The solar system's second planet paced across the face of the sun while millions around the globe watched. The rare event happens usually only once in a lifetime, and once helped early astronomers map... more 
The solar system's second planet paced across the face of the sun while millions around the globe watched. The rare event happens usually only once in a lifetime, and once helped early astronomers map the distance between the Earth and sun. (Kevin Frayer/AP Photo) less 
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| Photo By Kevin Frayer
Sun, Dec 2, 2012 12:00 AM EST