Transit events happen in pairs separated by more than a century

By Meghan Rosen

Web edition: December 13, 2012
Print edition: December 29, 2012; Vol.182 #13 (p. 30)

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Credit: SDO/Gsfc/NASA

2012 SCIENCE NEWS TOP 25: 22

Millions of eyes followed Venus’ steady June voyage across the sun’s surface, the coal-black dot sailing in front of a blazing golden orb (SN: 6/30/12, p. 11). The spectacle was the second in the recent pair of Venus transits; the first was in 2004.

At the beginning and tail end of Venus’ six-hour journey in 2004 — before and after the planet was completely silhouetted by the sun — astronomers saw something they had been hoping for: Faint wisps of Venus’ upper atmosphere winked clearly into view. This year’s event handed scientists another opportunity to learn about Venus’ gaseous clouds, like why they spin faster than the planet itself. “We got a second chance, and we tried to make the best of it,” says Jay Pasachoff, director of the Hopkins Observatory at Williams College in Williamstown, Mass.

Pasachoff’s team at Maui’s Haleakala Observatory snapped some 660,000 images of the transit using a coronagraph, a special telescope that blocks part of the sun’s brightness. In the next year, scientists will compare data from Haleakala with images from spacecraft, ground-based telescopes and several other coronagraphs positioned at different sites around the world. The Hubble Space Telescope even got in on the action, gathering light reflected off the moon’s surface during the transit to test a possible technique for detecting the atmospheres of faraway Earthlike planets. Pasachoff says he feels a strong obligation to future Venus-viewing astronomers. “They’ll be looking at our observations the way we looked back at observations from the 19th century.”

Those data were gathered the last times Venus crossed the sun, in 1882 and 1874, when observers focused on triangulating the Earth-sun distance. The planet’s next pair of transits won’t occur for another 105 years, in 2117 and 2125. 

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