Total eclipses happen because the moon, orbiting the earth at a distance of 240,000 miles, is just large enough to cover the apparent disc of the sun, 93 million miles away.

The moon's shadow creates a path of darkness along the earth's surface, about 60 miles wide and moving at about 1,700 miles per hour.

This is the shadow which, on 11 August, will cross Cornwall and south Devon. In Cornwall, the darkness will last two minutes.

Eclipses are dangerous because they encourage people to look directly at the sun, which can damage the retina.

The temptation is greatest during totality. The danger comes when the sun suddenly re-emerges, while pupils are still dilated for darkness.

The Department of Health says that, after the 1927 total eclipse, 13 people lost their sight. The only safe way of looking at the sun is to project its image through a pinhole onto a screen.