🚀 go-pugleaf

In article <1992Aug6.183457.3278@ucsu.Colorado.EDU> knapp@spot.Colorado.EDU (David Knapp) writes:
>In article <1151@kepler1.rentec.com> andrew@rentec.com (Andrew Mullhaupt) writes:
>>In article <1992Aug2.062235.12886@ucsu.Colorado.EDU> knapp@spot.Colorado.EDU (David Knapp) writes:
>>>In article <1138@kepler1.rentec.com> andrew@rentec.com (Andrew Mullhaupt) writes:
>>>It's Rayleigh scattering off oxygen, and that's about the end of it.
>>
>>- But the punch line is
>>that the light is scattered more with the higher frequencies, and this
>>dependence doesn't stop at blue, either.
>
>Right. Where it 'stops' is specie dependent.

Rayleigh scattering "stops" for visible light when the particle size is
about 4 microns, when you start using Mie scattering (as in 'mother of
pearl' clouds...) Needless to say, nitrogen and oxygen molecules are much
smaller than this, so for them Rayleigh scattering goes on up into the UV.

>>Now we have three things to do at this point. We want to claim that light
>>received in directions away from the sun is largely scattered sunlight,
>
>You'll have to explain why proximity isn't the answer here to me. Email  me
>if you like.

Read Olber's paradox in the sci.physics FAQ.

>>Then we need to fix the color of the sun.

>The 'color' of the sun?

Yes. The spectrum of the sunlight measured at the top of the atmosphere is
a rather lumpy curve, and it is not very rich in visible violet.

>> Rayleigh scattering on a planet
>>circling a hotter star than the sun would give a purple sky, and red
>>or even green sky for a cooler one. Jackson does not explain the color of
>>sunlight, but at least he plots the spectrum of the incident sunlight,
>>showing clearly that the sunlight doesn't have much violet to be scattered.

>'much'? How much is 'much'?  Scattering can involve two photon processes.
>Blue sky is not one of those, but you can have two infrared photons give you
>visible scattering. You can *certainly* have scattering of UV giving you
>visible wavlenghts and there are *loads* of UV in sunlight. I'm missing your
>point perhaps.

Mainly because there isn't much contribution from UV scattering to visible
in the atmosphere, and the fact that sunlight has _far_ less UV than visible,
according to Jackson's spectrum.

>My opinion is that you (and he...gads, I'm arguing with Jackson! I'm waiting
>for a lightning bolt...) are making too simple a situation out of a very 
>comlicated radiative transfer problem.

Hey, I'm the one arguing that the blue sky has a complicated explanation.
But maybe you didn't catch that when I posted:

 The actual number of physical phenomena which are involved here, according
 to present understanding, is quite large. In fact, the sky has many colors,
 and blue is only one of them.

>My understanding was that the Rayleigh scattering that gives a blue sky
>is due to UV.

No. This is not the case.

>How many stars have an absolute magnitude above detection by the human eye,
>that are red shifted enough to do this? I will posit not a heck of a lot if
>any.

You would be wrong. But altough the red shift is important, the main reason
for the dark sky at night is thought to be the finite age of the universe -
you can't see anything further out than roughly 20 billion years because the
light hasn't had time to get here yet.

>Unless you can point to a star (give a name and coordinates) that you should
>be able to see with your eye, that has red shifted all of its emission  out
>of the visible, I will maintain that it is silly.

Uh - visible starts are the ones with the names you'd know. But there are
plenty of things far enough away to be shifted out of the visible. The
universe is expanding, remember? Then all that is required for a star to
be shifted out of the visible is for it to be far enough away.

>>>It is the anti-stokes lines of Rayleigh scattering! Good grief! What more do
>>>you need? If you want to call it something else, go ahead, but study
>>>Rayleigh scattering and it will make more sense to you why the sky is 
>>>blue.
>>
>>Still stand by this?
>
>Yes. Nothing you've said (as I understand it) contradicts Rayleigh scattering
>as an explanation of a perceived blue sky.

>> Try explaining the blue color of the sky which occurs
>>at the zenith when the sun is setting using Rayleigh scattering.

>I don't undertand why you say this shouldn't happen according to Rayleigh 
>scattering.  If the atmospher *only* eperienced single scattering phenomenon,
>you might have a point. In fact, you can notice phase angle dependent
>*polarized* scattering with polaroid sunglasses. It is, as you say, at 
>90 degrees to the incident sunlight.

Because it cannot be explained by Rayleigh scattering. Ask Jearl Walker.
I quote from the _Flying Circus of Physics_:

	"The enhanced zenith blueness comes as a surprise. According to the
	 Rayleigh scattering, the zenith sky should be blue-green and then
	 yellow as the sun sets. The missing element in a simple Rayleigh
	 scattering picture is the absorption by the atmospheric ozone in
	 the red end of the spectrum. With the red end removed, the blue end
	 is left and appears richer. The geometry featuring such a blue
	 enhancement occurs when the sun is about 6 degrees below the horizon
	 and the light scatters from directly overhead the observer. The blue
	 is further increased by dust in the atmosphere because the dust
	 absorbs more red and yellow from the direct sunlight than the blue."

In other words, Rayleigh scattering _does not_ explain _this_ blue sky,
although it explains the blue in the sky most of the time.

Later,
Andrew Mullhaupt