Britons able to watch Venus crossing Sun for first time since 1283

Children will join amateur and professional astronomers to witness one of the rarest events seen from Britain -the planet Venus moving across the face of the Sun.

Children will join amateur and professional astronomers to witness one of the rarest events seen from Britain -the planet Venus moving across the face of the Sun.

So-called "transits" occur when the complex orbits of the Earth and Venus around the Sun result in all three bodies being lined up briefly in space, causing Venus to pass directly between the Earth and the Sun.

On the morning of 8 June - clouds permitting - anyone in Britain with a projection telescope and a piece of card, or access to the internet, will be able to observe the small black disc of Venus as it crosses over the southern face of the Sun.

Total transits of Venus, when its crossing can be seen from beginning to end, are extremely rare. The last complete transit of Venus occurred in 1283 and the next will not take place until 2247.

The last incomplete transit of Venus, when part of its path across the Sun could be observed, occurred in 1882. Transits, which should never be observed directly, have only occurred six times since telescopes were first used in the early 17th century.

This time schools across Britain will take part in a mass observation that repeats a seminal experiment performed by a little-known English astronomer called Jeremiah Horrocks, who in 1639 was the first person to predict and see a transit of Venus.

This time, the transit begins at about 6.20am on 8 June, shortly after sunrise, when the black disc of Venus appears to kiss the outer edge of the Sun.

Gordon Bromage, professor of astronomy at the University of Central Lancashire, Preston, said that the entire transit would take about six hours, with mid-transit due at 9.22am and Venus leaving the Sun at about 12.04pm.

"It's an extremely rare astronomical event. It's a very special period of six hours and will link people across the world," he said.

"It will link us with our past history and it will also link us with our future, because observing a transit of Venus like this is very like what we're going to be trying to do observing transits of other planets going around stars," Professor Bromage said.

Professional astronomers intend to use the event to test instruments designed to detect Earth-sized planets orbiting distant stars.

These telescopes exploit the fact that the brightness of light from a faraway star temporarily dims when an orbiting planet briefly passes in front of it - just like a transit of Venus.

Professor Allan Chapman, a historian of science at Oxford University, said that the great German astronomer Johannes Kepler correctly predicted a transit in 1631, but this was only visible from America, where people did not then have the telescopes to witness it.

However, Horrocks, a 20-year-old amateur astronomer, spotted something about the movement of the planets that Kepler had missed that led directly to the first observation of a transit.

"Kepler had not calculated that there would be a date in 1639 when it would happen and this is put down entirely to Horrocks' own originality," Professor Chapman said.

"From a mixture of his own meticulous observations of Venus, the Sun and the planets, combined also with his analysis of the errors of the best available astronomical tables, he realised that the inferior conjunction of 1639 - when Venus passes between the Earth and Sun - would neither be above it or below it but bang across its middle," he said.

Horrocks realised that there was about to be a transit just weeks before it happened and wrote to a friend, a clothmaker called William Crabtree in Salford, to keep an eye open.

"I beseech you therefore with all thy strength to attend diligently with a telescope," Horrocks wrote. Professor Chapman said that the correct prediction and observation of the first transit of Venus led to the birth of English astronomy.

"Horrocks got it dead right. In other words he predicted the transit. He then went on with Crabtree to draw a number of fundamental facts about the nature of the solar system which frankly were they done today would be classed as Nobel prize-winning discoveries," he said.

"He discovered first of all then when you saw Venus in transit across the Sun it was very small, much smaller than Kepler and Galileo and Tycho Brahe had suggested it should be.

"This led Horrocks to ask: why should an object appear to shine so brilliantly in the night sky but appear tiny in transit?" said Professor Chapman.

"What it shows very importantly is that Venus is a world. We might miss the significance of that but Horrocks was a Copernican and he was a staunch believer that the Earth moved around the Sun," he said.

"He was aware that in Copernicus's ideas, the planets were globes, rock solid like our own. In the astronomy of Ptolemy and the geocentric Greeks, the Earth itself was the only solid body in nature.

"Horrocks was therefore using the transit for what you might call a Copernican agenda. He is saying this object is rock solid ... and like a ball."

We know today that Venus can be considered a twin planet to Earth, but with a very different atmosphere, where a runaway greenhouse effect causes temperatures to soar to 470C.

17TH-CENTURY 'DABBLER IN ASTRONOMY' BEAT THE EXPERTS OF HIS TIME

Jeremiah Horrocks, the first person to predict and see a transit of Venus, lived in the Lancashire village of Much Hoole when, at the age of 20, he made his observation in 1639.

Although Horrocks was a self-funded, self-educated astronomer with a telescope costing half a crown (12.5p), historians believe he should be considered the father of English astronomy. Professor Allan Chapman of Oxford University suspects that Horrocks was working as a tutor or schoolteacher, and possibly a Bible clerk for his parish church, who in his spare time dabbled in astronomy. Horrocks collaborated closely with William Crabtree, a clothmaker living in Salford who shared his interest in the stars and planets. Professor Chapman said: "Both men were totally self-educated in astronomy, because although we know that Horrocks went to Emmanuel College, he tells us quite plainly that nobody was taught mathematics in Cambridge in those days." Most great continental astronomers had wealthy patrons, but not so Horrocks and Crabtree.

"The transit of Venus in 1639 was the first time that a pair of Englishmen had taken the whole of the great Continental tradition in astronomy and not only encapsulated it in terms of research but taken it one stage further," Professor Chapman said. "They had discovered things about nature that none of the great, officially patronised figures in astronomy had done. It was the beginning of English astronomy."