When Venus passes directly between the Earth and the Sun, this event is called a transit of Venus. Venus orbits the Sun at a smaller distance than does the Earth, so one might expect that Venus transits would occur often. However, the orbital plane of Venus is not parallel to that of the Earth, so transits occur very rarely - only four times every 243 years.
The last Venus transit occurred in June 2012; the next will occur in 2117.
Venus transits are of great historical significance, as they provided the first quantitative measure of the distance scales of the Solar System: the Astronomical Unit, where 1 AU is defined as the distance between the Earth and the Sun. Astronomers had started to measure the distances to nearby stars in units of AU – but the value of the AU was still unknown. Therefore, many expeditions were sent out from European nations to observe the Venus transits of 1761 and 1769, resulting in the first measurement of the AU. Further refinement in the measurement was then obtained when the next two transits occurred in 1874 and 1882.
Venus transits have also provided the first evidence of an atmosphere existing on a planet other than the Earth.