Every 243 years, Venus twice passes between the Sun and Earth. The two transits are eight years apart. The event is regular, predictable and very rare, and so is of great interest to astronomers. For a few hours, a black disc is visible passing across the face of the sun. By recording the exact time of the transit start from widespread points on Earth, it was possible to calculate by triangulation the distance to Venus and the Sun. Expeditions were sent all over the world to time the transits in 1761 and 1769, and from the combined data a close approximation of the distance to the sun (150m km) was achieved by Jérôme Lalande.
We are currently between two transits: the last was in 2004; the next will take place on 6 June 2012.