Yesterday was the last transit of Venus until 2117, making it a bona fide once- (or twice-, if you caught the last one in 2004) in-a-lifetime event. Only six other transits of Venus have been recorded, starting in 1639 with the observations of a young British astronomer named Jeremiah Horrocks. While Johannes Kepler predicted the transit of 1631, it wasn’t visible in Europe and it appears that no one observed it. Kepler thought we would have to wait another 130 years for the next chance to see Venus meander across the sun, but Horrocks caught an error in his hero’s calculations and realized it would happen again in 1639. (Now we know these transits always occur in pairs eight years apart.) Despite some inconvenient clouds, he managed to observe the event that year. Unfortunately, he wasn’t as lucky in his other endeavors, never formally graduating from Cambridge—making him something of an outsider scientist—and suddenly dying at age 22, just two years after observing the transit.
The next pair of transits happened in 1761 and 1769, and this time there were many more people watching the sky. Earlier in the century, Edmund “The Comet” Halley laid out a method to use data about the transit of Venus to calculate the distance between the Earth and the sun and, from there, the size of the solar system. (Here’s the best non-technical explanation I’ve found of how this worked.) His method depended on the transit being observed at different points around the world, so a bunch of European explorers set off for the ends of the earth with astronomers in tow in anticipation of the event. Read the rest of this entry »