Tales from the Transit of Venus

sad old sun
Sad Old Sun

Today is the transit of Venus, which, aside from being a totally rad astronomical event, is also the perfect excuse to tell my favorite story of an unlucky Frenchman (I have many). This is by no means new and, if you've ever taken an astronomy course, you've probably already heard it. It is perhaps the closest thing Astronomy has to a ghost story, told though the glow of a flashlight on moonless nights to scare the children. This is the story of Guillaume Le Gentil, a dude that just couldn't catch a break.

Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière was a Frenchman with an incredibly long name. He was also an astronomer, though he hadn't started out that way. Monsieur Le Gentil (as his friends called him and so, then, shall we) had originally intended to enter the priesthood. However, he soon began sneaking away to hear astronomy lectures and quickly switched from studies of Heaven to those heavens more readily observed in a telescope. Le Gentil happened to get into the astronomy game at a very exciting time. The next pair of Venus transits was imminent and astronomers were giddy with anticipation. Though the previous transit of 1639 had been predicted, it was met with little fanfare and only a few measurements. But the transits of 1761 and 1769 would be different. People would be ready. And the stakes were higher this time, too. Soon after the 1639 transit, Edmund Halley (he of the-only-comet-people-can-name fame) calculated that with enough simultaneous measurements, the distance from the Earth to the Sun (the so-called astronomical unit, or AU) could be measured fairly accurately. Since almost all other astronomical distances were known in terms of the AU, knowing its precise value would essentially set the scale for the cosmos. Brand new telescopes in hand, the astronomers of Europe set sail for locations all over the world.

Le Gentil had been assigned to observe the transit from Pondicherry, a French holding on the eastern side of India. On March 26th, 1760, he began his long sea voyage around the Cape of Good Hope towards India.

The voyage from France to India was a bit too long for the ship Le Gentil hitched a ride on and he only made it as far as Mauritius (a small island off Madagascar). Dropped off with all his equipment, Le Gentil was left waiting for any ship at all to take him to Pondicherry.

Perhaps it was the Curse of the Dodo or perhaps it was just bad luck, but while he was waiting, Le Gentil learned that war had broken out between the French and the British, making a trip to British India very difficult for a Frenchman.

Then the monsoon season started, meaning that even if he could find a ship, it would have to take a much longer route to India than initially planned and that it would be very difficult to make the journey before the transit occurred.

Then, he caught dysentery for the first time.

Finally, after months of waiting, Le Gentil (barely recovered from his illness) left Mauritius for India in February of 1761. Though time appeared to be running out, the captain of the ship he was on promised he would be there to observe the transit in June. About halfway to India, the winds switched directions and the ship was forced to turn back to Mauritius.

Le Gentil dutifully observed the transit of Venus in 1761 from a rocky ship in the middle of the Indian Ocean. The data were useless and he never attempted any analysis.

Although he missed the first transit, these things come in pairs separated by eight years. There was still another chance. And with all this time to prepare, there was no way he was going to miss the second one.

In fact, there was a bit too much time. But as a world-traveling 18th century man of science, Le Gentil had plenty of other interests to fill his days. He was particularly interested in surveying the region around Madagascar.

So he made a really nice map of Madagascar. And then he ate some bad kind of some kind of animal and came down with a terrible sickness. He describes this illness and its "cure" in his journals:

This sickness was a sort of violent stroke, of which several very copious blood-lettings made immediately on my arm and my foot, and emetic administered twelve hours afterwards, rid me of it quite quickly. But there remained for seven or eight days in my optic nerve a singular impression from this sickness; it was to see two objects in the place of one, beside each other; this illusion disappeared little by little as I regained my strength...

After recovering from both his sickness and the treatment, Le Gentil decided to begin his preparations for the 1769 transit of Venus. He calculated that either Manila or the Mariana Islands would be the ideal spot to observe. The Sun would be relatively high in the sky at both places when Venus passed by, meaning that the view would be through less atmosphere with a reduced chance of clouds passing through the line of sight. Le Gentil packed up his stuff and headed off to Manila, where he could catch another ship to get to the Mariana Islands. Arriving in Manila in 1766, the astronomer found himself exhausted from months of sickness and sea-voyage. So, when he was offered passage on a ship heading to the Mariana Islands, he quickly declined. That he chose not to depart Manila at that time was perhaps his one stroke of good luck in the entire journey. The ship sunk. Writing in his journal, Le Gentil appears to have developed that particular sense of humor that generally accompanies constant disappointment:

It is true that only three or four people were drowned, those who were the most eager to save themselves, which is what almost always happens in shipwrecks. I cannot answer that I would not have increased the number of persons eager to save themselves.

In any case, Le Gentil was in Manila with plenty of time to prepare for the next transit. Unfortunately, the astronomer may have over-prepared. Having arrived three years before the event, he now had three years to worry and second-guess his decision. It didn't help that the Spanish governor of Manila was kind of a crazy person. Not wanting to miss the observation of a lifetime owing to the whims of mildly insane strong man, Le Gentil packed up his stuff and headed to Pondicherry. Finally in Pondicherry, Le Gentil worked tirelessly to construct his observatory and make plenty of astronomical observations in preparation for the event. He had state of the art equipment and had fully calibrated and double checked everything. It was now nine years since his journey began and only a few days until the transit was scheduled to occur at sunrise on June 4th. The entire month of May was beautiful weather and pristine observing conditions, as were the first few days of June. Le Gentil likely went to bed on the 3rd of June fully confident that the next morning would be no different. He woke up early in the morning to begin preparations for his sunrise observations only to find clouds on the horizon. The clouds remained, obscuring the sun, all through the duration of the transit. A few hours after the end of the transit, the sun broke through the clouds and remained visible for the rest of the day. Le Gentil had missed his second transit in Pondicherry. He sums it up in his journal:

That is the fate which often awaits astronomers. I had gone more than ten thousand leagues; it seemed that I had crossed such a great expanse of seas, exiling myself from my native land, only to be the spectator of a fatal cloud which came to place itself before the sun at the precise moment of my observation, to carry off from me the fruits of my pains and of my fatigues

In Manila, the Sun rose in perfectly clear skies. Distraught, Le Gentil remained in bed for some weeks afterward. He soon caught a fever and missed the ship that was supposed to take him home. He recovered, but then came down with dysentery again. Barely recovered from his various illnesses, he managed to get a ride back to Mauritius. He caught a ship leaving the island in November of 1770. The ship was struck by a hurricane and almost completely destroyed. It managed to limp back to Mauritius. The second attempt proved more successful and Le Gentil finally "set foot on France at nine o'clock in the morning, after eleven years, six months and thirteen days of absence." Though he had finally made it home, he was not out of the woods quite yet. In his absence, Le Gentil's heirs had tried to declare him dead to gain their inheritance, his accountant had mishandled (and lost) a large chunk of his holdings, and the Academy of Sciences, which had sent him on his 11 year mission, had given his seat to someone else. It was not quite the welcome home he had hoped for. Despite his seemingly never-ending misfortune, things did turn around for Le Gentil. He married, had a daughter, and was reinstated into the Academy of Sciences. Presumably, he lived out the rest of his days in relative happiness. Le Gentil died in 1792. Keeping true to his style, this man who missed two of the most important astronomical events of his time fortunately managed to also miss the most important (and violent) political event of his time.


I have mainly used a very nice series of historical papers of Le Gentil's misadventures with the transit of Venus written by Helen Sawyer Hogg. The papers were originally published in the Journal of the Royal Astronomical Society of Canada and can be accessed through NASA's ADS (Part 1, Part2, Part 3, Part 4).

More Transit of Venus:

If you want to see the Transit of Venus without having to go on an eleven year voyage (or even leaving your room), check out the NASA live-feed from Mauna Kea.


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