Venus has its day in the sun

It is a sight not witnessed in Britain for more than 700 years.

The transit of Venus across the face of the sun took place today and was watched by thousands across the country. At 6.19am, the earth's closest planetary neighbour began to cut a diagonal path across the bottom of the sun.

Appearing as a black disc 30 times smaller than the sun's diameter, it moved from left to right over the course of six hours.

The event will not happen again in Britain until 2247. There will be one more transit on 6 June, 2012 - but it will be hidden from British sky-watchers.

Today saw the first Venus transit since 1882 but the last time it was observable from the UK was in 1283. On that occasion, long before the invention of the telescope, the phenomenon went unnoticed. The transit is similar to a solar eclipse, except that it is Venus rather than the moon that passes between the sun and earth.

Robin Scagell, vice-president of the Society for Popular Astronomy, said: "Some people might say: 'So what?' But the Venus transit is a unique chance to see our closest neighbour in space, and it gives us a better perspective of the scale of the solar system.

"We've all seen the models which show tennis balls flying round the sun. But Venus is a tiny dot against the background of the sun, a 30th of the sun's diameter. When you see that with your own eyes you get a feeling of the scale of the solar system and our place in it.

"The Venus transit is also something that hasn't been seen for 120 years. A lot of people will be glad they saw it for that reason alone."

The phenomenon can happen only when Venus lies directly between between the sun and the earth as it spins in its orbit.

Looking at the sun with the naked eye, or through a telescope or binoculars, is extremely dangerous and can result in blindness.

Experts warned observers to use a telescope fitted with a proper solar filter, or to project the image onto a screen that could be viewed safely. Most of the country had a good view of the transit.

Six have occurred since the invention of the telescope, in 1631, 1639, 1761, 1769, 1874 and

1882. The first of these was missed, despite being predicted, since it was not visible from Europe.

A transit was observed for the first time on 24 November 1639, by the young Lancashire amateur astronomer Jeremiah Horrocks.