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Transit of Venus: Measuring the Heavens in the 18th Century

from the Guardian (UK)

The 1761 transit of Venus was a watershed moment in the history of astronomy. It was the first time astronomers would have the opportunity to measure accurately the size of the solar system. The distance between the Earth and the Sun had been estimated, with variable degrees of success, since the Greeks, but this was different.

Thanks to a rare celestial alignment, Venus was to pass in front of the Sun, taking about six hours to cross the fiery disc. By recording the times of the start and end of the event from widely separated locations around the globe, trigonometry could be used to calculate the distance to Venus and the Sun. With that, Kepler's laws of planetary motion could be used to calculate the orbits of all the planets out to Saturn, the outermost known planet.

For societies that were still struggling with inadequate maps of their own countries, it was an unimaginable leap forward.