Sound ’cause of shadow spectacle’

By | May 27, 2008

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

“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.

‘Sonic boom’

The newest idea involves infrasound – sound with a frequency too low to be heard by the human ear.

“As the eclipse shadow moves through the atmosphere, the sudden disappearance of the Sun changes the Earth’s temperature,” Dr Eves, an astrophysicist who works for Surrey Satellite Technology Limited (SSTL), told BBC News.

This rapid cooling of the air sets up a difference in pressure. The potential energy associated with this pressure difference then escapes as high-intensity infrasound.

Dr Eves says the speed of the Moon’s shadow is generally supersonic and likens the phenomenon to the sonic boom of a jet breaking the sound barrier.

But the sound pulses are not generated as single events. Instead, they are created continuously along a “shock front” which moves ahead of the eclipse itself.

This infrasound “front” may create a pattern of peaks and troughs in the atmosphere, which changes the speed and direction of light waves – an effect called refraction – passing through it.

This in turn is responsible for generating the shadow bands seen on the ground.

Dr Eves says the effect could be similar to the way light and dark bands cross a swimming pool when the wind blows on a sunny day.

“If proven, it would be a something of a revelation that eclipses are a sonic as well as an optical phenomenon,” he said.

“None of the [existing] theories seem to take account of the fact that shadow bands change direction,” he explained.

But Dr Eves draws a comparison with the waves created when a ship travels through water. If this is correct, then it would explain why shadow bands seen before the eclipse would mostly travel in the direction of the eclipse shadow.

After the eclipse, the shadow bands would travel at angles in the same bay that waves diverge behind a ship.

Too quick?

Barrie Jones, who is director of the physics and astronomy department of the Open University, told BBC News: “I’m not sure how infrasound could generate the bands – it’s too fast.

He added: “Infrasonic waves in the atmosphere would move at the speed of sound, which would be something like 400m/s. Shadow bands move at wind speed, so they can be anything from stationary to a few metres per second.”

“The [accepted] theory works, there’s no need to seek an alternative,” said Professor Jones.

Stuart Eves thinks that demonstrating a role for infrasound might explain some other puzzling phenomena associated with eclipses.

For example, long period Foucault pendulums – designed to demonstrate the rotation of the Earth – have been known to swing wildly during eclipses.

Some researchers have proposed that gravitational effects may be responsible.

But Dr Eves thinks the disruption to pendulums may be caused by infrasound pulses causing the ground to vibrate, disrupting the pendulum’s rhythm.

In addition, animals, and in particular birds, have been seen to exhibit unusual behaviour. In the case of birds this includes premature roosting and apparent signs of distress or alarm.

Birds have auditory ranges that extend well beyond those of humans, and might be affected by low frequency sound pulses. – bbc

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