In 1761, participating in a big project with more than two hundred astronomers, the French astronomer Guillame Legentil set out to observe the Venus transit. But the place he was heading for was under occupation, and since he was not able to observe the phenomenon from the ship, he decided to wait and observe the next transit from the Philippines eight years later. But there was trouble in sight for the patient astronomer …

by Anne Mette Sannes

No doubt, it was the English astronomer Edmond Halley who, despite his death twenty years earlier, had created awareness for the next pair of transits that would take place in 1761 and 1769. Sites he recommended for observation were Hudson Bay, Norway and The Malucu Islands.

As early as 1716, he delivered a letter to The Royal Society where he urged future scientists always to observe Venus transits, to wish them luck and that observers never should be depressed by failed observations due to cloudy weather. He also asked that the transits in 1761 and 1769 should be used to measure the exact value of the astronomical unit by measuring the solar parallax.

And his appeal was heard! The transit was observed from more than 120 sites around the world by more than two hundred astronomers, a project led by Russian scientist Mikhael Lomonosov – the first one to discover the atmosphere of Venus.

One of the participants in this project was the French astronomer Guillame Legentil, or, to be more exact: Guillame Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière (!)

16th March 1760, he sailed from Brest, the northwest of France, to Pondicherry, a French colony south of India to observe the Venus transit. But, upon arrival, it appeared that Pondicherry was occupied by the British (war between England and France had broken out) and Legentil had to return to Mauritius. 6th June arrived, but because he was on the ship, he was not able to carry out the observations. Instead he decided to wait for the next transit, eight years later. After spending time on mapping the East coast of Madagascar, he decided to observe the upcoming transit from the Philippines and arrived in Manila in August 1766. But in July 1767, he was recommended by the French government to return to Pondicherry which was now possible due to the peace agreement established in 1763. He followed the recommendation, and in March 1768, he was back in Pondicherry where he built an observatory as he patiently waited for the upcoming transit in June the following year.

The day before Venus was to cross the solar disk, the weather was perfect, but the clouds came rolling in … 3rd June was cloudy and the astronomer who had patiently waited for so many years, saw nothing. An hour later the sun was shining. And as if that was not enough – the weather in Manila where he had originally planned to stay, was perfect during the whole transit!

This bad luck almost brought the poor astronomer out of his mind, but in the end he managed to pull himself together and sail back to France. Unfortunately, the ship was delayed due to dysentery, and by the time he finally could go, the ship was captured by a storm and he had to go ashore on Réunion, a French island east of Madagascar and wait until a Spanish ship brought him home to France. In 1771, eleven years after he left France, he arrived in Paris only to find out that he was declared legally dead by his employer, The Royal Academy of Science. He was listed as one who wanted to "encourage and protect the spirit of the French scientific research".

His private life was also suffering; his wife had remarried and his relatives had divided the assets between them. After a number of lawsuits, things started to normalize for the poor astronomer. He got his job back, remarried and lived happily for the next twenty-one years. It is said that both his wife and his daughter made him forget the unfortunate episode in 1769. But, no matter what happened, he was credited for discovering the objects M32, M36 and M38. In addition, he also discovered M2 or the Lagoon Nebula and was the first one to catalog the dark material in the constellation of the Swan, also called Le Gentil.

What had been learned from these two Venus transits?

British preparations for the Venus transit in 1761 started somewhat later than the French, but as they first got started, it was decided to send as many expeditions as possible. The expedition to St. Helena was a complete fiasco due to bad weather. But for Professor Winthrop, who had travelled all the way to St. Johns in Newfoundland, luck was on his side. The observations were carried out under perfect weather conditions and the results turned out to be one of the best.

Also in Sweden the weather was perfect, and Per Wargentin who both worked and lived at the Stockholm Observatory, observed the Venus transit and was probably the first one to mention the problem with the black-drop-effect. The effect occurs due to the enormous contrast in brightness between the precipitous night side of Venus and the intense solar disk. It looks as if Venus is melting together with the Sun's limb, and this effect makes it difficult to determine the exact time for Venus passing in or out of the solar disk.

In summary, the program around the Venus transit in 1761 was the biggest international program so far. However, the results were disappointing since the observations varied a lot due to the black-drop-effect.

The estimates of the solar parallax varied from 8.5 to 19.5 arc seconds, corresponding to a distance between 154 and 125 million kilometers. One of the reasons for this, was the lack of sufficient coordinates for all the observation sites. Another reason was the low number of measurements between the first and second contact. But all in all – the information had been useful and the astronomers had learned a lot.

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