The Eclipse - 11.11am, 11.8.99: What is a Solar Eclipse?; Heavenly bodies in motion

A mere fluke of nature has allowed us Earthlings to experience a total eclipse of our nearest star. A cosmic coincidence in time and space enables the Moon to completely block our view of the Sun.

Chance alone has enabled the Moon and the Sun to appear identical in size when viewed from the Earth. Although the Sun is 400 times bigger in diameter than the Moon, it is 400 times further away, allowing the lunar disc to cover the Sun's orb with a watchmaker's precision.

Solar eclipses occur because the Earth is orbiting the Sun, and the Moon is orbiting the Earth, both in an anti-clockwise direction. At some point the Moon will get in the way of the Sun. When it does, and the Moon is directly between the Earth and Sun at the beginning of the monthly lunar phase, there is a chance that the shadow cast by the Moon may fall on to the Earth, so causing an eclipse.

Although the Moon passes between the Earth and the Sun every month, the lunar shadow hits the Earth far less frequently. This is because the orbit of the Moon is tilted at an angle and only rarely does it line up precisely enough for the lunar shadow to fall on the Earth to result in an eclipse.

Sometimes only the shadow's "penumbra" reaches the ground, which means part of the Sun is still visible and only a partial eclipse occurs. More rarely, the whole Sun is obscured, and the shadow's full "umbra" cuts a swath across the globe, allowing everyone in its path to experience a total eclipse.

When the Moon is full, and lies behind the Earth, it is the turn of our own planet to cast a shadow on our natural satellite. This is called a lunar eclipse and, unlike a solar eclipse, can be seen at any point on Earth where the Moon is above the horizon.

On Wednesday, the umbra of the Moon will be travelling at a speed of about 2,000mph as it crosses the globe from west to east, starting in Nova Scotia, passing through Cornwall, south Devon, Alderney in the English Channel, northern France, across Romania, Turkey and Iran before fizzling out in the Bay of Bengal.

Although the Moon's umbra will be no more than a few hundred miles wide, the accompanying penumbra (and the partial eclipses it produces) will be nearly 3,000 miles across.

For those lucky enough to be in the path of totality, events in the South West will begin to unfold a few minutes before 10am, when the Moon begins to take a bite out out the top right-hand corner of the Sun - the point known as First Contact.

As the minutes unfold and the bite begins to get bigger, it may take another 45 minutes before a noticeable change in colour and light intensity begins to occur.

At about 15 minutes before totality the drama of a total eclipse starts to unfold. The Sun's heat dies away, the colours fade and birds maybegin their night-time rituals - evensong in the morning.

Just before totality, when the Sun is reduced to a thin crescent, the sky blackens quickly and the horizon to the west darkens ominously as the shadow of the Moon races silently forward at twice the speed of sound.

Temperatures can fall with the rapidly cooling atmosphere, producing a set of "shadow bands" or air currents that appear as parallel ribbons of light and dark across the ground. If the sky is clear and cloudless, it should be possible to see the stars and the brightest planets, such as Mercury and Venus.

Seconds before totality - the point known as Second Contact - the last few shafts of sunlight will sparkle against the edge of the lunar disc as the Sun shines through the deep valleys of the Moon. After these "Baily's beads", a final and intense sprinkle of brightness, called the Diamond Ring, will cause the final flashes of sunlight to stream through the mountains on the Moon's horizon.

Here, totality begins, the point known as Second Contact, which is timed to occur at eleven minutes past the eleventh hour on the eleventh day of August.

For about two minutes the Sun entirely disappears behind the Moon. A halo of light, the Sun's corona, will give a saintly glow to the black lunar face. A dim, eerie glow will descend around those experiencing totality on the ground and an array of colours will dance around the entire horizon like no other sunset on Earth.

The ghostly aura will end with a flash of another Diamond Ring, followed closely by a second set of Baily's beads. This point - the Third Contact - will mark the end of the last solar eclipse of the millennium.

Glossary

Baily's beads: Bubbles of sunlight breaking through the deep valleys of the Moon in the final moments before and after a total eclipse

Corona: The Sun's outer atmosphere which extends for millions of miles into space and appears as a halo around a totally eclipsed Moon.

Diamond Ring: The twinkling of the final bead of sunlight immediately before or after a total eclipse

First Contact: The point when the Moon begins to take a "bite" out of the Sun. Followed by Second Contact, the point when totality begins. Third Contact is when totality ends and the Sun makes its reappearance, and Fourth Contact: The point when the Moon passes completely away from the Sun.

Saros Cycle: The complicated pattern of lunar and solar eclipses that repeats itself every 6,585.32 days, named by Edmund Halley, the great 17th Century English astronomer, who took it from a Babylonian word.

Totality: The point when the Moon completely blocks the Sun. The longest possible period of totality is 7 minutes 31 seconds.

Penumbra: The shadow caused by a partial solar eclipse

Umbra: The shadow of totality caused when the Moon completely blocks our view of the Sun.