Dalo

Why is the sky blue on Earth

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8 minutes ago, John Cuthber said:

 

What is remarkable is your failure to understand it.

 

John, my failure is that I didn't read it.

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I wonder what the spectrum would look like on Mars through a prism. Anybody knows?

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The dormitive property of opium (Moliere, "Le malade imaginaire) will remain for ever the most acerbic critic of "savants' disguising their ignorance behind incomprehensible jargon. I would certainly not consider John William Strutt, better known as sir Raleigh, among one of the pompous 17th century characters,  since I am sure that his mathematical calculations amply justify his reputation.
Still, I cannot but wonder at the way his work is often presented. Take for instance this introduction from a respected site:
"The blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum."
I would say that it is a pregnant example of dormitive properties. As it is here explained, the sky is blue because... the sky is blue. or more explicitly, because the blue light is scattered more efficiently than other colors.
Am I glad this puzzle has been solved!

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51 minutes ago, Dalo said:

 As it is here explained, the sky is blue because... the sky is blue. or more explicitly, because the blue light is scattered more efficiently than other colors.
 

Those two sentences are not the same. The latter actually contains some physics. 

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35 minutes ago, swansont said:

Those two sentences are not the same. The latter actually contains some physics. 

I am sure it does. Analyzing empirical facts is an important aspect of physics. The second part is based on all kind of calculations while the first part is a simple observation. But then, the question is whether the second part is an explanation of why the sky is blue, or an explicitation of how it comes that the sky is blue: it is blue because short waves are more efficiently scattered than longer waves.

This is a dormitive property. It does not however make the calculations any less valid. 

It does not explain how the sky could not possibly be any other color than blue. That would be an explanation.

Edited by Dalo

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3 hours ago, Dalo said:

As it is here explained, the sky is blue because... the sky is blue.

I suppose someone might think that if they were not able to understand simple English. Personally, I think that is a good concise summary as an introduction to the detail that follows. I find it hard to believe it would appear to be “incomprehensible jargon” (except, perhaps, by someone wanting to hide their ignorance; rather than make an effort to understand, it is easier to dismiss it).

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1 minute ago, Strange said:

I suppose someone might think that if they were not able to understand simple English. Personally, I think that is a good concise summary as an introduction to the detail that follows. I find it hard to believe it would appear to be “incomprehensible jargon” (except, perhaps, by someone wanting to hide their ignorance; rather than make an effort to understand, it is easier to dismiss it).

Why are you being so hostile? 

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3 minutes ago, Dalo said:

Why are you being so hostile? 

I’m not. Why are you being so sensitive. 

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The sky is blue because the depth and density of our atmosphere is in the Goldilocks zone for showing that colour for most of the day. If the depth was much deeper, then all the blue would get scattered away and one of the longer wavelengths would predominate, as is witnessed towards sunrise and  sunset.

Edit: added

Edited by StringJunky

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3 hours ago, Dalo said:

I am sure it does. Analyzing empirical facts is an important aspect of physics. The second part is based on all kind of calculations while the first part is a simple observation. But then, the question is whether the second part is an explanation of why the sky is blue, or an explicitation of how it comes that the sky is blue: it is blue because short waves are more efficiently scattered than longer waves.

This is a dormitive property. It does not however make the calculations any less valid. 

dormitive means sleep-inducing, so I am rather confused about your point. 

3 hours ago, Dalo said:

It does not explain how the sky could not possibly be any other color than blue. That would be an explanation.

It does. It also explains why the sky is red at sunrise/sunset.

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Just now, swansont said:

dormitive means sleep-inducing, so I am rather confused about your point. 

It is related to the dormitive property of opium, an expression Moliere used to indicate that doctors in his time used very complicated jargon to express truisms.

2 minutes ago, swansont said:

It does. It also explains why the sky is red at sunrise/sunset.

Maybe it does. My impression is that the calculations show that in one case the sky is blue, and red in the other. They represent the mathematical expression of familiar observations. They represent empirical facts and not theoretical conclusions. And the most striking property of an empirical fact is that it just is the way it is. A theoretical fact assumes that something could not be otherwise based on other empirical, logical and theoretical facts.

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!

Moderator Note

There is plenty of science to critique in this thread. Let's keep our attacks aimed at ideas and ignorance, not people. 

 

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8 hours ago, Dalo said:

I wonder what the spectrum would look like on Mars through a prism. Anybody knows?

This question got lost in the discussion that followed. I think they are both related and it might help making things clearer, or at least more concrete.

Just to be clear, I wouldn't know how to answer it, so it is a genuine question.

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3 hours ago, Dalo said:

It is related to the dormitive property of opium, an expression Moliere used to indicate that doctors in his time used very complicated jargon to express truisms.

Maybe it does. My impression is that the calculations show that in one case the sky is blue, and red in the other. They represent the mathematical expression of familiar observations. They represent empirical facts and not theoretical conclusions. And the most striking property of an empirical fact is that it just is the way it is. A theoretical fact assumes that something could not be otherwise based on other empirical, logical and theoretical facts.

But it is the same theoretical concepts that predict a red sunset or sunrise as predicts the blue sky.  The scattering of the blue in the spectrum means that the wavelengths at that end of the spectrum are not coming from directly from the direction of the Sun when they get to you, but are scattered and bouncing around so that they end up coming at you from all directions in the Sky.   As the Sun nears the horizon, it passes through more and more atmosphere to get to you, and more and more of the blue end the spectrum is scattered out so the light coming from the direction of the Sun is redder and redder.    The red you see in the sky at sunset or sunrise is actually this redder light illuminating haze and clouds.  Another factor is that the scattering effect also depends on the size of the scattering particles.  At sunrise and Sunset the light is passing through air that is closer to the ground and more likely to have particulate matter in it which increases the filtering out of the upper spectrum.  (One of the best sunrises I every saw was in Montana a couple of years ago when they were having all the wild fires, And earlier this summer when we were getting smoke and haze here from wild fires,  the Sun looked blood red from the scattering effect)

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1 minute ago, Janus said:

But it is the same theoretical concepts that predict a red sunset or sunrise as predicts the blue sky.

You say prediction, I say description.

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13 hours ago, Dalo said:

Maybe it does. My impression is that the calculations show that in one case the sky is blue, and red in the other. They represent the mathematical expression of familiar observations. They represent empirical facts and not theoretical conclusions. And the most striking property of an empirical fact is that it just is the way it is. A theoretical fact assumes that something could not be otherwise based on other empirical, logical and theoretical facts.

It's both empirical fact and theoretical conclusion. That's one of the ways that science works. Make an observation and then develop a theory to explain it.

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3 hours ago, swansont said:

 

It's both empirical fact and theoretical conclusion. That's one of the ways that science works. Make an observation and then develop a theory to explain it.

Agreed. I will just mention in passing, without in anyway inviting a new discussion, that there is a continuum with, at one end, observations for which we seek an explanation, as is the case for the color of the sky, and, at the other end, un-observed or even unobservable phenomena which the theory predicts, as most facts are in, for instance, Quantum theory.

Regarding the color of the sky the following quote of Raleigh might be of interest:

"It is now, I believe, generally admitted that the light which we receive from the clear sky is due in one way or another to small suspended particles which divert the light from its regular course... Whenever the particles of the foreign matter are sufficiently fine, the light emitted laterally is blue in color... the standard of linear dimension, with reference to which the particles are called small, is the wave-length of light." John William Strutt, On the light from the sky, its polarization and its color. 1871. (freely available on the web)

I find these lines particularly interesting because they present a means of testing the theory beyond Earth atmosphere, on Mars for instance. Would particles of the same size as found in our sky yield the same color in Martian sky?

I am genuinely curious as to the answer.

Edited by Dalo

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3 hours ago, Dalo said:

Agreed. I will just mention in passing, without in anyway inviting a new discussion, that there is a continuum with, at one end, observations for which we seek an explanation, as is the case for the color of the sky, and, at the other end, un-observed or even unobservable phenomena which the theory predicts, as most facts are in, for instance, Quantum theory.

Regarding the color of the sky the following quote of Raleigh might be of interest:

"It is now, I believe, generally admitted that the light which we receive from the clear sky is due in one way or another to small suspended particles which divert the light from its regular course... Whenever the particles of the foreign matter are sufficiently fine, the light emitted laterally is blue in color... the standard of linear dimension, with reference to which the particles are called small, is the wave-length of light." John William Strutt, On the light from the sky, its polarization and its color. 1871. (freely available on the web)

I find these lines particularly interesting because they present a means of testing the theory beyond Earth atmosphere, on Mars for instance. Would particles of the same size as found in our sky yield the same color in Martian sky?

I am genuinely curious as to the answer.

If the makeups were similar, The Martian sky would be likely be darker, as the thin atmosphere would scatter much less light.   However, because Mars is so dry, there is a lot of magnetite dust suspended in the atmosphere which changes the way the sky looks.  The only time you will see the typical Rayleigh scattering is when the Sun is near the horizon and passing through more atmosphere.  Then you will see a bit of blue in the sky around the Sun like in this picture.

https://en.wikipedia.org/wiki/Astronomy_on_Mars#/media/File:Mars_sunset_PIA00920.jpg

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16 minutes ago, Janus said:

If the makeups were similar, The Martian sky would be likely be darker, as the thin atmosphere would scatter much less light.   However, because Mars is so dry, there is a lot of magnetite dust suspended in the atmosphere which changes the way the sky looks.  The only time you will see the typical Rayleigh scattering is when the Sun is near the horizon and passing through more atmosphere.  Then you will see a bit of blue in the sky around the Sun like in this picture.

https://en.wikipedia.org/wiki/Astronomy_on_Mars#/media/File:Mars_sunset_PIA00920.jpg

Thank you for the link. And what would you conclude from the comparison between Earth and Mars? Would you say that the size of the particles is determinant, as Raleigh said in the quote above?

To be honest, I did not read his other articles (yet) and I do not know if he kept the same assumption throughout his long career.

edit: Do you know maybe of any reference  of the spectrum through a prism on Mars?

Edited by Dalo

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3 hours ago, Dalo said:

Thank you for the link. And what would you conclude from the comparison between Earth and Mars? Would you say that the size of the particles is determinant, as Raleigh said in the quote above?

To be honest, I did not read his other articles (yet) and I do not know if he kept the same assumption throughout his long career.

edit: Do you know maybe of any reference  of the spectrum through a prism on Mars?

The spectrum created by a prism on Mars would be pretty much the same as that seen on the Earth, with a slight difference in the absorption bands due to the difference in the composition of the atmosphere it passed through to reach the surface.

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Just now, Janus said:

The spectrum created by a prism on Mars would be pretty much the same as that seen on the Earth, with a slight difference in the absorption bands due to the difference in the composition of the atmosphere it passed through to reach the surface.

Is that a prediction, or have experiments taken place that show these results?

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32 minutes ago, Dalo said:

Is that a prediction, or have experiments taken place that show these results?

Multiple experiments have been performed by passing light through various gasses and examining the resulting spectrum.

Mars' atmosphere has had a spectral analysis done of it.

Martian probes have been equipped with spectrometers to analyze collected samples and none of these results have indicated that a prism on Mars behaves any differently than one on the Earth. 

So i would say that it is a prediction grounded on very strong experimental evidence.

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14 minutes ago, Janus said:

Multiple experiments have been performed by passing light through various gasses and examining the resulting spectrum.

Mars' atmosphere has had a spectral analysis done of it.

Martian probes have been equipped with spectrometers to analyze collected samples and none of these results have indicated that a prism on Mars behaves any differently than one on the Earth. 

So i would say that it is a prediction grounded on very strong experimental evidence.

okay. Do you maybe have a link or reference for me? I would really appreciate it.

edit: You will understand that spectrograph's would not really be conclusive. We do not need to go to Mars for that. It is already undeniably proven that different elements show different absorption patterns. What I would find interesting is to see an experiment comparable to that originally done by Newton whereby a ray of sunshine is let through a small opening and then through a prism. That would be the definite proof that the atmosphere and/or the prism play no role in the composition of the spectrum.

It would be a little bit like Armstrong's experiment where he dropped a feather and a heavy object on the moon, and they fell at the same speed. That was in fact the first empirical proof of the Galilean and Newtonian concept of gravity outside of earth conditions. So, your "prediction" is certainly understandable. What goes for Earth should be valid on Mars also. Still, conviction is not proof.

Edited by Dalo

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Here is what Goldstein (polarized light, 2011) says about the color of the sky:

"Light from the sun interacts with the molecules of our atmosphere such that the light that we see coming from the dome of air over our heads is scattered sunlight (or moonlight)."

But then I wonder: The Sun is many times larger than Earth, and its light encompasses about half of the planet at once. Why do we need scattering to explain the color of the sky? Why would it be relevant since light would be everywhere at the same time?

Edited by Dalo

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31 minutes ago, Dalo said:

Here is what Goldstein (polarized light, 2011) says about the color of the sky:

"Light from the sun interacts with the molecules of our atmosphere such that the light that we see coming from the dome of air over our heads is scattered sunlight (or moonlight)."

But then I wonder: The Sun is many times larger than Earth, and its light encompasses about half of the planet at once. Why do we need scattering to explain the color of the sky? Why would it be relevant since light would be everywhere at the same time?

That's why all the sky is blue (where you can see it)

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