Mysterious bands of shadow which sometimes pass across the ground during an eclipse might be produced by sound pulses, according to a new theory.

- 21 / 05 / 2008 22:49

"Shadow bands" have been observed travelling across the ground before and after totality - when the Moon completely covers the Sun.

Many attribute these regular light and dark bands to atmospheric turbulence.

But astrophysicist Dr Stuart Eves thinks the phenomenon could be down to something called infrasound.

One astronomer who has studied "shadow bands" was sceptical of the new idea, however. Professor Barrie Jones, from the Open University in Milton Keynes, said that sound travelled too fast to be responsible for the phenomenon.

Prior to the eclipse totality, the bands are usually seen to pass over the ground in the direction in which the eclipse is travelling.

After totality, the bands are often seen spreading at an angle to the path of the eclipse.

Early theories suggested this effect was due to diffraction of the Sun's rays around the limb of the Moon. But this theory has fallen out of favour.

The theory currently favoured by many astronomers is that the bands result from illumination of the atmosphere by the thin solar crescent a minute or so before and after the eclipse totality.

This means that the light from a distant point can reach a particular place on the ground by a variety of paths, each one is bent in a different way as it passes through the atmosphere.

Thus in some places, the light waves reinforce and the light level is enhanced, whilst in others the waves tend to cancel each other out and the light level is reduced.

When the effects of all the paths taken through the atmosphere are taken together, the result is a ragged banded pattern of light and shade - shadow bands.

bbc