Evening Star Goes Black in Rare Celestial Event

Posted on May 23, 2012. Filed under: Science | Tags: , |

The shadow of Venus last passed across the sun in June 2004. After another transit by the planet this June 5 and 6, the spectacle will not recur until 2117.


On June 5, skywatchers will have their last chance to glimpse a rare celestial spectacle, a “transit of Venus,” in which the planet passes directly between Earth and the sun. Venus will take six hours to march across the star’s face, appearing as an inky black dot in silhouette against the looming solar disk.
After that, the sun-shadowedVenusian outline will disappear until 2117. Becausethe planet’s orbit is slightly off-kilter, its solar transits come in pairs spaced eight years apart, with more than 100 years between pairs.
During the most recent transitpair of 1874 and 1882, observers around the world focused on triangulating the Earth-sun distance. They tried to time precisely when Venus entered and exited the sun’s disk, so they could calculate the size of the sun (a complicated endeavour, it turns out, since an optical effect that blurred the boundary between planet andsun muddied the timing measurements). The most recent transit happened in 2004 — only the sixth such performance seen through telescopes — and it revealed that large portions of the Venusian atmosphere are visible to Earthly observers.
Now, scientists are hoping not only to study Venus itself during the transit, but are using the crossing to inform observations of far-off exoplanets that similarly betray their presence by passing between their star and Earth. “Over 100 years ago, astronomers couldn’t have anticipated this transit question,” says astronomer Jay Pasachoff of Williams College in Williamstown, Mass., who will observe the transit from Hawaii.
Pasach off and his colleagues are also deploying nine instruments to locations including Japan, Kazakhstan, and Norway, to study the sunlight filtered through Venus’ toxic clouds.
But such observations aren’tso simple. “Big mirrors and sensitive detectors are not good things to point at the sun,” explains planetary astronomer Heather Knutson of Caltech. Instead astronomers will use the Hubble Space Telescope to capture sunlight reflected off the face of the moon during the transit. In that light are signatures of chemical compounds in the Venusian atmosphere, Knutson says. “It’s the first time we can make this measurement for a truly terrestrial planet.”

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