In the 18th century, astronomers used the solar transit of planet Venus to calculate the distance from the Earth to the sun. After the 1882 Venus transit, John Philip Sousa scored a march in its honor. Since it was first predicted in the 17th century, we've only had a chance to view the phenomenon six times. And Tuesday's our day ... Edward Murphy, associate professor of astronomy at the University of Virginia, explains more about the transit in the video.