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It's V-Day 2012, the day Venus transits the sun, appearing as a black dot against the star's searing surface. It's a show that won't make its celestial revival until December 2117.
For many sky watchers, this event is drawing eyes because it is rare and intimately tied to the history of humanity's quest to take the measure of its corner of the cosmos. In the 18th and 19th centuries, scientists risked their lives traveling to far-flung corners of the globe to carefully record the track of the first lady of the solar system across the sun. Back then, it was the best means available for trying to estimate the Earth's distance from the sun and derive a size for the solar system.But for some astronomers, Tuesday's transit offers an unique opportunity to use Venus as a surrogate for Venus- and Earth-size planets orbiting other stars, as researchers hone their skills at analyzing potential havens for life outside the solar system.
For others, the transit could help unravel a longstanding mystery about Venus itself: What drives the "super rotation" of its atmosphere? Venus spins on its axis once every 243 days. Its atmosphere performs the same feat in four days, with winds traveling at nearly 450 miles an hour.
Venus's transits owe their century-scale frequency to the orbital paths Earth and Venus trace around the sun.
Seen in relation to the plane of Earth's orbit, Venus's orbit has a slight tilt – about 3.4 degrees. Depending on where the two planets are in their respective orbits, that leaves Venus trailing the sun toward the horizon at nightfall or rising ahead of the sun before dawn. But on Tuesday the two planets' orbits are aligned in such a way that Venus will pass directly between Earth and the sun during Venus's inferior conjunction, or closest approach to Earth.
Venus's transits occur in pairs, separated by eight years – Tuesday's transit is paired with one in 2004. The pairs repeat their appearance twice in a 243-year cycle, with one return time spanning 105.5 years, and the other 121.5 years. So, if you miss this one, tell your children to tell their children to take heart. We're in a part of the cycle with the shortest return time – in December 2117.
