Once in a lifetime

On 8 June the planet Venus will cross the sun's path, creating a unique light show in the sky.

The spectacle, which millions of amateur astronomers are expected to watch, has been recorded only five times in human history.

The last full transit by Venus across the sun was in 1882 and it will not occur again until 2247.

This time, it will begin at about 6:19am when Venus will appear as a tiny speck at one side of the sun. The transit will last six hours.

Dr Stewart Eyres, a senior lecturer in Astronomy at the University of Central Lancashire, said: "Venus will be about one 30th the size of the sun, but will be clearly visible.

"It will be an impressive sight and it is scientifically important for us, as it is so rare."

A transit occurs when Venus and the earth, whose paths round the sun tilt at different angles, line up exactly where their orbits cross. The full transit can be seen from Europe, Africa and Asia. Other countries including Japan and Australia will be able to see the beginning, but the sun will set before the end.

The 1769 transit led to the discovery of Australia and New Zealand - Captain James Cook was travelling to Tahiti to get the best possible view when he encountered land.

Experts are warning people not to use plastic glasses to view the transit, as it could lead to permanent eye damage. Dr Eyres said: "Plastic glasses were fine for the eclipse but this is a six-hour event, and using them could lead to blindness.

"We would advise people to get some card, put a hole in it with a pin, then put a piece of paper behind it, and aim the whole thing at the sun. That way you'll see the whole event safely."

Amateurs will not be the only people studying the transit. Nasa hopes it could hold the key to finding extraterrestrial life.

"In a few years, space probes and land-based telescopes will start to try to observe alien worlds in transit around other stars," said Nasa scientist David Crisp. "But first we need the Venus transit to test our techniques."

Sensors will measure tiny fluctuations in the amount of light given out by the sun as Venus passes in front of it. Nasa then hopes to use this data to hunt for potential planets in faraway galaxies, by monitoring the light emitted by their suns.