# Why is the sky blue on Earth

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Just now, John Cuthber said:

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

that the sky is blue is a fact. Now we have to explain scattering of light when light is already everywhere.

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9 minutes ago, Dalo said:

Now we have to explain scattering of light when light is already everywhere.

Most of the light is going out from the Sun. Light coming from anything else must have been scattered.

How is this difficult?

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1 minute ago, John Cuthber said:

Most of the light is going out from the Sun. Light coming from anything else must have been scattered.

How is this difficult?

how is this clear?

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13 minutes ago, Dalo said:

that the sky is blue is a fact. Now we have to explain scattering of light when light is already everywhere.

If here were no scattering, then the sky would be black apart from the little bit where the sun was.

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2 minutes ago, Strange said:

If here were no scattering, then the sky would be black apart from the little bit where the sun was.

This is a circular argument. I am asking why we need scattering if light is already everywhere and you answer that otherwise the sky would be black. But that this exactly the issue. If light is already everywhere then how could the sky be black?

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4 minutes ago, Dalo said:

This is a circular argument. I am asking why we need scattering if light is already everywhere and you answer that otherwise the sky would be black. But that this exactly the issue. If light is already everywhere then how could the sky be black?

For you to see a color, light has to be going toward you. The sky can't be blue if the light is not scattering in your direction.

The sky is black on the moon, even though light is everywhere.

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1 hour ago, Dalo said:

...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?

It's rather a long way away, so, it's effectively pretty small relative to the Earth;  most of the incident rays are diverging just before they enter its atmosphere.

Edited by StringJunky

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8 minutes ago, Dalo said:

This is a circular argument. I am asking why we need scattering if light is already everywhere and you answer that otherwise the sky would be black. But that this exactly the issue. If light is already everywhere then how could the sky be black?

You can only see light when it arrives at your eye. This will only be the case for rays that come directly from the sun to your eye. Therefore you will only see a small disk where rays come directly from the sun. Light arriving from the sun to other parts of the sky will be on a trajectory that does not intersect your eye, therefore you will not see the light from those rays. Unless they are scattered and arrive at your eye.

As someone else said, how hard is this? Are you pretending to be dumb for entertainment purposes?

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6 minutes ago, swansont said:

The sky is black on the moon, even though light is everywhere.

so is the interstellar space, or vacuum.

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13 minutes ago, Strange said:

You can only see light when it arrives at your eye. This will only be the case for rays that come directly from the sun to your eye. Therefore you will only see a small disk where rays come directly from the sun. Light arriving from the sun to other parts of the sky will be on a trajectory that does not intersect your eye, therefore you will not see the light from those rays. Unless they are scattered and arrive at your eye.

As someone else said, how hard is this? Are you pretending to be dumb for entertainment purposes?

I think he's hanging onto some erroneous fact which is hindering his understanding.

8 minutes ago, Dalo said:

so is the interstellar space, or vacuum.

Where does the blue come from? Think of an atmosphere as like a sheet of, translucent plastic throwing rays in all directions as they pass through.There will be a mix of diverging and converging rays.

Edited by StringJunky

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3 minutes ago, Dalo said:

so is the interstellar space, or vacuum.

Exactly. Because there is nothing to scatter the light.

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Just now, Strange said:

Because there is nothing to scatter the light.

or there is nothing to be illuminated.

Let us take all potential observers away, and put a light sensor among the 'scatterers". We can say by looking at the sensor afterwards whether it has received light or not. That does not depend on our vision organ and its idiosyncrasies. We could also have the sensor indicate the wavelength of light received, and then imagine that all particles in the air are such sensors. All the sensors, I think, would tell us the same story, that they have received light, and that it was blue.

This way we will have confirmed that the sky is blue without once having appealed to scattering or vision.

Am I missing something here?

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5 minutes ago, Dalo said:

or there is nothing to be illuminated.

Let us take all potential observers away, and put a light sensor among the 'scatterers". We can say by looking at the sensor afterwards whether it has received light or not. That does not depend on our vision organ and its idiosyncrasies. We could also have the sensor indicate the wavelength of light received, and then imagine that all particles in the air are such sensors. All the sensors, I think, would tell us the same story, that they have received light, and that it was blue.

This way we will have confirmed that the sky is blue without once having appealed to scattering or vision.

Am I missing something here?

You need the atmosphere to selectively absorb the wavelengths to get a colour, otherwise it will just be white.

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8 minutes ago, Dalo said:

or there is nothing to be illuminated.

Which is pretty much the same thing.

8 minutes ago, Dalo said:

All the sensors, I think, would tell us the same story, that they have received light, and that it was blue.

The colour will depend on the direction the light comes from. The light (directly) from the Sun will be white. The light scattered from other directions will depend on the colour of the sensors it has been reflected from.

8 minutes ago, Dalo said:

Am I missing something here?

Obviously. But I have no idea what!

Edited by Strange

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13 minutes ago, Dalo said:

or there is nothing to be illuminated.

Let us take all potential observers away, and put a light sensor among the 'scatterers". We can say by looking at the sensor afterwards whether it has received light or not. That does not depend on our vision organ and its idiosyncrasies. We could also have the sensor indicate the wavelength of light received, and then imagine that all particles in the air are such sensors. All the sensors, I think, would tell us the same story, that they have received light, and that it was blue.

This way we will have confirmed that the sky is blue without once having appealed to scattering or vision.

Am I missing something here?

Yes.

Do you understand this

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and we are back to the unanswered questions of a couple days ago.

That is what I said on Tuesday:

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.

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1 minute ago, Dalo said:

And we are back to the unanswered questions of a couple days ago.

Which questions?

2 minutes ago, Dalo said:

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.

You were given examples where spectra are used on Earth, on Mars, in space, etc. We also understand the mechanisms by which a prism separates the different frequencies present in the light. So there is no sensible reason to expect different results.

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Just now, Strange said:

So there is no sensible reason to expect different results.

Let us leave it at that for until the empirical proof can be found on Mars. There is no need to continue this discussion any longer.

There are many things in the theory (of the dual nature) of light, that I find disturbing. I will continue expressing my doubts and learn from the reactions how to refine the questions I have. I might end up as strong a believer in the theory as you all are, or not.

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6 minutes ago, Dalo said:

Let us leave it at that for until the empirical proof can be found on Mars.

Spectrometers have been used on Mars. Why deny it?

6 minutes ago, Dalo said:

I will continue expressing my doubts and learn from the reactions how to refine the questions I have.

Asking random questions based on misunderstandings and lack of knowledge is probably not the most productive route. A structured course would be far more effective. You can find lots of free courses on line. Or go old school and buy a book.

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@Strange

You must be very unhappy with your life if you need to bash people all the time. You should take a more scientific attitude towards opponents in a debate, instead of attacking them personally all the time. It does not do you honor.

edit: like I said, spectrometers do not tell you anything about the atmosphere, only about the elements themselves. They cannot therefore be used as an argument.

Edited by Dalo

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23 minutes ago, Dalo said:

You must be very unhappy with your life if you need to bash people all the time. You should take a more scientific attitude towards opponents in a debate, instead of attacking them personally all the time. It does not do you honor.

I don't think I have attacked anyone. But feel free to report the relevant posts to the moderators.

Quote

edit: like I said, spectrometers do not tell you anything about the atmosphere, only about the elements themselves. They cannot therefore be used as an argument.

You asked if a prism (which is basically the same as a spectrometer) would work the same way on Mars. The answer is yes. Because they are in use there. Why does that not satisfy you.

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Just now, Strange said:

Because they are in use there. Why does that not satisfy you.

do you have a reference, or a link? That would be a fitting end to this conversation.

Just now, Strange said:

I don't think I have attacked anyone.

Is that you said when you were a kid? "who? Me?!"

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1 minute ago, Dalo said:

do you have a reference, or a link? That would be a fitting end to this conversation.

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Just now, Strange said:

no prisms, only, among other things, spectrometers.

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Just now, Dalo said:

no prisms, only, among other things, spectrometers.

And you expect a prism to behave differently but a spectrometer not to? What is the theoretical justification for that?