Re: [Phys-L] Where is the sky?

[John Denker <jsd@av8n.com>]

On 08/22/2013 08:08 AM, Bob Sciamanda wrote:
> This gives an answer to my query by asserting that only the UPPER 
> atmosphere produces the blue sky of scattered sunlight. ==>
>
> http://en.wikipedia.org/wiki/File:Sunset_from_the_ISS.JPG

That photo is nowhere near being the answer (or even "an" answer)
to the original question.  The result in the photo is mostly a 
consequence of details of the geometry of a special situation.

Even on the ground you can observe the same effect at sunset:
Light coming straight from the sun is redder (i.e. less blue)
while light scattered through larger angles is bluer.

You can do a better-controlled experiment by going up in an
airplane.  At 18,000 feet you are above half of the mass of
the atmosphere (and above nearly all of the dust), and at
36,000 feet you are above three-fourths of the mass.  You
are not however above most of the scattering.  This drives
home the point that the scattering comes from /fluctuations/
in the density.  The amount of fluctuation /per unit mass/
goes like √N/N, so less-dense air has more scattering per
unit mass. As previously discussed, it is partially true
but quite incomplete and misleading to say that the light 
is scattered by air "molecules", since the amount of
scattering is not simply proportional to the number of
molecules.

We expect the upper layers to contribute /somewhat/ more 
than their share to the sky light, but only somewhat.


> How is it that the blue sky of scattered sunlight appears to come
> from a highly localized source confined to a distant hemispherical
> surface?

Even in the photo, the source of the blue light is not
"distant" from the surface, not very distant compared 
to its thickness, and certainly not distant compared to
the radius of the earth.  The source is not a hemispherical 
surface.