Transits
Venus continues to blaze as the "evening star." The planet pops into view in the southwest not long after sunset, and sets about three hours later. It's near its brightest for its current evening-sky appearance, which won't last much longer. Venus is dropping toward the Sun, and will cross between Earth and Sun in January.
This crossing happens five times every eight years. Because Venus and the Sun appear so small as seen from Earth, most of the time Venus passes just above or below the Sun. But last year, Venus passed across the Sun. It looked like a tiny black dot sliding over the Sun's face.
This type of event is called a transit, and it provides important information that may help astronomers in their search for planets in other solar systems.
Two Sun-watching satellites observed the transit. Their instruments detected a slight dip in the Sun's brightness -- about one-tenth of one percent.
Astronomers have already discovered several planets using this same technique -- watching for a planet to pass in front of its parent star, causing a slight decrease in the star's brightness. The Venus transit shows that the planet doesn't have to be all that big to produce a detectable difference.
A spacecraft that's scheduled for launch in 2008 will watch hundreds of thousands of stars in search of these periodic dips in brightness -- dips caused by passing planets. More about the Kepler mission tomorrow.
Script by Damond Benningfield, Copyright 2005
For more about the Kepler Mission, visit NASA's website at http://kepler.nasa.gov.
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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