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The 2012 Venus Transit

30 May 2012, 23:57 UTC
The 2012 Venus Transit
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For those who are unaware, roughly twice each century, Venus passes directly between the Sun and Earth, and are called transits. In 1677, after observing a transit of Mercury across the face of the Sun, Edmund Halley concluded that future transits could be used to determine solar parallax, and thus the distance to the sun, but that the rare Venus transits would be more useful. Halley then devised a system in which observers at multiple locations of widely separated latitudes would merely need to precisely time the transit from the moment the planet appears entirely on the disk of the sun until the moment the planet appears to make contact with the limb of the sun upon exiting. Obviously, this required that observation posts be located in zones where the entire transit was visible. Jean-Nicolas Delisle, overseeing French preparations for the 1761 Transit of Venus, recognized that cloud cover at either point of contact would negate the entire operation. Instead, Delisle posited that parallax could be determined by timing either the beginning of transit (ingress), or the end (egress), and would reduce the chance of the interference of weather. Observing just one stage of transit, however, meant that it must ...

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