UPDATE: This other NASA site shows you what it will look like from your location, and also provides a live feed.
Today you’ll get the rare chance to see a transit of Venus across the Sun. You’ll probably be dead by the time the next one occurs. A piece at the Guardian will answer all your questions, including these:
What is the transit of Venus?
A rare astronomical event that happens when Venus travels across the face of the sun and appears as a small black dot on its surface.
Here’s what it looks like (WARNING: DO NOT LOOK DIRECTLY AT THE SUN WITHOUT EYE PROTECTION!):
When does it happen?
Transits occur in pairs eight years apart. There are two in December that repeat every 121.5 years, and two in June that repeat every 105.5 years.
The last transit of Venus of the 21st century occurs on Tuesday and Wednesday (5 and 6 June 2012) depending on where you are viewing from. The transit starts at 11.04pm BST (6.04pm ET) on Tuesday, when it will be visible from the US. The final hour of the transit will be visible from the UK just before 5am BST on Wednesday (12am ET US), clear skies permitting. The transit will not happen again until December 2117.
How long does the transit last?
Venus takes nearly seven hours to cross the face of the sun, but the event is divided into four “contacts” that mark different phases of the transit. Venus makes first contact when it encroaches onto the disc of the sun. Twenty minutes later, on second contact, the planet will be fully silhouetted. On third contact, at 5.37am BST (12.37am ET), Venus will begin to leave the sun, and the transit will be over on fourth contact at 5.55am BST (12.55am ET).
NASA will host a live streaming of the transit (from Hawaii) that you can see here; it starts at 5:45 p.m. EST in the US or 11:45 p.m. London time. The site links to other live webcams in Europe and America, which of course operate at different times.
Besides being a rare spectacle, the transit was used in what may have been the first international scientific collaboration: 200 astronomers observing the transit in 1769 and using trigonometry (see the method here) to calculate, from the transit time, the size of the solar system. Another article in the Guardian describes the endeavor (do read it: it’s informative and has some quite dramatic incidents):
The major British expedition for the 1769 transit was Captain Cook’s voyage to Tahiti. Following success with the measurements on the island, Cook sailed on. Cook’s return to England in 1771 allowed the final calculation of the sun’s distance to be made. The combined results from all the various missions were within about 4% of the modern accepted value of 93m miles (150m kilometres). At the next pair of transits, in 1874 and 1882, the accuracy was improved to 1%.
Only since the Second World War have radar experiments with radio telescopes bettered those results. Yet the 18th century transits will always stand out. They represent the first time in history that the distances to the Sun and planets were measured, and the first global scientific collaboration.