The transit of Venus takes place when the planet passes in front of the sun. While Venus and the earth roughly line up with the sun every 584 days, their orbits are tilted in relation to each other, meaning that a transit only takes place on the rare occasions when the two planets and the sun are almost exactly in line.
On average, transits of Venus take place around 80 years apart, but when they do so, it is in a pair of events which are eight years apart.
The last transit was on 8 June 2004 and the transit on 5-6 June 2012 is the second of this pair.
The Royal Astronomical Society issues the following guidelines: *Never look directly at the sun, with or without a telescope or pair of binoculars, without using a safe solar filter. To do is very dangerous and could result in permanent blindness.