It is 6.19am, a black spot transits the sun ... and Venus lovers are in heaven

At 6.19am BST on Tuesday a tiny black disc will creep across the face of the sun. Venus, the second rock from the sun, will move directly between the Earth and its parent star in a rare celestial phenomenon known as the transit of Venus.

The six-hour event will be watched - on projected screens, or via television and internet - by hundreds of millions. No human alive has ever seen a transit of Venus; the last was in 1881. The last one visible in its entirety in Britain was in 1283. The next transit visible from the UK will be in 2247.

This transit will be visible from the UK, Europe and Africa in the morning; it will be seen in the middle of the day across the Middle East, Russia and India, and during the late afternoon from Australia and the far east. Paradoxically, no one will see the event directly, with the naked eye: the sun is too dangerously bright and Venus too small for any sense of dramatic eclipse.

The transit of Venus is a key to the understanding of the solar system. The first observers used their measurements to calculate the basic measuring stick of the solar system: the astronomical unit, or distance from the Earth to the sun.

By the 18th century, astronomers began to coordinate their efforts to observe the transit from many points around the globe, using the technique of parallax to make ever more accurate estimates of the astronomical unit. Their plans were in many cases foiled by war and weather.

But in 1769, James Cook sailed to Tahiti in the Endeavour to observe a transit and then went on to map the Pacific, claim Australia and New Zealand for Britain, and change the course of history.

"Whereas in the 18th century it was necessary for European powers to send expeditions all over the world to observe this transit, by the 19th century there were observatories established all over the world," says Alan Chapman, a historian at Oxford University.

"We also have something else in 1882. The world is linked by railways, electric telegraphs - the Victorian internet - steamships and so on. So it was possible to know the San Francisco value of the 1882 transit in the evening papers in London, by means of the electric telegraph."

Astronomers today can measure solar distances by radar. But they value the transit of Venus all the same. "It will link us with our past, and it will also link us with our future," says Gordon Bromage of the University of Central Lancashire. "Because observing a transit of Venus is very like what we are going to try to do, observing transit of planets going around other stars."

The orbits of Venus and Earth are tilted slightly. So although Venus overtakes the Earth on the inside every 584 days, only four times every 243 years does Venus appear to cross the face of the sun.

The cycle starts with two December transits eight years apart. The 121.5 years later, there are two June transits, eight years apart. The next transit, in 2012, will happen during the British night.

The first person to observe a transit of Venus was Jeremiah Horrocks, a 20-year-old Puritan at Much Hoole in Lancashire who projected the shadow of the planet crossing the disc of the sun in 1639.

Horrocks calculated that the sun was far bigger, and far further away, than the giants of science Copernicus and Kepler had imagined. His work was published decades after his death by a Polish astronomer in what is now Gdansk.

Three electronic telescopes set up by Sheffield Hallam University's star centre will relay the transit to two giant plasma screens in a marquee in the city's Peace Gardens.

The government's chief medical officer, Liam Donaldson, stressed that any attempt to watch the event directly could be dangerous. "People who look directly at the sun during the transit, even for five seconds, risk permanent eye damage, even blindness," he said.

Where days outlast years

· Venus takes 243 Earth days to rotate, and 225 to orbit the sun - so a Venusian day is longer than a Venusian year
· It rotates in the opposite direction to its orbit, so the Venusian sunrise is in the west and the sunset in the east
· The planet is drier than the driest desert on Earth, and the surface temperature is 450C (842F) - twice the maximum of a kitchen oven, and hot enough to melt lead
· Atmospheric pressure is 90 times Earth's, so anyone on the surface would feel as though they were under 900 metres of ocean
· The planet is shrouded in clouds of sulphuric acid, and radar from orbiting spacecraft have detected more than 1,000 volcanoes with diameters larger than 12 miles on its surface

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