On June 5th, 2012, Venus will pass across the face of the Sun, producing a silhouette that no one alive today will likely see again.
Transits of Venus are very rare, coming in pairs separated by more than a hundred years. This June's transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117. Fortunately, the event is widely visible. Observers on seven continents, even a sliver of Antarctica, will be in position to see it.
Venus transits are of great historical significance because they gave astronomers a way to measure the size of the Solar System.
The transits of the 18th century enabled astronomers to calculate the distance to the Sun by timing how long it took for Venus to cross the solar disc from different locations on Earth and then using simple trigonometry.