Sound 'cause of shadow spectacle'

Sticks · #1 (**permalink**) 22-May-2008, 09:42 AM

Quote:

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.

hhEb09'1 · #2 (**permalink**) 22-May-2008, 11:06 AM

Quote:

Originally Posted by Sticks

From BBC News Online

Thanks for bringing this up, Sticks. Interesting.

Quote:

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.

I wondered about that when I read it. Wouldn't the speed of sound at the surface be slower than the earth's rotation, for a lot of the earth? Up to about latitude 45. I know the geometry is not that simple, but still. Then I read the following quote:

Quote:

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.

So, I'd like to understand Barrie Jones' argument a little more. Oops, here it is:

Quote:

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."

I'm confused.

O no:

Quote:

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.

Whew, that would explain everything

publiusr · #3 (**permalink**) 30-June-2008, 11:20 PM

Cool!

billslugg · #4 (**permalink**) 01-July-2008, 02:55 AM

I am pretty sure that shadow bands are produced by interference between bright spots on different parts of the sun. As for any sound associated with them, it might possibly be caused by a heating of the air in the nodes.

hhEb09'1 · #5 (**permalink**) 01-July-2008, 09:35 AM

Quote:

Originally Posted by billslugg

I am pretty sure that shadow bands are produced by interference between bright spots on different parts of the sun.

Would that work? How?

novaderrik · #6 (**permalink**) 01-July-2008, 05:20 PM

so now shadows can have their own sonic boom?
has anyone ever noticed this happening with the shadows of supersonic aircraft?

01101001 · #7 (**permalink**) 01-July-2008, 05:34 PM

Quote:

Originally Posted by novaderrik

so now shadows can have their own sonic boom?
has anyone ever noticed this happening with the shadows of supersonic aircraft?

If this is something shadows do, I'd expect an aircraft shadow to produce an infrasound effect (in my book, the definition of "sonic boom" wouldn't include this) many orders of magnitude less than that of the Moon, for the difference in shadow size, and corresponding lesser solar-energy disruption, resulting in way less air displacement. The sonic effect of the aircraft's shadow might well be so small as to be undetectable even with fine instruments.

StupendousMan · #8 (**permalink**) 01-July-2008, 07:40 PM

Quote:

Originally Posted by billslugg View Post
I am pretty sure that shadow bands are produced by interference between bright spots on different parts of the sun.

Quote:

Originally Posted by hhEb09'1

Would that work? How?

If the lunar limb were to move across a bright point source of light,
observers on the Earth's surface would see bands of light race
across the ground due to diffraction; see this page for details:

http://spiff.rit.edu/richmond/occult/bessel/bessel.html

The Sun isn't a point source, of course, but in the stages of a solar
eclipse just before totality, the solar limb becomes very thin, and
finally a small set of point sources. The result can still be a set
of shadow bands.

Kaptain K · #9 (**permalink**) 02-July-2008, 12:57 AM

Thank you StupendousMan. Very good and understandable explanation. Should put this one to rest!

01101001 · #10 (**permalink**) 02-July-2008, 01:55 AM

Quote:

Originally Posted by StupendousMan

[...] observers on the Earth's surface would see bands of light race
across the ground due to diffraction; see this page for details:

http://spiff.rit.edu/richmond/occult/bessel/bessel.html

So I wonder how did the article come to contain:

Quote:

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.

sabianq · #11 (**permalink**) 02-July-2008, 02:17 AM

Hi all, In the audio world, there is a type of interference called "comb filtering" and it analogous to the interference patterns one sees in light when it is passed over the edge of something sharp like a razor blade (called diffraction ^^^). could this be what is being seen? Well stupendous man showed us that that ain't the case.

Maybe the reference to Infrasound means frequencies less than 20 hz and these frequencies incidentally have a wave length at 1 atm that is between 340 meters (1 hz) and 17 meters (20 hz)

are these shadows this size? ^^^^ I have never seen one.

sabianq · #12 (**permalink**) 02-July-2008, 02:18 AM

could someone post a picture, these sound really cool

sabianq · #13 (**permalink**) 02-July-2008, 02:28 AM

Quote:

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 [sic] per second."

Yes, but a standing wave could bee seen to not be moving or moving very slowly if the modulation is constant. if the frequency bands are constructed with multiple frequencies, it could very easily appear to move forward and backwards and even stand still.

What would cause standing waves in the atmosphere is another question though.

fascinating....

DyerWolf · #14 (**permalink**) 02-July-2008, 01:20 PM

Quote:

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

This is the first I'm hearing of this. Any truth to it? Anyone know of links to papers on the phenomena?

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