Today’s transit of Venus, in which our sister planet appears to cross the disk of the sun, will be the last till 2117.  As observatories and millions of people watch the rare planetary alignment, few may know the stories of astronomers who predicted them and explorers who risked life and limb to observe them.

Watching the 7-hour event live on the internet (see is a privilege that was unavailable the last time the paired events occurred (they come in pairs 8 years apart, separated by a more than a century).  Because some parts of Earth are in darkness when they occur, Europeans often had to travel far to get to places where they could watch.  Only 4 pairs of transits have been observed by humans since Johannes Kepler predicted them: the pair of 1631–1639, the pair in 1761–1769, the pair of 1874–1882 (for which John Philip Sousa composed a special march), and this pair in 2004–2012.

Science Daily described how 18th century explorers had a much tougher time when they realized that important measurements could be made about the size of the solar system by observing the transit of Venus:

The idea galvanized scientists who set off on expeditions around the world to view a pair of transits in the 1760s. The great explorer James Cook himself was dispatched to observe one from Tahiti, a place as alien to 18th-century Europeans as the Moon or Mars might seem to us now. Some historians have called the international effort the “the Apollo program of the 18th century.”

Bolton Davidheiser, in his 1971 book Science and the Bible (Baker Book House, out of print) told a couple of lesser-known anecdotes about some of the observers of the previous pair of transits in the 17th century:

The great astronomer John [Johannes] Kepler had predicted mathe­matically that on December 6, 1631, the planet Venus would pass in front of the sun. Kepler himself did not live to see this day, but a Frenchman named Pierre Gassendi, prepared to observe the phenomenon. He watched in vain, for Venus made its transit across the face of the sun after the sun had set in Europe….

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