ROBERT SIEGEL, HOST:
In the continental United States, this is your last chance to see the transit of Venus. It won't happen again for another 105 years. The transit takes Venus between the Earth and the sun. It's like an eclipse, though Venus will look something like a dark pea drifting across a bowl of carrot soup. The transit starts just after 6:00 p.m. Eastern time, and it's more than just a celestial curiosity.
As NPR's Christopher Joyce reports, the transit has revealed secrets of the universe.
CHRISTOPHER JOYCE, BYLINE: German astronomer Johannes Kepler first predicted a transit of Venus for December 1631, but he died a year before he could see it. That astronomical honor fell to a British prodigy named Jeremy Horrocks. The transits came in pairs, and Horrocks pegged the second transit for 1639. He saw it, then died less than two years later.
Lest one think the transit is bad luck, plenty of others have watched since then with no apparent ill effect. In fact, astronomer Gordon Chin says the transits of 1761 and 1769 drew a huge following.
GORDON CHIN: There was a massive, almost global, effort to observe the transit of Venus.
JOYCE: Captain James Cook, the English explorer, sailed with scientists to Tahiti to observe the transit. Other fanned out across Europe and Asia, wrestling with the best telescopes and timekeepers that money could buy. What they wanted was the answer to a big question: How far away is the sun from us. That's what astronomers now called the Astronomical Unit. They studied the transit and made a number of calculations.
CHIN: And from all the data that was compiled at that time, they estimated the Astronomical Unit was about 95 million miles, which was not a bad estimate.
JOYCE: They used a form of triangulation. Surveyors or sailors used the technique as well as astronomers. You measure the angle to a faraway object from different points on the Earth. For Venus, astronomers also used the planet's transit to track across the sun's face to get more angles. Then they used trigonometry to calculate the distances to Venus and the sun.
Chin, who works at NASA's Goddard Space Flight Center, says the discovery of the Astronomical Unit was a big deal. Astronomers now, for the first time, could measure the whole solar system.
CHIN: It provided the scale of the solar system. A couple hundred years ago that was the forefront of science.
JOYCE: And a Russian astronomer also noted a halo of light that surrounded Venus just as it crossed the edge of the sun. That was the first evidence of an atmosphere on another planet.
So is there anything more to learn from Venus on the move? Well, in fact, the Hubble space telescope is watching the current transit of Venus at the request of astronomer Jean-Michel Desert, among others.
JEAN-MICHEL DESERT: We are using, again, the transit of Venus to study this time other solar systems.
JOYCE: Other solar systems around other stars. Here's how it works: Astronomers can detect faraway planets as they pass in front of their own suns - just as Venus is doing now with our sun. They found really big ones, but they're interested most by planets the size of Earth and Venus.
DESERT: It's typical of a kind of planet that we are discovering nowadays, so Earth-size planets around sun-like stars. And the next step will be to study the atmosphere.
JOYCE: You can find out about a planet's atmosphere by looking at the way sunlight passes through it. That's what the Hubble telescope is doing; the sun's rays through Venus' atmosphere revealed its atmospheric signature. Astronomers can compare that with the signatures of planets outside our solar system.
The transit takes about seven hours. You can watch but don't look directly at the sun, even Hubble can't do that. The telescope instead is focusing on a crater on the Moon, using it as a mirror to catch the reflected rays of the sun coming through Venus's atmosphere. You could call it a Venus sky trap.
Christopher Joyce, NPR News. Transcript provided by NPR, Copyright National Public Radio.