Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that generally repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 commensurabilities.
The last transit of Venus was on 5 and 6 June 2012, and was the last Venus transit of the 21st century; the prior transit took place on 8 June 2004. The previous pair of transits were in December 1874 and December 1882. The next transits of Venus will be on 10–11 December 2117, and 8 December 2125.