Transits of Venus—the movement of Venus across the face of the sun—occur in pairs that are eight years apart and then don’t happen again for more than a hundred years. Before the transits in 2004 and 2012, the last two Venus transits were in 1874 and 1882, and there won’t be another pair until 2117 and 2125. These rare alignments have been important for scientific research. Of particular significance, Venus transits provided observers with data that eventually led to a very close estimate of the astronomical unit—the distance between the earth and the sun.