3.2. The transit of Venus

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The transit of Venus
One of the first joint scientific ventures was carried out in 1761- scientists hoped to be able to measure the Transit of Venus (which occurs twice with an interval of 8 years then no more for a century). If it was measured from selected points on earth, scientists would be able to calculate the distance from the Earth to the Sun, and so to calibrate distances to all other bodies in the solar system. In 1761, scientists from Britain, France, Sweden Russia, Italy and Germany set out to different parts of the globe from which to measure the transit. Guillaume le Gentil of France began his journey in 1760, but due to various problems, was still at sea at the time of the transit, and a ship is the worst place for calibrating.
Undaunted, he went on to India, spent 8 years preparing a new viewing station and tested and retested his instruments. On the day, just as Venus began to pass, a cloud appeared over the sun for the entire duration of transit, so he saw nothing. He returned to France and discovered that his relatives had declared him dead in his absence and taken all his property!
Unfortunately, the scientists produced too much information, and much of it was contradictory. This was the reason for Captain Cook's later voyage to Tahiti. With Cook's measurements, the Frenchman Joseph Lalande calculated a mean distance of 150 million km (nowadays known to be 149,597,870,691 km). The last transit of Venus was 5th-6th June 2012, and the next one will be in 2117.