We’re just hours away from the last transit of Venus until the year 2117. A transit is when a planet passes in front of the sun, revealing itself as a tiny black dot on the sun’s face. Transits in the 18th century helped us figure out the dimensions of the solar system, which are now well known. But the 2012 transit will still provide information that could help astronomers understand distant exoplanets.

“During a transit you get a tiny bit of light that passes through the planet’s atmosphere, if it has an atmosphere. And so, if you tease that out you then can start doing all kinds of interesting science on the atmosphere itself.”

Mark Anderson, author of The Day The World Discovered The Sun, about the 18th-century Venus transits.

“So there are going to be telescopes all over the world, and what they’re all going to be looking for is the about 0.001 percent of the sun’s light that has also passed through the atmosphere of Venus during the transit. That’s interesting to astronomers—they can work backwards from that light and figure out, well, what is the chemistry in Venus’s atmosphere.

“So what the Venus transit allows us to do is to study a known system, where we know Venus, we understand its atmosphere relatively well, we understand the sun’s light relatively well. The one thing we don’t have a good picture of is what does it look like when Venus transits directly in front of the sun and we study that light very carefully. Then you can take the things that we learn from studying the transit of Venus and apply it to a transit of another planet elsewhere in the galaxy and maybe even someday study its atmosphere.

“So that’s why the Venus transit is really helpful, because it lets us do kind of a test case where we understand all the parameters, and then we apply all that knowledge to studying these planets elsewhere in the galaxy.”

—John Matson

[The above text is a transcript of this podcast.]