Cutting-edge astronomy in the 18th century wasn't just a matter of technology. It could also mean valiant effort, great hardship and even death. There were many such stories in relation to the international collaborations for the transits of Venus.

Edmond Halley (1656-1742) had proposed a method of solving the problem of the size of the Solar System. It used trigonometry based on measuring a transit of Venus from different locations on Earth. The idea took hold with a view to the transits of 1761 and 1769. Although Halley didn't live to see it, astronomy was ready. (You can find out more by clicking on the links at the bottom of this article.)

Perhaps the most dedicated – but least successful – participant in the transit observations was French astronomer Guillaume Le Gentil who spent eleven years away from home. His fame now rests on his nearly proverbial bad luck. Yet he was an able astronomer who worked at the Paris Observatory, and had been elected to the French Academy of Sciences at the age of 28. Not only was he was a keen observer and a regular contributor to the Academy's Mémoires, but he had also observed the 1753 transit of Mercury.

The header image shows the central region of the star cluster M36, discovered by Le Gentil. Credit: Wikisky

The French Academy, with the support of the King, chose astronomers to observe the 1761 transit, and Le Gentil was one of them. He was to go to Pondicherry, a French settlement on the southeast coast of India. This meant sailing all the way around Africa to Mauritius, then finding a ship to India. He set off in March 1760 for the transit on June 6, 1761.

Off to a good start, Le Gentil arrived in Mauritius in July, but international politics confounded his undertaking. Britain and France were at war and because of disputed territory in the Indian Ocean region, there were no ships sailing to India. Le Gentil knew that if he didn't get away soon, the monsoon winds would greatly delay him.

However, in what seemed like the nick of time, a French frigate arrived on its way to India. A ship like this should be able to reach Pondicherry in two months, even with adverse winds. But it didn't. It got blown everywhere except where he wanted to go. Then, not far from his destination, they discovered that the British had taken Pondicherry, and the captain decided to return to Mauritius. They arrived back on June 23.

Transit day, June 6th, had been a clear day, but they were at sea. Le Gentil needed a steady platform for his telescope, and precise timing couldn't be done with a pendulum clock on a rolling ship. He saw the transit, but his observations were useless. So did he look for passage home? No. After all, there was going to be another transit in eight years. Think of all the traveling time he could save by not returning to France and coming back again. So he told the Academy that he would stay and use the time to study the “geography, natural history, physics, astronomy, navigation, winds, and tides.”

Le Gentil also considered the best place to observe the 1769 transit, finally deciding on Manila in the Philippines. Before leaving Mauritius in the spring of 1766 he requested letters of recommendation for the Spanish governor in Manila from the Court of Spain. Nonetheless, from the start, the governor was hostile and unhelpful. He was not only a tyrant but also suspicious of the French. When Le Gentil's requested letters of support arrived in July 1767, the governor claimed they'd arrived too quickly and accused the astronomer of forgery. Le Gentil feared for his safety in Manila and was having doubts about the weather, so decided he would go to Pondicherry, now back in French hands.

On arrival in Pondicherry in March 1768, Le Gentil was warmly welcomed by the governor who also had an observatory built for him. Learning about Brahmin astronomy kept Le Gentil busy, along with his other researches.

He was very hopeful for June 4th after perfect weather throughout May, and indeed up through June 3rd. But at the critical time — very early on the 4th — the wind changed, it clouded up, and it rained. Nothing could be seen. When the transit ended the sky gradually cleared, and there was brilliant sunshine for the rest of the day. It's not surprising that Le Gentil took to his bed for two weeks and couldn't even bear to write in his journal.

By the way, it turned out that the weather in Manila was perfect.

Le Gentil then suffered bouts of severe illness and couldn't travel. He was anxious to get home because he'd had word that his family insisted he was dead and wanted to divide up his estate. In March 1770 he made it as far as Mauritius, but was too ill to travel farther. He finally set off in November, but the ship ran into a hurricane and was so badly damaged it was lucky to get back to Mauritius.

Finally, his return home was by Spanish frigate to Cadiz where he arrived in August 1771. The last part of the journey was by land, crossing the Pyrenees to France.

He did find that his estate was about to be divided. More painfully, he had lost his seat in the Academy of Sciences, the organization for which he had made the expedition. But most of his possessions were saved and the King intervened to restore him to the Academy.

Le Gentil lived for another two decades, and in that time he returned to the Paris Observatory, married, and had a daughter of whom he was very fond. He also published two volumes of work related to his voyages. He died in 1792, a year before the Reign of Terror of the French Revolution executed thousands of people, including some members of the Academy.

So in the end perhaps he was not so unlucky after all.

Reference:
Helen Sawyer Hogg, “Le Gentil and the transits of Venus, 1761 and 1769,” Royal Astronomical Society of Canada, https://cseligman.com/text/atlas/LeGentil.pdf