Mauritius and the French astronomer who missed the Transit of Venus

From the end of the 16th century, French astronomers routinely sailed to India to harvest astronomical techniques that could be brought back to France.

One of such quests was to understand the transit of the planet Venus.

Twice in a hundred-plus years, planet Venus has its shining hour when it moves across the face of the Sun.

The transit of the ‘morning star’ is a rare spectacle that happens in a predictable pattern of two – occurring twice within an eight-year period followed by a gap of 121 and a half years.

Astronomers of the time knew how important the transit was to unlocking the secrets of the universe, especially in the light of Kepler’s prediction of the 1761 transit and Edmond Halley’s suggestion of placing observers at different points on earth to calculate the distance between the earth and the sun (one astronomical unit) using trigonometry, triangulation and parallax methods.

France sent astronomer Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière (1725–92) to Pondicherry which was ideally located to see the transit.

After three months at sea, he arrived in Mauritius. The island was still a French colony then and French colonisers were well settled on the island.

As he was about to pursue his journey he soon found out that the Seven Years War between England and France had begun – Pondicherry was under siege by English warships.

He set out nevertheless, but somehow reached a wrong destination. He failed to receive permission to go to Pondicherry and was compelled to sail back to Mauritius.

The transit occurred while he was at sea. Le Gentil was unable to determine his longitude or make precise time recordings, rendering his observations scientifically useless.

He decided to wait in Mauritius till the next transition on June 4, 1769. He used the waiting period to calculate the best places to view the transition.

He also explored Madagascar; describing Fort-Dauphin in the South and Foulpointe on the East coast, where he performed observations on the local flora and fauna. He climbed the mountains of Bourbon Island (now Reunion Island) and mapped it.

He left Mauritius after few years later to Manila, but was accused by the Spanish governor of to Manila being a spy and fled to Pondicherry in time for the second transit.

The night before the transit, the weather was clear. But during the transit, monsoon clouds rolled in, and again, Le Gentil missed recording the event.

Misfortune followed him. After 11 years, when he got back home, he discovered that he had been officially declared deceased, his possessions gone to relatives, his wife remarried and his fellowship at the Royal Academy of Sciences terminated.

And so, a town on the coast of the Bay of Bengal figures in the astronomical archives as the place where one man’s chance to make history was shattered twice.

More stories like this at Science Beyond Borders, an exhibition that is on at Bangalore International Centre (11am to 8pm) till May 21, 2022.