Venus will cross the face of the sun on June 5, 2012, a sight that will be visible from parts of Earth. This is the last transit for more than 100 years.

Vincent Yu, Associated Press

For most of us, last viewing of Venus transit

  • Article by: BRIAN VASTAG
  • Washington Post
  • June 5, 2012 - 6:45 AM

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

Horrocks is 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 worldwide -- and in space -- will turn sunward as the seven-hour transit begins at 5:03 p.m. CDT. 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.

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

Several years later, Horrocks found an error in Kepler's figures -- that Venus would make a transit in December 1639. Horrocks jumped at the opportunity. He set up a telescope -- a relatively new invention -- and projected a 6-inch image of the sun onto paper.

The spot appeared on schedule. Horrocks traced it as it moved, the first record of this rarity. But it was winter in England and soon the sun set -- with Venus moving just a smidge of the way across the sun. Shortly thereafter, Horrocks died, at just 22.

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.

The most famous scientists and explorers of the day -- Captain James Cook -- chased the event. This was the big science of the time. And all for a single goal: to figure the size of the solar system. "This was the measurement to make," Pesnell said.

Earlier in the 18th century, English astronomer Edmund Halley -- of comet fame -- had struck on a way to make this measurement with the transit. It required precise timing, with observers -- stationed as far apart as possible -- noting to within a second when the disk of Venus enters the sun and leaves it.

In one of the first examples of international scientific cooperation, the observations were collected -- albeit with some dispute over some of the specifics -- and sent to a scientist in Paris who made an estimate of the distance between the Earth and the sun, said Lawrence Marschall, a professor of physics and astronomy at Gettysburg College in Pennsylvania.

The estimate was, all things considered, not far off -- about 100 million miles (the actual distance is 93 million miles), much more than the scientists of the time had believed.

Space was, indeed, vast.

© 2018 Star Tribune