Modern astronomical observatories are located on barren far-away mountaintops with dry, thin, freezing-cold air. But modern astronomers travel in relative comfort to get to these observatories. Air travel is fast, frequent-flier lounges are plush, and food and lodging at major observatories rival those of a comfortable hotel. And once observing is done, an astronomer can fly home in a day to join colleagues for lunch at the faculty club.
But in 1760, it was different.
In these early days of the Age of Reason, traveling astronomers braved war, stormy seas, and deadly tropical disease to reach their destinations. And they were often away from home for several months, or even a couple of years.
But no astronomical expedition comes close to matching the epic hardship and back luck that fell upon the French astronomer Guillaume le Gentil, who braved eleven years of travel around the Indian Ocean in an attempt to observe the transit of Venus across the Sun’s disk.
The transit of Venus was a huge deal in the 18th century. Fifty years earlier, Sir Edmund Halley worked out a method to figure the distance from the Earth to the Sun by measuring the time it took for Venus to pass across the Sun’s disk from various parts of Earth. Once the distance from the Earth to the Sun was known, astronomers could calculate the distance to other planets in the solar system, and perhaps even to nearby stars. This was turning point in human understanding of the universe.
But a transit of Venus is a rare event, occurring at eight-year intervals just once per century. One transit was to occur on June 6, 1761 and another on June 4, 1769. Guillaume le Gentil was dispatched in March, 1760 at the request of the French Academy of Sciences to observe the transit from the French colony at Pondicherry, India. He was one of hundreds of European astronomers who traveled across the world to observe the transit. le Gentil sailed around the Cape of Good Hope to what’s now called Mauritius, and waited months to secure a ride on another ship to Pondicherry. He found a ship. But while enroute he discovered the British, who were at war with the French, had seized Pondicherry and he would have to return to Mauritius. The transit of Venus occurred when he was still at sea, and while he observed the event, he obtained no useful measurements on the moving ship.
He knew the next transit was eight years away, so he stayed in Mauritius and nearby Isle de Bourbon (now called Reunion Island). He mapped the coast of Madagascar, learned about indigenous culture, and collected samples of the natural history of the region. He planned carefully for the next transit, determining that Manila was the best nearby place to see it. He found a Spanish ship to take him to Manila. And he looked forward to returning to Europe eastward from Manila past Mexico and around South America, completing his trip around the world.
le Gentil arrived in Manila in March, 1766 to prepare for the transit. But he was not welcome by the Spanish governor of the colony. Seeking council from France, he was advised to return to Pondicherry to avoid trouble with the Spanish. He could have refused, but he believed India had a better chance of clear skies during the event. Once back in India, he calibrated his instruments and enjoyed a month-long spell of perfectly clear skies before the transit. But on the morning of June 4, 1769, just as the transit of Venus began, the sky filled with clouds for a few hours, just long enough for le Gentil to miss the entire event. He later learned that his colleagues in Manila enjoyed the transit under perfectly clear sky.
This was all too much for le Gentil, and his bad luck sent him to the brink of insanity. At the very least, he was exhausted by years of hard travel and wished to return home. So late in 1769, he pulled himself together, packed his equipment, and set sail for France.
But his troubles were not over.
In early 1770, before leaving Pondicherry, he contracted dysentery. Determined to press on, he set sail in March 1770, still quite sick. His illness at sea was overwhelming, so he landed again in Mauritius to recover, where among other events, he watched a valued colleague die of tropical fever. But le Gentil recovered his health and secured a place on a trading ship to return to France in July. The ship delayed sailing until November– hurricane season– and first set eastward from Mauritius to Reunion Island instead of westward around the Cape of Good Hope towards Europe. The ship was caught in a massive storm in early December, when its rudder was badly damaged and its mast nearly sheared off. The ship back barely made it back to Mauritius on January 1, 1771.
The hapless le Gentil almost gave in to despair. But he persisted, and in March 1771 he gained berth on a Spanish ship which was repeatedly delayed by more storms near the Cape. He rounded the Cape in May, and after many tense encounters with English and Spanish ships, who were preparing for war with France, finally landed in his home country on October 8, 1771 after enduring an absence of eleven years, six months, and thirteen days to observe an astronomical event that lasted about six hours.
Ah, to be home! Given le Gentil’s abnormally long absence and lack of correspondence, his countrymen were shocked to see him alive. And so, apparently, was his family. While he was away, his wife had remarried and his relatives declared him dead and ransacked his estate. What’s more, he lost his place in the French Academy of Sciences, the same institution that sent him on his odyssey in the first place.
It took years of litigation and the intervention of the King of France to set things right. But le Gentil resumed a normal life. He remarried, regained his position in the Academy, and lived for another 21 years. And as with most trips gone awry, the pleasure lies not in the trip itself but in the telling of tales after returning home. He immortalized his adventures in his two-volume memoir “A Voyage in the Indian Ocean”.
While le Gentil enjoyed robust health in his later years, he died of a sudden serious illness in 1792 at the age of 67. He was, at least, saved from the escalating horror of the French Revolution, which did not treat kindly the French Academy and its members.
Aside from his epic story, which has been retold for more than two centuries, le Gentil is also remembered by a crater on the Moon and by a large dark nebula in Cygnus near the Northern Coalsack. He was also the subject of a play by Canadian writer Maureen Hunter in 1992.
As you will learn in an upcoming article, the next transit of Venus occurs on June 6, 2012. Plan accordingly…