AN ASTRONOMICAL event so rare that no living person has witnessed it will be visible in the skies above Britain next month when Venus moves across the face of the Sun.

At 6.19am on Tuesday, June 8, shortly after sunrise, the planet will appear as a small black disc silhouetted against the Sun as it makes its first “transit” since December 6, 1882.

For the next six hours the Earth’s nearest planetary neighbour will cut a slow left-to-right diagonal path across the southern part of the Sun before disappearing at 12.24pm.

Given good weather, the entire phenomenon will be observable from Britain — the first time this has happened since 1283, when no one knew to watch for it. While another transit will take place on June 6, 2012, this will not be seen by British skywatchers, who must wait until 2247 for another chance.

Transits occur so infrequently that only five have been observed, and they have led directly to some of the most important discoveries in astronomy and geography.

The distance between the Sun and the Earth, 93 million miles, was first calculated with approximate accuracy during an 18th-century transit, and Captain James Cook reached Australia and New Zealand when dispatched to the South Pacific to observe the transit of 1769.

This time, scientists plan to take advantage of the opportunity to test technology intended to find Earth-like planets orbiting distant stars.

When such “exosolar” planets make transits of their own parent stars, the event produces a minute dip in the brightness of that star’s light as seen from Earth.

On June 8 astronomers will turn instruments designed to measure this effect towards the Sun to discover whether Venus’s transit affects its brightness as predicted.

Similar equipment will be launched on the French Corot probe next year, on Nasa’s Kepler mission in 2007, and possibly on the European Space Agency’s Eddington spacecraft, which has been postponed indefinitely because of a shortage of funds.

“Observing the transit of Venus is very like observing the transit of planets around other stars,” said Gordon Bromage, Professor of Astronomy at the University of Central Lancashire, who is co-ordinating transit events for the Royal Astronomical Society. “We are going to be looking for planets like our own using this method, and it is a great opportunity to try things out.”

The transit will also help scientists to study the Sun’s outer atmosphere, or corona: all the instruments on the Soho solar observatory satellite are to be aligned with the black edge of Venus as it crosses the Sun, providing temperature data of unprecedented accuracy.

Hundreds of events are being staged at observatories and universities across the country to allow the public to watch the transit, which is expected to be the biggest event in popular astronomy since the total solar eclipse in 1999.

As with the solar eclipse, scientists have warned people not to look directly at the Sun during the transit, but to use eclipse viewing glasses or project the image of the Sun on to a screen using a small telescope. Venus travels between the Earth and the Sun approximately once every 18 months, but this rarely produces a transit as the orbits of both planets are slightly angled.

At present, transits occur at intervals of 8, 121.5, 8 and 105.5 years, and this pattern will continue until 2984.

A transit has happened only six times since the invention of the telescope in 1608 made the phenomenon predictable and observable. The event of 1631, however, which was correctly forecast by Johannes Kepler, was not visible from Europe and was thus missed by astronomers.

A partial transit was observed for the first time on November 24, 1639, by Jeremiah Horrocks, a 20-year-old self-taught Englishman who had used Kepler’s tables to calculate that another transit was due. Further transits took place in 1761, 1769, 1874 and 1882. After next month’s transit, the next to be visible from Britain will not take place until 2247.

“It is significant because of its rarity, because of its importance through history and the way it links us to the future,” Professor Bromage said.

Venus, which will be 43 million miles away from Earth on June 8, will appear as a black disc about one thirtieth of the diameter of the Sun. It will first enter the Sun’s disc at the 8 o’clock position and leave near the 5 o’clock position.

“First contact”, when Venus starts to obscure part of the Sun, will begin at 6.19am, and it will take about 20 minutes for “second contact” — the phase in which the planet’s entire disc is visible — to commence. As this happens, observers may be able to see a phenomenon known as the “black drop effect”, in which the black disc of Venus appears linked to the edge of the Sun by a dark “neck”, as if it were shaped like a pear.

This effect is caused by the way in which the brightness of the Sun fades towards its visible edge, and confounded early attempts to use the transit to measure the distance between the Sun and the Earth.

A list of events in Britain to mark the transit, compiled by The Royal Astronomical Society, is available at www.transit-of-venus.org.uk, along with a downloadable diagram of the Sun during the transit.

If a telescope is used to project the Sun on to this diagram, and viewers record the precise times at which Venus moves into particular positions, it will be possible to calculate the distance to the Sun.

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