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.

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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 again crossed 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 — turned sunward as the roughly seven-hour transit began 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. 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: 121¹ / ₂ years, then eight years, then 105¹ / 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.

The first occurred on schedule, although no one in Europe could watch; the six-hour event happened at night there. Kepler missed it, too; he was dead by then.

Several years later, Horrocks was checking Kepler’s figures when he found an error, which revealed that Venus would make another transit in December 1639.

Horrocks jumped at the historic opportunity. He set up a telescope — a relatively new invention — and projected a six-inch image of the sun onto paper. His figuring showed that the black spot of Venus should nudge into the bright edge of the sun around 3 p.m.

The spot appeared on schedule. Horrocks traced it as it moved, the first record of this heavenly rarity. But it was winter in England and soon the sun set. Venus had moved just a smidge of the way across the sun, William Sheehan and John Westfall recount in their 2004 book “The Transits of Venus.”

Horrocks died tragically young, just 22, shortly thereafter.

The next set of transits, in 1761 and 1769, triggered scientific drama around the world. European powers sent 100 expeditions to Siberia, the South Pacific, Indonesia, India and other remote locales where the entire transit could be seen.