A page from the Philosophical transactions of the Royal Society of London (1770) with observations by Samuel Dunn of the 1769 transit of Venus. A transit occurs when Venus passes directly between the Earth and the Sun. Though Venus passes between the Earth and the Sun every 584 days, for an observer on Earth it usually appears to be either above or below the sun. Between 1600 and 2000 C.E. only six transits occurred, but there are two this century; the first June 8, 2004 and the second is tommorrow, June 5th, 2012!
NASA has more information on observing the transit safely, as well as a live webcast! http://venustransit.nasa.gov/2012/transit/index.php
(via Chasing Venus: Observing the Transits of Venus, 1631-2004)
Unlike the event eight years before, a fair amount of the 1769 transit would be visible in London. (NASA Goddard Space Flight Center Sun-Earth Day)
Historic Transit of Venus
British explorer James Cook and astronomer Charles Green drew these stages of the transit of Venus in 1769. Cook and Green observed the transit from Tahiti on June 3, 1769.
Like Neil deGrass Tyson writes on twitter, the Venus transits are rarest astronomical events, and they occurs in pairs eight years a part, about every 100 years.
CREDIT: Museum of the History of Science, Oxford
Afbeelding van den weg der planeet Venus Nicholas Ypey, 1761. Rare Book and Special Collections Division, Library of Congress. A beautiful drawing of the transit of Venus of 1761, by Nicholas Ypey. Although the coronal detail on the sun is not actually observable, the path of the transit is accurately depicted.