(Publicado por News Telegraph)

A once in a lifetime chance to see a transit of Venus, the passing of Earth’s sister planet in front of the sun, takes place next month for the first time in 122 years.

For six hours on June 8, the small, intense silhouette of the Earth’s sister planet will edge slowly across the disk of the sun. No living person has seen a transit of Venus, one of the rarest events of the astronomical calendar. The last took place in December 1882 and there have been just six since the invention of the telescope.

Transits of Venus once helped the great astronomers of the 17th and 18th centuries to unlock the secrets of the solar system – revealing the distance between the Earth and sun. They confirmed that Venus was a rocky planet like the Earth and suggested that it too had an atmosphere.

In 1769, a mission led by Capt James Cook on the Endeavour to observe a transit of Venus from Tahiti marked the start of the first systematic exploration of the Pacific.

Prof Gordon Bromage, of the University of Central Lancashire and spokesman for the Royal Astronomical Society, said: “Something wonderful, something marvellous is happening on June 8 and will be witnessed and experienced by millions of people all over the world. It is an extremely rare astronomical event.”

Next month’s transit will begin shortly after sunrise at 6.20am when the sun hovers low above the eastern horizon. It will take about 20 minutes from “first contact” until the planet is fully silhouetted.

The planet will then cut a diagonal path across the southern part of the solar disk. The transit will be over by about 12.24am, although the precise timings differ across Britain.

Venus is sometimes described as the Earth’s twin sister. The two planets are similar in size, mass and composition and are neighbours in the inner solar system.

However, Venus is one of the most inhospitable planets of them all. It is covered with thick clouds of sulphur and sulphuric acid, its atmospheric pressure is 90 times that of Earth and, thanks to a runaway greenhouse effect, its surface temperatures reach 450°C.

Venus orbits the sun closer than the Earth but because its plane of orbit is different to the Earth’s, it rarely transits the sun from our point of view.

Since 1631 transits have occurred in a cycle of eight, 121.5, eight and then 105.5 years. This pattern will continue until 2984. The next few transits – in 2012, 2105, 2113 and 2233 – will not be visible from Britain. The next that can be seen above the UK takes place in 2247.

The transit can be seen through a solar filter, or by projecting the image of the sun through a telescope on to a piece of white paper. Pinhole cameras are unlikely to work with a transit of Venus because the projection of the sun is not big enough.

Peter Bond, of the Royal Astronomical Society, said: “No one should ever look directly at the sun with or without a telescope or binoculars without using a safe solar filter. To do so is very dangerous and is likely to result in blindness.”

Historically transits of Venus helped astronomers discover the nature of the solar system. Next month’s will test instruments designed to look for planets around far distant stars.

It will also be observed by the SOHO observatory which studies the sun and sits 940,000 miles from the Earth. It will observe the planet passing in front of the sun’s corona, or outer atmosphere.