An Etymological Dictionary of Astronomy and Astrophysics

A rare phenomenon that happens when the planet Venus passes directly between the Sun and Earth and is therefore seen against the solar disk. Such a passage occurs every 122 or 105 years and when it happens the next occurrence is after 8 years. Only seven transits of Venus have occurred since the invention of the telescope:
in 1639, 1761, 1769, 1874, 1882, 2004, and 2012. The next one will be in 2117.
The reason for this rarity is that the Earth and Venus do not orbit the Sun in the same plane. Their orbital planes have a relative inclination of about 3°. The first observation of the Venus transit was in 1639 by the English Jeremiah Horrocks (1618-1641). See also → black drop.

See also: → Venus; → transit