Transit of Venus: Early 'tape measure' for size of our solar system (+video)

A Frenchman commonly known by the name of Le Gentil (full name: Guillaume-Joseph-Hyacinthe-Jean-Baptiste Gentil de la Galaisière!) departed France more than a year in advance of the 1761 transit, and headed for Pondicherry, India. First he survived a hurricane and a bout of dysentery, and then Pondicherry was captured by the British (it was at the height of the Seven Years' War between England and France).

As such, Le Gentil was prevented from landing in India and was forced to watch the transit out at sea from the deck of a rolling ship, making his observations practically useless. Ultimately, he ended up on the island of Mauritius.

Passionately fond of astronomy, Le Gentil made the decision to make Mauritius his home base for the next eight years to await the Venus transit of June 1769. He spent those eight years exploring the geography and history of the Indian Ocean. Ultimately, he decided to observe the transit from Manila, but was then instructed by his sponsor, the Academy of Sciences in Paris, to go to Pondicherry instead.

The weather is usually superb at Pondicherry in June, but on the day of the transit, a "vexatious cloud" covered the sun, preventing Le Gentil from observing the transit. Worse yet, he was later exasperated to learn that in Manila, the sky was crystal clear.

Sadly, Le Gentil decided to return to Paris. But along the way he suffered through two shipwrecks and when he finally arrived in France, he discovered that he was presumed to be dead by his heirs, who were busily dividing up his estate. 

Captain James Cook (1728-1779), called by some "The Great Ocean's greatest explorer," observed the 1769 transit from the black sand beach now called Venus Point, northeast of Papeete, on the island of Tahiti. Cook was specifically chosen because he was among the very few who knew how to work out longitude. During the voyage, he opened his secondary secret orders, which revealed he was to search for "Terra Australis Incognita," the unknown southern land (known to us as Australia) and claim it for Britain.

A look to the future

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