National Aeronautics and Space Administration

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A 'transit' occurs whenever one object moves in front of another object as viewed from your particular location in space. We observe this phenomenon thousands of times every day because we live in a 3-dimensional world that is crowded by many different kinds of objects as we move past them. In astronomy, transits are actually rather rare. The most dramatic ones we hear about involve the moon passing in front of the sun. because the moon and sun appear to be about the same size, we call these transits 'eclipses'. On rare occasions, the planet's Mercury and Venus can be seen as small 'dots' passing across the face of the sun. The light from the sun actually dims by a small amount when this happens, but only astronomers can measure this dimming. As it turns out, we can measure this dimming of distant stars and use it to discover planets orbiting them. The NASA Kepler mission has detected over 700 of these 'transiting exoplanets' since 2009 in a study of over 150,000 stars.

The image to the left shows a lovely transit of the moon across the sun as viewed by the STEREO spacecrafts. Thanks to the many interplanetary spacecraft deployed by NASA, we have the opportunity to see many more amazing transits in the future now that we are not limited by the perspective from only one planet!

In this eClips episode, we discover how scientists used the last Venus transit in 2004, and a geometric technique called parallax, to verify the distance between sun and Earth. Find out what scientists hope to learn the next time Venus makes a shadow on the face of the sun in 2012!