Venus to cross Sun in rare spectacle

The crossing of Venus across the face of the Sun is such a rare and spectacular event that in 1769 Captain James Cook sailed the HMS Endeavour half way around the world to witness it.

Venus passing between the Earth and the Sun on June 8, 2004

But Britons will have a once-in-a-lifetime opportunity to view the phenomenon, which has happened just three times since the 18th Century, from their own homes this week.

Early on Wednesday morning the planet – usually one of the brightest lights in the night sky – will be clearly visible as a black disc slowly passing against the background of the rising Sun.

The transit of Venus, one of the most impressive and eagerly anticipated events in the astronomical calendar, will mainly take place during night-time in Britain but can be viewed during its final stages for about an hour immediately after sunrise.

In other parts of the world such as Hawaii and Australia the entire six and a half-hour spectacle will be visible and observatories around the world will live stream the event over the internet.

Members of the public can view the transit without a telescope but must wear protective "eclipse glasses" or use a piece of paper with a pin prick in it to project an image of the transit onto a screen behind them, experts warned.

Looking at the sun with the naked eye or even through a camera lens risks causing permanent eye damage and even total blindness.

Wednesday will mark only the sixth time the event has been witnessed since the invention of the telescope more than 400 years ago, although it was also documented by the Babylonians and by the ancient Greek mathematician Pythagorus.

It will also be the last chance for virtually anyone alive today to see it. The last transit was in 2004 but because the events always occur twice in quick succession before gaps of more than 100 years, next week's crossing will be the last until 2117 and the last to be visible in Britain until 2125.

During the 18th and 19th centuries explorers including Captain Cook sailed across the globe to watch transits take place from widely separated locations so that they could combine their observations and calculate the distance between the Earth and the Sun.

Modern-day astronomers will use the rare opportunity to test methods designed to study the atmosphere of planets in other solar systems, which can only be done as they cross in front of their own suns.

By testing their equipment on the familiar territory of Venus, scientists will be able to learn how reliable it is and fine-tune it for use in the search for life elsewhere in the Universe.