On 5–6 June 2012 a very rare astronomical event took place: the planet Venus passed directly between the Earth and the Sun, appearing as a small black dot against the face of our parent star. These transits of Venus occur in pairs eight years apart, but each pair is separated by more than a century. The last one was in 2004 and, after June 2012, the next won’t be until December 2117.
Historically, transits of Venus were used by astronomers to give the first accurate measure of the distance between the Earth and the Sun. To be sure of observing these twice-in-a-lifetime events expeditions were sent around the world in the 18th and 19th centuries and the story involves many famous characters. Captain Cook was sent to Tahiti to observe the transit of 1769, and King George III had Kew Observatory built so that he could view the transit himself (the telescope he used will be on display in the Royal River exhibition at the National Maritime Museum).
By the 21st century the distance to the Sun was well-established, with confirmation by radar studies and space missions. But the 2004 transit still excited the interest of the press and public and was even the subject of a series of photographs by Turner Prize-winning artist Wolfgang Tillmans, some of which could be seen in the Royal Observatory’s free exhibition Measuring the Universe (now closed). And now the idea of transits has acquired a new significance for astronomers as they are used to discover new planets orbiting distant stars.
'When the last transit season occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows.' William Harkness 1882
Safety first! You should never observe the Sun directly, even with the naked eye, as this can permanently damage your eyesight. But there are lots of ways to observe the transit of Venus safely, by using special solar filters or even by projecting an image of the Sun onto a sheet of paper. See these websites for advice on safe ways to watch:
The transit took place between roughly 22:00 GMT on 5 June and 04:55 GMT on 6 June, although exact timings depended on your location on the Earth.
From the UK only the last hour of the transit was visible just as the Sun rose on the morning of 6 June. In Greenwich visibility ran from sunrise at 04:45 British Summer Time to the end of the transit at 05:55 BST. Times varied very slightly for the rest of the UK.
- Get the app: take part in a worldwide effort to reproduce the historic transit observations of the 18th and 19th centuries by downloading the free VenusTransit smartphone app
- Follow the voyage: the Australian National Maritime Museum’s replica of Captain Cook’s ship HMB Endeavour sailed to Lord Howe Island to observe the transit on 6 June 2012. A 21st-century crew of keen sailors, enthusiasts and astronomers echoed the voyage of the ship’s famous predecessor and its crew of 243 years ago. You can view their journey on the HMB Endeavour website
- Discover new worlds: NASA’s Kepler spacecraft uses the Transit Method to discover new planets around distant stars. You can help astronomers in their search by accessing real NASA data at the Planet Hunters website – you may even find a new planet yourself!