|Venus at the beginning of its transit of the Sun |
(image: NASA/Reuters, via http://www.guardian.co.uk/)
A transit of Venus occurred June 5-6, 2012. This phenomenon (in which the planet Venus can be witnessed crossing the sun) is relatively rare. None occurred throughout the entire twentieth century, and the next pair of transits will not take place until December 2117 and December 2125. In the eighteenth century, measurement of a pair of transits of Venus enabled astronomers to calculate the size of the Solar System for the first time.
See NASA's coverage of the transit of Venus:
Follow the Guardian live-blog of the transit: