The transit of Venus, then and now

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.

One of NASA’s many amazing pictures of the 2012 transit of Venus.

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.

The ends of the earth were more accessible to Europeans in the 1760s than they had ever been before, since many of them had been or were in the process of being colonized. Politics and science collide in a particularly unfortunate way in the story of Guillame Le Gentil, whom University of Colorado astronomer Doug Duncan called “the most unfortunate astronomer in history” on The Madeleine Brand Show. Le Gentil sailed to the French colony of Pondicherry in India to observe the 1761 transit of Venus, but by the time he arrived war had broken out between France and England and the English were occupying Pondicherry. Turned away from his intended destination, Le Gentil missed the event. (In fact, most attempts to observe 1761 transit were failures.)

Rather than sail home to wait for the next one, he decided to stick around the Indian Ocean for the next eight years, spending time in Madagascar and the Philipines before returning to Pondicherry, which was once again under French control by 1769. But when the big day finally arrived, the sky over Pondicherry was too cloudy for him to see anything. Despondent, Le Gentil sailed for home—and was promptly shipwrecked on Ile Bourbon, a small island east of Madagascar. By the time he finally made it back to France, no one had seen or heard from him for 11 years and he had been legally declared dead. The French king eventually commanded that all of his possessions be returned to him, though his former wife was allowed to remain with the man she had married during his absence. (For even more twists and turns of the Le Gentil story, check out Cocktail Party Physics.)

Fortunately, other astronomers had better luck observing the 1769 transit. So many, in fact, that author Mark Anderson dubbed their efforts “the world’s first big science project” and wrote a book about it. Data from the various expeditions was collected and used to calculate a distance between the earth and the sun of 95 million miles—only 2 million miles off from the actual distance of 93 million. Today, we have much more precise ways of measuring astronomical distances and most people watch the transit of Venus just for fun, though some astronomers are hoping to use the event to refine the search for exoplanets. One of the best times to detect planets outside of our solar system is when they are transiting their own stars, so by precisely measuring the amount of solar energy Venus blocks during its transit as well as studying what happens to sunlight filtered through its atmosphere, scientists can better understand how to look for far away planets and determine which of them are most likely to support life.

I watched yesterday’s transit of Venus from my friend Jef’s beautiful backyard garden in Claremont. We set up a telescope (complete with solar filter, of course) in time to catch the ingress, which in southern California happened just after 3 p.m.

Celestron and its solar filter helped me watch the transit of Venus yesterday.

We checked on the planet’s progress periodically over the next few hours, each time realizing anew how difficult it is to find even an object as big as the sun in a telescope. When the roof of the house eclipsed the sun around 6:30, we took the solar filter off Celestron and used it to watch the transit with our own eyes for a bit longer. As far as astronomical events go, it wasn’t as dramatic as a solar eclipse or a meteor shower, but it was still a haunting reminder of the scale and beauty of celestial mechanics and a small way, in the words of Doug Duncan, to “feel like [we're] a part of the universe we live in.”

My transit companions Eric and Jef. Jef didn’t let a broken leg stand in the way of participating in this once-in-a-lifetime event.

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