John Stringer presents a guide to one of the last great astronomical events of this millennium.
On Wednesday, August 11, 1999, a total eclipse of the Sun will be visible from much of Cornwall, parts of Devon, the Isles of Scilly, and Alderney in the Channel Islands. These are in the "zone of totality", from which you will have the best chance of seeing the eclipse in the United Kingdom.
At 11.11 am, the black disc of the Moon will fit exactly over the Sun. Half the sky will go dark, while the half opposite the eclipse should remain reasonably light. This event will take place in most pupils' school holidays; but the eclipse presents great opportunities to teach children more about the Earth in space and to prepare them for an experience that will be spectacular in the zone of totality - and noticeable elsewhere in the UK.
A total eclipse like this one was last visible from the UK mainland in 1927; the next one will not happen until 2090.
What is an eclipse?
The Sun and the Moon appear to be the same size in the sky. But the Sun is around 400 times bigger in diameter than the Moon, and is 400 times further away from the Earth.
It is this coincidence that allows the Moon to obscure the Sun during a solar eclipse. You can demonstrate the eclipse on an overhead projector using two pieces of card. One has a circle cut out of it to represent the Sun and it is placed on the projector.
The other is a circle with a radius slightly larger than the circle cut out for the Sun. Simply slide one across the other to make an eclipse.
The picture above shows how the Moon comes between the Sun and the Earth to cast a shadow on the Earth.
* Time watch
A solar eclipse always goes through the same stages, from first to fourth contact. So it is possible to predict what will happen precisely. These are the times of these stages for August 11, 1999, and what will be seen at each of these times.
09.57 am: First contact
The Moon takes a bite out of the Sun's disc. Gaps between leaves act like pinhole cameras focusing hundreds of images of the Sun on the ground beneath trees. In the last few seconds before totality, the Moon's shadow will be seen approaching from the west and the sky will darken dramatically.
11.10 am: Second contact
The Sun is hidden by the Moon. The remainder of the Sun's bright disc is broken into blobs of light called Bailey's Beads as sunlight flashes through valleys on the edge of the Moon. The final bright bead, in conjunction with the Sun's inner corona, gives the appearance of a diamond ring.
11.11 am: Totality
The sky appears purple; the horizon is orange or maroon. You can see stars and planets in the middle of the day. The Sun's glowing corona, and long streamers of blazing gas circle the Moon. The Moon exactly covers the Sun.
11.13 am: Third contact
The diamond ring and Bailey's Beads return. The Moon's shadow is rushing towards the eastern horizon. The Sun brightens rapidly.
12.32 pm: Fourth contact
The Moon clears the Sun. Its silhouette disappears. The eclipse is over.
* Lunar eclipses
The Moon is a ball of rock; although it appears to shine, it has no light of its own. It reflects the light of the Sun. When the Earth is between the Sun and the Moon, the Earth blocks most of the light of the Sun. For a time, the Earth's shadow covers the Moon. Some light is scattered towards the Moon, so it appears a deep copper colour.
Both solar and lunar eclipses occur every 18 months, on average. But a lunar eclipse can be seen from anywhere on Earth where the Moon is above the horizon, so a much larger portion of the Earth's surface can see a lunar eclipse than a solar eclipse.