The transit of Venus happens every 100 years. The last one was in June 2012, and the next one occurs in 2117. This happens as Venus directly passes between the Earth and the Sun. This once-in-a-lifetime astronomical alignment has been witnessed eight times since the telescope was invented. These eye-witness-accounts occurred in 1631, 1639, 1761, 1769, 1874, 1882 and 2004, and the last was in June 2012.
The one-hundred-year pair-pattern for the transit of Venus is due to the orbital nodes of Earth and Venus. Since these orbital nodes are changing, transits which used to occur in May and November are now happening in June and December. Fred Espenak of NASA has released a Six Millennium Catalog of Venus Transits; and, it contains predictions based on geocentric calculations.
The transit of Venus has helped astronomers calculate Earth's distance from the Sun: It is calculated using the astronomical unit or AU. This transit was proposed by English astronomer Edward Halley in 1716. Scientists continue to study transits to this day, to discover extrasolar planets and exoplanets beyond the solar system. The Kepler Mission was launched by NASA in 2009 to find exoplanets; and, some are equal to the size of the Earth.