory. He never returned and on October 22, 1792, Le Gentil died in Paris, France.

The Academy never filled the vacancy following his death. The lunar impact crater Le Gentil located in the south-southwest part of the Moon was named after him.