Aphrodite’s Commute (or, the Transit of Venus)

3 06 2012

As we wind up our year in the UCSC Science Communication Program, it’s a time of transition. Not only for the ten of us, as we wrap up our final stories, multimedia projects, and internships, but also for the solar system. This week marks a rare celestial event, which happens just twice every hundred or so years: the transit of Venus.

Photo of the 2004 transit of Venus (photo by Mswggpai, Wikimedia Commons)

Earth’s fiery twin will cast a tiny black dot as it crosses in front of the Sun on June 5th or 6th, depending on where in the world you’re viewing it. Here in North America, it takes place June 5th, starting at 3:06pm in California. The last transit occurred in 2004, and this year’s will be the last your lifetime, as the next doesn’t occur for another 105 years. See the end of this post for tips on watching it.

The transit of Venus isn’t merely an astronomical curiosity – it played a crucial role in measuring the size of the solar system. A Brit named Jeremiah Horrocks first calculated and observed the transit in 1639, along with his friend William Crabtree. Horrocks turned his room into a giant pinhole camera, by putting a piece of cardboard with a hole cut in it in his window and projecting the black dot of Venus onto a white sheet. By measuring the size of the dot of Venus, Horrocks calculated the distance between the Earth and Sun to be about 60 million miles (97 million km). His number was only two-thirds the actual distance, 93 million miles (150 million km), but it was closer than any previous estimate.

Fellow Englishman Edmund Halley later described how to obtain a more precise estimate of the size of the solar system, which involved timing when Venus slid past the Sun’s borders. Yet Halley wouldn’t live to see the next Venus transits in 1761 and 1769. Transits occur in a pattern of 8 years apart, then 121.5 years, then 8 years, then 105.5 years. The whole pattern repeats every 243 years. The 1769 transit marked a major milestone in international scientific collaboration, because Halley’s calculations required observations from scientists around the world. One observer was the explorer Captain Cook, who voyaged to Tahiti to observe the transit as part of a secret mission to explore Australia.

Unfortunately, they had a tough time timing the exact entry and exit of Venus on the face of the Sun, due to something called the “black drop effect.” At the time when the black disk of Venus was just passing the Sun’s inner edges, a black teardrop shape appeared, connecting Venus to the Sun’s border. The effect is thought be caused by optical disturbances in the Earth’s atmosphere or the viewing apparatus.

Nevertheless, observations since then honed the estimate of the solar system’s size, which modern methods such as radio telemetry and radar have confirmed.

So why not turn your eyes skyward as Venus waltzes through the glare of our Sun next week. Here are some tips for viewing the transit:

• Here’s a map of places where the transit will be visible

• You can find out your local transit time here, **keeping in mind that daylight savings time adds an hour to the times shown.

• Not surprisingly, it’s not a good idea to stare directly at the Sun. Use proper eye protection.

• The best way to see the transit is with a telescope or binoculars with a solar filter. Failing that, you can use eclipse glasses or #14 welding glasses, or even a pinhole camera. You won’t be able to see subtle effects like the black drop with these methods, though.

• For a fun and engaging history of the transit, listen to the Big Picture Science radio show and podcast “Mass Transits”.

This will be my last post for this blog. Like Venus, our SciCom program has come full circle. It’s been a pleasure learning the craft of science writing here among the Central Coast redwoods. To all reading this, best wishes on your own transits!




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