Black-out over Cornwall

By Malcolm Gibb

No, not a power failure, a total eclipse of our nearest star, the Sun. On 11th August 1999 Cornwall will be the Mecca for that group of people who study the sky, astronomers!

The Sun is 400 times the diameter of the Moon and 400 times further away from the Earth. This means that both the Moon and the Sun appear the same size when seen from the Earth. It is this coincidence which makes a total solar eclipse possible, when every so often, the orbits of the Sun, Moon and Earth line up exactly. Unfortunately it's not quite as straightforward as that! If the Moon's orbit was in the same plane as the Earth's, an eclipse would occur at each new and full Moon but the Moon's orbit is inclined at 5 degrees to that of the Earth's, so eclipses only occur occasionally. When the Earth is the middle body it casts a shadow on the Moon (a lunar eclipse). When the Moon comes between the Earth and the Sun it casts a shadow on part of the Earth (a solar eclipse). Total solar eclipses occur about every 18 months or so but are only visible from a narrow band of the Earth's surface at any one time, partial eclipses can be observed over a much wider band.

The eclipse, which will be observed in Cornwall, is a total eclipse and will last just over two minutes, but what a two minutes! First, a word of warning! NEVER look at the Sun directly with the naked eye, or worse still, through binoculars or a telescope. Always use a dark filter, a piece of welder's glass is recommended. No doubt the souvenir shops in Cornwall will cash in on this. Another way to observe the Sun is to project its image through binoculars or a telescope onto white card. Once the eclipse is total it will be safe to look directly at it. The eclipse itself starts when the Moon begins its progress over the face of the Sun. It takes about one and half-hours from then until the total eclipse. About fifteen minutes before totality an eerie darkness will fall over the land, birds and animals will think that night is falling and act accordingly. In the last few seconds before totality you may see what is known as the diamond ring effect, just before the last part of the photosphere (the Suns surface) disappears. Once the Moon completely covers the surface of the Sun, if you have binoculars you may be lucky enough to see a solar prominence. These are clouds of relatively cool gas, lifted and held up by magnetic fields. The feathery plumes and streamers of the Suns outer atmosphere, or corona will be visible; extending outwards, sometimes reaching several times the diameter of the Sun. Bright stars will be visible in the darkened sky. Soon the diamond ring appears again and the spectacle is over. Two minutes of breathtaking beauty. In just over an hour's time, the Moon moves clear of the Sun and the eclipse is over.

Of course, you may see nothing except darkness falling if the British weather decides to be cloudy, but if the sky is clear it's a natural phenomenon worth travelling for.