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'Transit' of Venus will Treat Skygazers to Rare Astronomical Event - 2004-06-05

Tuesday, June 8, we'll be treated to a rare astronomical event: the transit of Venus. For the first time since 1882, Venus will pass directly between the Earth and the Sun. Nearly everyone on the planet will be able to see at least some part of this highly unusual sky show, as the tiny black dot of Venus slowly travels across the blazing face of the Sun. A new exhibit in Washington focuses on the history of the transit of Venus.

Even though Venus comes between the Earth and the Sun every 584 days, a transit isn't something you see every couple of years.

"Because Venus' orbit is slightly tilted, it actually most of the time will pass either above or below the sun, from where we're situated," said Ronald Brashear. "So it's just these rare occasions where it actually crosses the face of the Sun."

That's Ronald Brashear, the curator of a new exhibit at the Smithsonian Institution called "Chasing Venus." Through books and artifacts, it documents efforts by astronomers in the past to observe the transit of Venus. As we toured the exhibit, he filled me in on the history of transit observations.

The first people known to have seen the transit of Venus were a self-taught English mathematician named Jeremiah Horricks, who predicted the 1639 transit, and a friend of his, William Crabtree. Famed astronomer Johannes Kepler had predicted one eight years earlier, but his calculations were off a bit.

Venus transits come in pairs, about eight years apart, and the pairs are separated by a little more than a century.

In the 18th and 19th centuries, the transit of Venus captured the imagination of a world that was learning about its place in the cosmos. Astronomers were beginning to sort out the objects in the solar system, but Ronald Brashear says they had no way of determining distances in space.

Brashear: The discovery in the 18th century that you really could effectively use this to measure the Earth-Sun distance propelled it into a major astronomical event. Because at that time nobody really knew the absolute distances of anything in the heavens. So for the first time, there was a promise of actually knowing how far away something was in the universe. And so, we really went into a great paroxysm of excitement with astronomy.

Chimes: And the astronomical Unit, A.U., the distance between earth and the sun, that's the key to a lot of other measurements?

Brashear: Yeah, in some ways some people have called it the Holy Grail of astronomy in that a lot of distances in the solar system and the nearby area are expressed in terms of the Earth-Sun distance.

To compute that critical Earth-Sun distance, astronomers needed to have at least two sets of observations from widely-separated places on Earth, so expeditions were sent out, including one headed by famed English explorer, Captain James Cook, whose journals are in the Smithsonian exhibit.

"Most people don't realize that Captain Cook's first voyage in 1769 was really designed to observe the transit of Venus from Tahiti," he said. "That was just a small moment in the expedition, and a number of other scientists, astronomers, botanists, went along as well to take specimens, do some other scientific work. That's what it's really known for. Many people have forgotten it really went out to observe the transit of Venus."

In fact, the place where Cook set up his instruments is still known as Point Venus.

Mr. Brashear describes the 18th century transits as?"?the golden age of the transit of Venus. You have the potential there; people didn't realize the pitfalls yet, so it was a great time of excitement."

Fast-forward to the 19th century transits - in 1874 and 1882 - which gave astronomers a new tool to use, photography. But the technology was still in its infancy and couldn't overcome the limitations of an optical problem called the "black drop effect," which made it hard to determine exactly when Venus crossed the edge of the Sun. That moment was key to the calculation of the Astronomical Unit.

"So, it really didn't work out as the great tool make the transit observations any better, and as a matter of fact by this time there were other methods that had been devised since [the last transit in] the 1760s that had greater promise for determining the distance from the Earth to the Sun," said Ronald Brashear.

The Astronomical Unit, by the way, is about 150 million kilometers.

Tuesday's transit will last about six hours, starting just after 0500 UTC. Times will vary slightly, depending on where you are. If you decide to watch it yourself, please don't look directly into the sun. It could permanently blind you. If you have any doubts, check with an expert.

After Tuesday's event, the next transit of Venus will be on June 6, 2012.