On Tuesday 8 June there will be an astronomical event so rare that no one alive has ever seen it before: the planet Venus will pass between us and the Sun. This is called a Transit of Venus, and it has been observed only five times in the entire history of the universe.

Because Venus lies between us and the Sun, you might think transits would happen often, but the orbit of Venus is tilted relative to ours; so most of the time Venus is a bit above or below the Sun.

What will happen on Tuesday morning? At about 5.20 a.m. Venus will appear as a tiny black dot creeping into the bottom left edge of the Sun, and will travel slowly across the bottom of the Sun’s face, taking about five hours to complete the journey. This will be visible all over Europe, as long as there are no clouds in the way.

The simplest and safest way to see the transit is on television. On BBC1 on Tuesday morning I shall be at Greenwich with news of the event, with live links to other places, including Egypt, where we expect clear skies.

Much the most fun is to observe the transit yourself, but you need special equipment. NEVER LOOK DIRECTLY AT THE SUN. Even a quick glance at the Sun can seriously damage your eyes. There is advice on the website www.open2.net, or you can watch the Stardate programme on BBC2 on Saturday afternoon, which will provide all the information you need.

Why have so few transits been seen? First of all, you cannot see a transit without a telescope, and the telescope was not invented until about 1600; so no transits could have been seen before then.

In 1639 a young man called Jeremiah Horrocks spotted what he thought was a mistake in an old astronomical table. If he was right, there would be a transit of Venus during the afternoon of 24 November, and he would be able to see it from the village of Much Hoole in Lancashire, where he was assistant curate. Unfortunately that day was a Sunday; so he had to spend most of it in church, and the afternoon was cloudy. But after the third service he sprinted back to his room, the clouds parted, and he ‘beheld a most agreeable spectacle’ - a black dot on the face of the Sun. He had written beforehand to his friend William Crabtree, and these two were the first people ever to see the phenomenon.

Transits of Venus are a bit like buses; you wait for ages, and then two come along together. The next transits after 1639 were in 1761 and 1769; this time the astronomers were ready, and hundreds were out with their telescopes. Captain James Cook was sent all the way round the world by the Royal Society to observe the 1769 transit from the island of Tahiti.

By noting the times of the transit, from several places on Earth, the scientists were able to calculate the distance from the Earth to the Sun. They got 95 million miles, close to today’s accepted figure of 93 million miles. Look at the website to find out how you can get your own personal measure of the distance to the Sun.

There were transits in 1874 and 1882, and the next one is on Tuesday. After that there is a transit in 2012, but it will be invisible from Europe, and there won’t be another until 2117; so this Tuesday really does represent a once-in-a-lifetime chance. Don’t miss it!