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Transit of Venus: a tale of two expeditions

Guillaume Le Gentil sailed the seas for many years to catch a glimpse of Venus in transit. Brocken Inaglory

On Wednesday, as you’ll no doubt know by now, a rare celestial event will occur. Venus will pass between the earth and the sun - the transit of Venus. You might also already know that this cosmic spectacle is not without cultural significance …

Some 240 years ago, astronomers from England, France and Austria were sent across the globe to observe the transit. Captain James Cook was part of the English expedition.

Cook also had secret sealed orders from the British Admiralty. After the Venus transit observations were complete the expedition would go in search of the mooted southern continent, Terra Australis Incognita. En route he mapped the east coast of Australia.

But Cook’s success obscures the misfortunes of another man, the French astronomer Guillaume Le Gentil and his cursed obsession to observe the transit.

The goal of the transit voyages

The 18th century transits (1761 and 1769) of Venus inspired international teams of astronomers to travel to the far reaches of the known world, to measure the size of the solar system.

Even today, observing a transit from multiple locations across Earth improves not only the precision of the measurement, but gives better odds against bad weather and bad luck.

Captain James Cook, circa 1775. Nathaniel Dance-Holland/Wikimedia Commons

But this venture was extremely dangerous in the 18th century, as the transits were only observable from some of the most inaccessible places at the time: South Africa, Siberia, North America, the Indian Ocean, the South Pacific and Central America.

As noted, the two 18th century transits were separated by eight years. Each observation promised to determine the size of the solar system. But the first transit in 1761 occurred during the peak of the “Seven Years’ War” between England and France.

A spirit of scientific collaboration was difficult to promote during the conflict.

Cook’s voyage to the South Pacific and Australia was one of the first attempts at a major international scientific collaboration to improve estimates for the Astronomical Unit.

Although Cook and Le Gentil were funded independently by the English Royal Society and the French Academe, the data from all observations was to be combined. In reality the two expeditions were in a friendly competition to show off their scientific prowess.

While Cook’s expedition was heralded a great success, the other, led by Le Gentil, endured great disappointment.

The luck of Le Gentil

Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière. An unheralded hero? Brocken Inaglory/Wikimedia

Le Gentil was commissioned by the French Academe to observe the 1761 transit of Venus.

After three months at sea, Le Gentil arrived at the French colony of Mauritius to secure a voyage to Pondicherry, only to find the Indian territory under siege from English warships.

Although he had papers granting safe passage, Le Gentil failed to receive permission, and the transit occurred while he was sailing back to Mauritius. This clearly made the task of precision timing impossible and the opportunity was lost.

Rather than give up, Le Gentil decided to stay in Mauritius for another five years, planing the best places to observe the next Venus transit in 1769. He also used the time to map the east coast of Madagascar.

With the war over and letters of support from the Governor of Mauritius and the French Academe, Le Gentil boarded a Spanish ship bound for Manila in May of 1766 … things were looking up.

Or so it seemed.

Fort Venus on the Island of Tahati. Drawings illustrative of Captain Cook's First Voyage, 1768-1771 located in the British Library

Unfortunately, after all his meticulous planning and a three-month voyage, Le Gentil was accused by the Spanish Governor of being a foreign spy. He fled Manila for Pondicherry, to observe the transit from the Indian Ocean destination.

On the night before the 1769 Venus transit, the weather in Pondicherry was clear and everything was set. But, after nine years waiting for this fateful moment, the wind changed and the transit, clouded out, was unobservable.

Compared to Gentil’s nightmare, Captain Cook’s expedition went smashingly well. Assisted by fellow-astronomer [Charles Green](, they set up at Fort Venus in Tahiti, the main observation site.

Cook published his transit observations for the Philosophical Transactions of the Royal Society, although he only used data obtained at Fort Venus. By combining observations from four other sites in addition to the Fort Venus data, Oxford astronomer Professor Thomas Hornsby calculated a useful value for the solar-parallax of 8.78" (arcseconds).

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars. NASA

Le Gentil’s return to Paris

After two failed attempts at observing a Venus transit, spanning a total of 11 years, Le Gentil decided to return to France – a journey that took more than a year.

He had to contend with sickness and a hurricane damaging his ship.

Le Gentil finally returned to Paris where he was greatly acclaimed for his memoirs and scientific studies spanning a decade spent at Mauritius, Madagascar, and India.

One can only wonder how deflated Le Gentil’s felt that a series of remarkable misfortunes prevented him observing a transit of Venus – not to mention the fame and praise heaped on Captain Cook. Le Gentil would have been painfully aware of his friendly competitor’s sucesss.

If you have the opportunity to observe the transit of Venus on Wednesday, don’t be a Le Gentil: make like Captain Cook and watch the skies for something truly remarkable.

Further reading:

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