Transit of Venus: your last chance to see it before 2117

A small black dot will grace the face of the sun as it rises (over Europe) on Wednesday, when Venus makes a rare and historic journey across the burning disc of our parent star.

Few people alive today will have another chance to witness the transit of Venus, as the laws of orbital mechanics do not bring the planets into position again until December 2117.

In previous centuries, nations dispatched astronomers to their farthest territories to record the transit in progress. In doing so, they embarked on the first global scientific collaboration in history and answered the pressing question of the size of the solar system.

This year's observations from powerful telescopes will help scientists learn more about planets far beyond our solar system, and may even help spot those with atmospheres that are similar to Earth's and capable of harbouring life.

To celebrate the event, national space agencies, universities and amateur astronomers will point telescopes at the sky and trace Venus's seven hours in the sun from 11.04pm on Tuesday until 5.55am BST the next morning.

The transit occurs when Venus moves directly between the Earth and sun, an event that happens less than once a century. It takes place in pairs eight years apart (the first in this pair was in 2004). Only the final stages of the transit will be visible from Britain, and then only if skies are clear on Wednesday morning, when the sun rises at 4.46am in London and 4.30am in Edinburgh.

The start of the transit will be visible from North and Central America and the north-western countries of South America. Parts of Asia and Australia will see the entire show.

Despite the early start, the Royal Astronomical Society said enthusiasts across the UK had organised transit parties and viewings from Ayrshire to St Austell and County Antrim to Whitby. Members of Ayrshire Astronomical Society are heading for a remote location off an unmarked road with three telescopes to witness the spectacle.

The equipment needed to watch the transit safely has its own language, with Graham Longbottom, president of Ayrshire Astronomical Society, speaking of Colorado Hydrogen Alpha solar scopes and Baader-filtered white light scopes. Though Venus will be visible to the naked eye, observers should never look at the sun directly, and even with eclipse goggles only for a few minutes at a time. One way to watch the transit safely is to project an image of the sun on to a screen, using a telescope or binoculars.

Subject to Scottish weather, Longbottom expects to watch the transit for two hours, though he is unsure how many will join him. "There is a lot of interest in astronomy at the moment as a result of TV coverage, but 4am on a Wednesday morning is likely to test the resolve of all but the really committed, and that includes society members," he said.

The first observations of a transit of Venus came from Jeremiah Horrocks in Much Hoole, a tiny village in Lancashire. On 24 November 1639, Horrocks watched as the planet traversed the sun after projecting its image on to a sheet of paper through a small telescope. He died two years later aged only 22.

The scientific importance of the transit was made clear by Edmund Halley, Britain's second astronomer royal, who in 1716 called on nations to join forces and record the event from positions around the world. Timing the transit from different spots on Earth allowed astronomers to calculate the distance from our planet to the sun, and so work out the size of the solar system.

Halley's essay was visionary, written nearly 50 years before the next transit was due in 1761. At the time, astronomers knew only relative distances in the solar system, for example, that Jupiter was five times further from the sun than Earth. Their best estimate of how far Earth lay from its star was 55m miles. "They didn't know the distance from Earth to the sun, and that was a base unit. It was like having a map without the scale," said Andrea Wulf, author of the 2012 book Chasing Venus: the Race to Measure the Heavens.

"What was so different was that no observation on its own would work, they had to be paired up. You had to send astronomers to as many, and as far apart, places as possible," said Wulf. "This was the first truly global international collaboration which lays the foundations of modern science."

The path Venus takes across the face of the sun varies depending on where the transit is viewed from. Halley's method called for pairs of astronomers a known distance apart to time the start and end of the transit. Taken together, the astronomers used these figures to calculate the separation of the Earth and sun using trigonometry.

The British sent James Cook on the Endeavour to witness the transit from Tahiti, where his crew became so enamoured with the locals they made only cursory notes on the event. Others fared worse. The French astronomer Guillaume le Gentil was barred entry to Pondicherry for the first transit and watched hopelessly from sea. He stayed in the area to watch the second transit in 1769, only for cloud to obscure his view. On returning home, he discovered he had lost his job, and his heirs had divided up his estate, giving him up for dead.

Halley's plan was a success despite the hardships of those who set out to observe the transit. The astronomers shared their records and eventually arrived at a new measurement for the distance between Earth and the sun of 93m to 97m miles. Today, the accepted distance is 92.96m miles.

"The transit has this remarkable history, going back to Horrocks and the amazing efforts that were made to observe it in the 18th and 19th centuries. It was the first enterprise in big science," said Lord Rees of Ludlow, the astronomer royal. "Very careful measurements of what happens when the transit starts and ends may reveal that you can in principle learn something about the planet's atmosphere and such like, and about the atmosphere of the star itself," he added.

Rees said it was unlikely he would watch the transit this year though. "Down here in the south we have even less chance of seeing it than in Scotland. And it's very early in the morning," he said.

Key facts

When does it happen?

The last transit of Venus of the 21st century occurs on 5 and 6 June 2012 depending on where you are viewing from. The transit starts at 11.04pm BST, when it will be visible from the US. The final hour of the transit will be visible from the UK just before 5am BST on 6 June, clear skies permitting. The transit will not happen again until December 2117.

How long does the transit last?

Venus takes nearly seven hours to cross the face of the sun, but the event is divided into four "contacts" that mark different phases of the transit. Venus makes first contact when it encroaches onto the disc of the sun. Twenty minutes later, on second contact, the planet will be fully silhouetted. On third contact, at 5.37am BST, Venus will beginto leave the sun, and the transit will be over on fourth contact at 5.55am BST.

Where can I see it?

The whole transit is visible from Alaska, and parts of northern Canada, and from New Zealand, much of Australia, Asia and Russia. In the US, the transit will be in progress as the sun sets on 5 June. In East Africa, Europe and Scandivia, the transit will be under way as the sun rises on 6 June. Much of South America and western Africa will not see the event.

How can I watch it safely?

Never look directly at the sun, it will damage your eyes. You can use eclipse viewing glasses that carry a CE mark and are not damaged or worn, but only for a few minutes at a time. Venus is large enough to see with the naked eye and will appear as a spot about 1/32 the width of the sun. It is not safe to look at the sun through regular sunglasses. For a better view, use a small telescope or a pair of binoculars to project an image of the sun on to a screen.

Can I watch online?

Nasa will broadcast a live webcast of the transit from the Mauna Kea Observatories in Hawaii.