The story
Venus passes in front of the Sun in pairs of transits eight years apart, with more than a century between pairs. Edmond Halley proposed in 1716 that timing a transit from distant places on Earth would give the distance from Earth to the Sun, the base unit of the solar system. Kepler's laws gave the relative distances of the planets, so one measured distance would fix all the others. In 1761 and 1769 astronomers from Britain, France, Russia and other nations traveled to Siberia, India, the Cape of Good Hope, Hudson Bay, California and the Pacific.
The work was hard. Observers saw a smear called the black drop effect that blurred the moment of contact, and bad weather ruined some sites. In 1761 the Russian scholar Mikhail Lomonosov noticed a glow around Venus and concluded the planet has an atmosphere. For 1769, James Cook took the ship Endeavour to Tahiti, where astronomer Charles Green watched the transit, before Cook went on to explore the Pacific. Combining all the results, Jerome Lalande estimated the distance at about 153 million kilometers in 1771, within about 2 percent of the modern value, 149.6 million.
Why it mattered
- Produced the first good measurement of the astronomical unit, fixing the scale of the solar system.
- Organized the first large international scientific collaboration, spanning nations at war.
- Sent James Cook to the Pacific, leading to his later voyages of exploration.
- Led Lomonosov to propose that Venus has an atmosphere, an early idea in planetary science.
Sources
- The Transits of Venus ASSA
- Venus Transit Observations of 1761 IMCCE
- Mikhail Lomonosov and the discovery of the atmosphere of Venus during the 1761 transit Cambridge University Press
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