The Transit of Venus

Posted: Mon May 28 2012

It happens twice a century, is cosmically 'irrelevant' and still gets astronomers excited. Paul F Cockburn looks at the transit of Venus

Clouds permitting, on Wednesday June 6 2012, early-rising Londoners could catch a brief glimpse of something quite special - the planet Venus passing between the Earth and the Sun, appearing as a small black dot on our parent star. 'This one really is your “last chance to see”,' explains Dr Marek Kukula, public astronomer at the Royal Observatory, Greenwich. 'The next one is not until December 2117 - very few, if any, people alive today are going to see that.'

Yet, apart from its comparative rarity - transits happen in pairs, eight years apart, separated by more than a century - why are astronomers still so excited by the transit of Venus? 'When watching a planet slide in front of the Sun, the mechanics of the heavens are laid open for anyone to see,' Kukula says. And, as the exhibition 'Measuring the Universe', currently running at the Royal Observatory, explains, the transit of Venus has played an important historical role in helping astronomers calculate the size of the solar system.

'In the eighteenth century, the transits were among the first examples of international big science collaborations,' says Kukula. 'Despite a lot of national tensions within Europe, scientists were communicating with each other across borders. These were also early examples of government-sponsored science; Captain Cook's first voyage to the South Seas was funded so that he could observe the 1769 transit from Tahiti.' On the way back, of course, he went on to discover New Zealand and Australia.

There were practical considerations in all this. 'By measuring the transit accurately, you gained a new level of understanding in how the heavens worked, and that actually was very useful for navigation,' says Kukula. 'Even in the eighteenth century, though, the observations were not quite good enough to really nail it. For the nineteenth-century transits - in 1874 and 1882 - you have Greenwich sending out numerous expeditions around the world. Armed with photography, they can at last make accurate observations.'
So, by the time the next transit came along in 2004, its initial scientific value had gone. 'As a cosmic yardstick, the transit is completely irrelevant now,' admits Kukula. However, astronomers have found a new use for the transit of Venus.

'In the twenty-first century, we use transits of planets around other stars to find these new worlds,' Kukula explains. 'Since 2009, Nasa's Kepler satellite has discovered literally hundreds of new planets by watching for the slight dip in light caused as the planet moves in front of the parent star. So, in 2012, many astronomers are going to be watching the transit of Venus very intently. Knowing what the atmosphere of Venus is like - we've already sent spacecraft there - the question is, how does that atmosphere change the sunlight passing through it?'

Finding out should help astronomers to 'reverse engineer' their data and so calculate the composition of the atmospheres of planets outside the solar system. 'What I really like about this is that - in the seventeenth, eighteenth and nineteenth centuries - the transit was our way
of understanding the true scale of the universe and our place within it - it's big and we're very small!' Kukula
says. 'In the twenty-first century it's still all about these big philosophical questions: who are we, and how do we fit into the cosmos that we can observe? The transit may well be our handle on whether we're alone in the universe.'

'Measuring the Universe' runs at the Astronomy Centre at the Royal Observatory Greenwich (ROG), until Sept 2. Free. For more information on the transit, visit

Viewing the transit of Venus safely in London

The last hour of the 2012 transit will be visible from London on June 6, from sunrise (at 4.45am) to approximately 5.55am. You will need a clear view of the eastern horizon, avoiding trees and tall buildings — plus, of course, no clouds in the way.

You should never observe the sun directly through a telescope or binoculars, as this can permanently damage your eyesight. However, there are numerous ways to observe the transit safely using special solar filters. Check the ROG website for more information.

The safest, most inexpensive method is by projection. A pinhole or small opening is used to form an image of the sun on a screen placed about a metre behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun on to white card - just don't use either to look directly at the sun.

Your local astronomy society might be running events around the transit. Track them down via the Federation of Astronomical Societies at: