SCIENCE IN THE NEWS DAILY
Evening Star Goes Black in Rare Celestial Event
from Science News
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-shadowed Venusian outline will disappear until 2117. Because the 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 transit pair 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 and sun 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.