Solar eclipses

A total solar eclipse is one of the most spectacular astronomical events that can be seen from Earth.

Such an event occurs when the Moon passes directly between the Sun and Earth. An eclipse does not occur at every new moon because the Moon's orbit often passes above or below the Sun instead of directly across it. During an eclipse, the Moon's shadow traces a curved path across the surface of the Earth. Any person standing in the path of the shadow will see the sky and landscape go dark as the Moon blocks out the sunlight.

Solar eclipses can be total, partial, or annular, depending on how much of the Sun is covered by the Moon. Total eclipses occur when the Moon is exactly in line between the Earth and Sun and exactly covers the disc of the Sun. If the Moon is not exactly in line between the Earth and Sun, a partial eclipse occurs. An annular eclipse occurs when the Moon is far enough away from Earth that its apparent size is smaller than the Sun's. Hence a bright ring (annulus) remains visible, and the Sun's corona cannot be seen.

There are as many as two total solar eclipses a year, and sometimes as many as five, but few people have a chance to see them. The paths along

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Moon's shadow

Figure 3.9 How an eclipse of the Sun occurs.

Figure 3.10 The solar corona as seen during a total solar eclipse. (Photo: J. Wilkinson)

which eclipses can be seen are narrow, and the darkest period, or totality, can last only about seven and a half minutes at most.

Among the features of a total eclipse are the so-called Bailey's Beads. These are seen just as the Moon's black disc covers the last thin crescent of the Sun. Sunlight shining between the mountains at the Moon's edge looks like sparkling beads. The Diamond Ring effect is a fleeting flash of light seen immediately preceding or following totality.

At the time of totality, an observer with a small telescope can see the Sun's prominences as long flame-like tongues of incandescent gases around the edge of the Moon's disc. Also during totality, the corona can be seen as a region of glowing gases stretching out from the blacked-out Sun.

Care must be taken when observing the Sun, even during an eclipse - and advice should be sought to protect your eyes from damage.

Future total solar eclipses are due on 22 July 2009 (Asia); 11 July 2010 (South Pacific ocean).

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