<P> A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) the solar disk . During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun . The duration of such transits is usually several hours (the transit of 2012 lasted 6 hours and 40 minutes). A transit is similar to a solar eclipse by the Moon . While the diameter of Venus is more than 3 times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth . </P> <P> 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 . </P> <P> 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 . </P> <P> Venus transits are historically of great scientific importance as they were used to gain the first realistic estimates of the size of the Solar System . Observations of the 1639 transit, combined with the principle of parallax, provided an estimate of the distance between the Sun and the Earth that was more accurate than any other up to that time . The 2012 transit provided scientists with a number of other research opportunities, particularly in the refinement of techniques to be used in the search for exoplanets . </P>

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