Open Eye: Don't leave it until 2090 - this spectacle will be well worth seeing

Cornwall is usually crowded in August, but one day in particular will be particularly busy . Barrie Jones explains everything you need to know about the total solar eclipse

On 11 August this year the UK will be visited by a magnificent spectacle not seen from these shores since 1927. This is a total solar eclipse, in which the Moon blocks out the Sun completely. I have seen five total solar eclipses, and I can assure you that it is worth making a considerable effort to see one.

Imagine that you are within the narrow track on the Earth's surface from within which the total eclipse will be seen. The first indication of the eclipse is a tiny notch apparently missing from the edge of the Sun; this is the Moon beginning to get in the way.

The notch grows, and after about an hour, with the Sun reduced to a thin crescent, the light level has dropped noticeably. It has all become rather eerie, with the sky a dark blue, and sharp shadows, as if there is a searchlight above.

In the final minute or so before totality, the light level drops dramatically, and elusive bands of light and shade, called 'shadow bands' might be seen dancing across the landscape. Then, about 70 minutes after the first notch in the Sun appeared, the bright solar surface is completely obscured by the Moon and the eclipse is total.

Now, the pearly-white ribbons of the Sun's atmosphere are seen extending far outwards, the sky is a very deep blue, the brightest stars and planets are visible, and all around the horizon sunset colours are seen. The temperature will have dropped, and plants and animals behave as if night has fallen.

After a few minutes that seem like seconds, the end of totality is heralded by Baily's beads, the aptly-named phenomenon where tiny chinks of the solar surface shine through lunar valleys at the edge of the Moon. One of these chinks grows to dominate, to produce the equally aptly named diamond ring. The whole experience reduces many to tears of wonder.

Until the cause was understood, a total solar eclipse was regarded by many people with alarm, and even today there are unfounded fears. During a recent radio phone-in I was asked why it was dangerous for pregnant women to be bathed by the light of the eclipsed Sun. Well, it isn't dangerous to pregnant women.

In a total solar eclipse the bright surface of the Sun is hidden from view, so we have lost the familiar source of daylight rather than gained something nasty. The only danger of total solar eclipses is to our eyes.

It is always dangerous to stare at the Sun, with permanent eye damage, even blindness, the outcome. The difference in solar eclipses is the strong temptation to stare. The only time it is safe to do so is in the few minutes when the Moon has completely obscured the Sun's bright surface. At other times, when the Moon has only partially covered the Sun, precautions are a must. The simplest precaution is to use glasses specifically designed for solar eclipses, certified as safe, and in pristine condition.

What is the mechanism of an eclipse, and why has it been so long since there was one in the UK? The Moon orbits the Earth, and the Earth orbits the Sun. A solar eclipse occurs when the Moon gets exactly between the Earth and the Sun.

The shadow of the Moon then falls on the Earth, and from a point on the Earth's surface within this shadow a total eclipse is seen. At any instant this shadow is a small patch on the Earth's surface, up to a few hundred kilometres across. As the Earth rotates, and as the Moon moves around its orbit, this patch gets drawn out into a narrow track across the Earth's surface. At the centre of the track totality lasts longest, becoming shorter with increasing distance from the centre.

That the track is so narrow is a consequence of a curious cosmic coincidence. In a total solar eclipse the Moon is only just large enough to cover the Sun. This is not because the Moon itself is a bit bigger than the Sun, but because the Moon is about 400 times smaller, but also about 400 times closer, and so, in the sky, the Moon and Sun appear to be about the same size. In the same way, a penny held at arm's length can obscure a football further off.

The precise track drawn across the Earth's surface depends on exactly where the Earth is in its rotation, and exactly where the Moon and the Earth are in their orbits. The track is thus different from one eclipse to the next. For an area the size of the UK, a total solar eclipse occurs on average somewhere once every 44 years. In 1927, North Wales and northern England were favoured; this time it will be southern Cornwall and Devon, and Alderney.

The Moon takes 29.5 days to orbit the Earth (new Moon to new Moon), so you might think that there will be an eclipse somewhere on Earth every month. However, the Moon's orbit is not in the plane of the Earth's orbit, and so, at new Moon, when the Moon is broadly in the direction of the Sun, it will usually be either slightly above the Sun or slightly below the Sun as seen from any terrestrial vantage point, and there will be no total eclipse. Only when the alignment is better, can the Moon's shadow encounter the Earth.

Where should you go for the 11 August eclipse? The track of totality is not confined to the UK, and the prospects for clear skies improve as you go eastwards. In Cornwall there is rather less than a 50:50 chance of seeing totality, and this does not improve substantially until Romania.

Weather prospects are excellent in Turkey, and also in northern Iran. Cornwall and Devon will certainly get extremely congested, and I would not recommend anyone to try to enter and leave these counties on 11 August.

Though the weather prospects are little better in France, the track is north of Paris, so is not far away, and the congestion will probably be merely severe, rather than appalling. But if you want to witness a total solar eclipse from the UK, 11 August 1999 will almost certainly be your only opportunity - the next one on the UK mainland is 23 September 2090.

There are a lot of publications on solar eclipses; some include the sort of scientific activities that you could undertake. Here is a short selection.

The RGO Guide to the 1999 Total Eclipse of the Sun, Steve Bell, (RGO 1997, ISBN 0 905 087 03 8)

UK Solar Eclipses from Year 1, Sheridan Williams, (Clock Tower Press 1996, ISBN 1 85142 093 2)

The Cambridge Eclipse Photography Guide, J M Pasachoff and M Covington, (Cambridge University Press 1993, ISBN 0 521 45651 7)

Totality: Eclipses of the Sun, M Littman and K Willcox, (Univ of Hawaii Press 1991, ISBN 0 8248 1371 5)

The main UK eclipse website can be found at: http://www.eclipse.org.uk

You can also find useful information on the Science Museum website

http://www.nmsi.ac.uk

Barrie W Jones is a member of the Astronomy Group at the Open University

Safety first

There are a number of vital rules regarding the observation of solar eclipses to avoid eye damage. The most important is never to observe the uneclipsed or partially eclipsed Sun unless you have taken proper precautions.

There is no space here to give sufficient details of proper precautions. The matter is discussed more fully in, for example, the RGO Guide mentioned above, and the eclipse website.