On June 5th, 2012, Venus will pass across the face of the sun, producing a silhouette that no one alive today will likely see again.
Transits of Venus are very rare, coming in pairs separated by more than a hundred years. This June’s transit, the bookend of a 2004-2012 pair, won’t be repeated until the year 2117. Fortunately, the event is widely visible. Observers on seven continents, even a sliver of Antarctica, will be in position to see it.The nearly 7-hour transit begins at 3:09 pm Pacific Daylight Time (22:09 UT) on June 5th. The timing favors observers in the mid-Pacific where the sun is high overhead during the crossing. In the USA, the transit will be at its best around sunset. In astronomy, transits take place when a smaller body passes in front of a larger one. Although the Sun, Venus and Earth roughly line up every 584 days, the orbits of the Earth and Venus around the Sun are tilted with respect to one another. This means that Venus normally appears to pass above or below the Sun. A transit only takes place on the rare occasions when the three bodies are almost exactly in line. On average, transits of Venus happens only every 80 years or so. This average figure is however very misleading, because transits occur in a ‘pair of pairs’ pattern that repeats every 243 years. First, two transits take place in December (around December 8th), 8 years apart. There follows a wait of 121 years 6 months, after which two June transits occur (around June 7th), again 8 years apart. After 105 years 6 months, the pattern repeats. Prior to the last transit on 8 June 2004, no living person had seen a transit of Venus (the previous one was on 6 December 1882). The forthcoming transit will take place on 5-6 June 2012, but only the final stages will be visible from the UK. The entire event will be seen from eastern Asia and Australasia, the Pacific Ocean and the north-western parts of North America. The next transit of Venus after that will not take place until 2117, so 2012 will be the last chance in most of our lifetimes to see this extraordinary celestial event.