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)
Year in Review 2012: Astronomical Wonders
Once-in-a-lifetime astronomical events and monumental scientific achievements drove people to look to the skies and online on Yahoo! throughout 2012.