Sound 'cause of shadow spectacle' Article from BBC News

[Guest guest]

Dear Wendy & list-found this fascinating article on BBC.

Sound 'cause of shadow spectacle'

 

The bands appear before and after eclipse " totality "

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.

 

If proven, it would be a something of a revelation that eclipses

are a sonic as well as an optical phenomenon

 

Dr Stuart Eves, SSTL

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.

Regards

Jiyer

[Guest guest]

Dear Jiyer,

 

Many thanks for that! Unfortunately I've only managed to glance through

it quickly, but hope to sit back and read through it (at my leisure)

soon...

 

Best Wishes,

Mrs. Wendy

http://JyotishVidya.com

___

 

 

-

" ranjiaiyer " <ranjiaiyer

<jyotish-vidya >

Thursday, May 22, 2008 4:15 AM

Sound 'cause of shadow spectacle' Article from

BBC News

 

 

Dear Wendy & list-found this fascinating article on BBC.

Sound 'cause of shadow spectacle'

 

The bands appear before and after eclipse " totality "

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.

 

If proven, it would be a something of a revelation that eclipses

are a sonic as well as an optical phenomenon

 

Dr Stuart Eves, SSTL

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.

Regards

Jiyer