Transit of 1761, Part IV
Brilliant Venus is just clinging to view in the dawn sky. It climbs into view about 45 minutes to an hour before sunrise. And it's so bright that, with a clear horizon, you shouldn't have much trouble spotting it.
Venus shines so brightly in part because it's enveloped by clouds atop its thick atmosphere -- an atmosphere that was discovered 250 years ago this week.
On June 6th, 1761, Venus passed across the face of the Sun. Scores of astronomers around the world watched the event, called a transit. Most hoped to use their observations to help measure the distance to Venus, which in turn would provide the best measurement yet of the distance to the Sun.
At the Saint Petersburg Observatory, though, Russian astronomer Mikhail Lomonosov had an additional goal -- to measure Venus's diameter. But as he watched the transit unfold, he made an even more important discovery: He found that Venus has an atmosphere.
As Venus neared the solar disk, Lomonosov noticed that Venus's "edge" looked a bit fuzzy. And for a moment, a "ring of fire" partially encircled the planet. The conclusion was obvious: Venus must have an atmosphere, which was "bending" sunlight around it.
Today, astronomers use transits to discover atmospheres around planets in other star systems. They can't see the same "ring-of-fire" effect, but they can detect the atmosphere, and even learn something about its composition -- all from a brief passage across the face of a star.
Script by Damond Benningfield, Copyright 2011
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