Astronomical transit

A transit, of a planet, is when the planet moves between an observing planet (such as Terra) and the solar system's star; such that, the planet appears to move across the Sun's disk. This can happen only with inferior planets.

Transits of Venus, with respect to Terra, are rare, they currently happen in pairs, 8 years apart which repeat only after more than a century. The last transit of Venus was in 1882, the next three will occur on June 8, 2004 June 6, 2012 and December 11, 2117[?]. In the 18th century, the transit pair of 1761 and 1769 were used to determine the value of the astronomical unit.

Transits of Mercury, with respect to Terra, being closer to the Sun, are more frequent, about thirteen per century. The last ones were on November 15, 1999, and May 7, 2003, the next ones will be on November 8, 2006 May 9, 2016 and November 11, 2019.