June 5, 2012 | David F. Coppedge

Venus Transit Recalls Adventures of Yore

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 Space.com) 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.
According to Kepler, a transit would not occur again for over a hundred years. But an English boy in his teens, Jere­miah Horrocks, did some figuring of his own and found that Venus should repeat its performance in just a few years. Go­ing over his calculations, he found that indeed it was so and that Venus again should pass in front of the sun on Decem­ber 4, 1639. He was too timid to mention this to anyone except his best friend, William Crabtree.
Modem astronomers can tell at what time of day such a phenomenon will be visible at any place on earth where it can be seen, but the calculations of Jeremiah Horrocks told him only the day, and it was to be a Sunday. If he saw this transit he would be the first to do so, for no one ever before had observed Venus move across the disk of the sun. After this day no one on earth would have an opportunity to see it for a hundred twenty-one years. Besides the rarity of the event it had important theoretical implications in the science of astronomy.
But the transit was due to occur on a Sunday and “the in­ward voice seemed to tell him that the Creator Himself is more worthy of worship than the phenomena He has insti­tuted for admiration.” He watched the sun without interrup­tion from sunrise until it was time to go to church. He went to church. When he returned he hastened to his telescope. The transit had just begun! Where his friend Crabtree was watching, the sky was cloudy but it cleared long enough for him to see it also and to confirm the observation of Horrocks.
It is interesting to note, in contrast to this, the experience of a Frenchman named Legentil who went to India to observe the next transit of Venus a hundred twenty-one years later. Because a war was in progress his ship was delayed and he did not reach land until after the transit was over. As the following transit was to occur only eight years later, he de­cided to remain in India and wait. When the day of the transit arrived the sky was cloudy and he saw nothing of it. After being shipwrecked twice on his way home, he arrived in France to find his heirs preparing to divide his posses­sions.
Between that time and the present, Venus has crossed the face of the sun twice. The next transit will occur on June 8, 2004. It is indeed remarkable that a boy in his teens could make a calculation of greater exactitude than the great Kepler. It also is remarkable that he would risk missing such an event instead of missing church one Sunday.

See our biography of Johannes Kepler, the first astronomer to predict transits of Venus.

If you missed the 2004 transit, don’t miss this one–it will be the last in your lifetime.   Live Science tells how to observe it safely (do NOT look at the sun directly).  Share the experience with children if you can; it will be a nice memory and a teachable moment about the clockwork regularity of the heavens, and how our Privileged Planet gives us an ideal platform for making scientific discoveries.

Exercise: Watch The Privileged Planet after the transit, and discuss what factors apply to making the event observable from Earth.  Discuss the discoveries made using transits of Venus (see Astrobio.net for ideas), and then discuss what today’s scientists hope to learn by today’s transit (see Space.com and JPL).


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