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Venus at SunriseThis Tuesday or Wednesday, depending on where you live, sky-watchers around the world will be able to see a cosmic spectacle known as a transit of Venus. The events are so rare that only six Venus transits have been observed since the invention of the telescope more than 400 years ago. (See a telescope time line.)Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than Earth, can undergo this unusual alignment.The last Venus transit was in 2004—above, the planet glides across the rising sun in a picture taken during the event from the North Carolina coastline. After 2012, we won't see another transit of Venus until 2117.(Find out how to see the 2012 transit of Venus.)"People watching this event through some form of safe solar viewer will see the small, dark silhouette of Venus crossing the sun's face over the course of about six hours," said Jay Pasachoff, an astronomer at Williams College in Massachusetts. (Read a Q&A with Pasachoff about Venus transits.)"The effect won't be visually impressive, but that black dot against the sun is a remarkable thing to see."—Andrew Fazekas

Photograph by David Cortner, Galaxy Picture Library/Alamy