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June 5, 2012

As Venus crosses the sun, a rare treat for skywatchers

The annals of European astronomy burst with famous names: Copernicus, Kepler, Halley.

Jeremiah Horrocks? Not so famous. But in 1639, the young Englishman became the first person known to witness one of the rarest events in the heavens: the passing of Venus across the face of the sun — a transit.

On Tuesday, Venus will again cross the sun, for just the sixth time since. It won't do so again until 2117, making this the last transit of Venus for nearly everyone alive today.

Telescopes around the world — and up in space — will turn sunward as the roughly seven-hour transit begins at 6:03 p.m. Eastern time. As the black dollop of Venus inches along, scientists will examine the planet's atmosphere and gather clues that may help them find Earth-like planets circling other stars.

"This is a full-court press," said Jay Pasachoff, a transit tracker leading an expedition to the Haleakala Observatories high atop Maui in Hawaii. Pasachoff, chairman of the astronomy department at Williams College in Massachusetts, is also coordinating observations across a global network of solar telescopes.

In space, NASA's most advanced sun-spotter, the Solar Dynamics Observatory, will stream the event to computer screens while banking gigabytes of data. "We are going to give the world the best data ever seen from a Venus transit," said Dean Pesnell of NASA's Goddard Space Flight Center in Greenbelt, Md. A special NASA Web site is preparing for a million viewers.

Transits of Venus are so rare because the planet's orbit is tilted relative to the Earth's. The two planets line up with the sun only four times every 243 years. (The timing between transits is odd: 121 1/2 years, then eight years, then 105 1/2 years, then eight years again.)

Johannes Kepler — that master of orbital mechanics — was the first to puzzle most of this out. In 1627, he predicted a transit would occur in December 1631, and then not again until 1761.

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