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dstebbins

Why is the day sky blue?

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Ok, now, before anyone blurts out the answer some of you are certainly thinking ... yes, I know that the reason it's blue as opposed to pink (like on Mars) is because of our oceans. That's not what baffles me.

 

I'm wondering why a clear, daytime sky has ANY color at all, other than black.

 

When it's night, we see the stars as just a bunch of white dots scattered across an otherwise black void. The night sky is black ... because that's the color of nothingness. The stars are, absolutely, shining light in all directions. The only problem is ... there's nothing in the deep void of space for those stars' lights to bounce off of (at least, nothing large enough, and close enough, for use to be able to see it; after all, we don't exactly have the best seats in the house).

 

So, why should it be any different during the day? We don't see the stars during the day, not because they aren't there, but because the sun is just so bright that it blocks out all the other stars while it's out.

 

But during the day, there's no solid object for the sun's light to bounce off of, giving us the blue "ceiling" (for lack of a better word).

 

Even astronauts, when they're out in space, look out the window and see what looks like a perpetual night, even when the heat next to the window tells them that they're clearly facing the sun. Because again, nothing to bounce the light off of.

 

And if you're telling me that it's the atmosphere that the sun's light is bouncing off of, well then why isn't it that way even during the night?

 

This random idea just popped into my head for no reason! Can somebody answer it for me?

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Rayleigh scattering. Light scatters off of the molecules in the atmosphere, anf the process is stronger at higher frequency. Light not going toward you initially is scattered. Red sky at sunrise and sunset is from the same process - the blue is scattered away.

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^ Plus the number of violet photoreceptors in our eyes.

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And the oceans have nothing to do with it.

And, following off of this, the Martian sky is red because there are red rust particles physically floating in the air. It's not because the sky is somehow reflecting the color of the ground, and neither is the Earth's sky blue because it is reflecting the color of the oceans.

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and it is sky blue because the sky is blue.

The colour we see, when considered objectively, should be more towards purple but our eyes have more sensitivity in the blue than violet, so, coupled with our brain's evolved tendency towards maximising visual differentiation for a survival advantage it is seen more as blue. Rather than being a perfect reflector of the world around us our brain constructs its own reality in a way that suits it best.

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The colour we see, when considered objectively, should be more towards purple but our eyes have more sensitivity in the blue than violet, so, coupled with our brain's evolved tendency towards maximising visual differentiation for a survival advantage it is seen more as blue. Rather than being a perfect reflector of the world around us our brain constructs its own reality in a way that suits it best.

We really do make our own reality.

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dstebbins, I think you mix up a few things.

 

In outer space the sun is not blocking the starlight. It is only that if something is lighted by the sun (the spaceship, the moon landscape, the earth), it is much brighter than the stars. So our eyes adapt to bright light and we so not see the stars anymore. Same when astronauts take pictures: the shutter speed becomes so short, that there are no visible stars anymore. If an astronaut blocks the sunlight with his fist, or just looks in a direction where there is nothing bright lighted by the sun, he sees the stars, much brighter than we do.

 

On earth, on the other side, light is scattered by the atmosphere, especially blue, and there is bright light everywhere. It is brighter than all the stars. During the night, the stars themselves are not so bright that there is much scattered light. So now we can see the stars. On the other side, during the full moon, it gives so much light that we see significantly less stars then on a moonless night.Now the moonlight does not make the sky blue, just some kind of grey light. The reason is that it is not so bright as the sun. So only the more light sensitive rods in our eyes see the scattered light. But these are not colour sensitive, so we see the light as grey, instead of blue.

 

Two more observations that might help to understand these phenomena a little:

  • If you see far away mountains, they seem to be blueish. Of course they are not. There is just much more air between you and the mountains, and this air scatters the blue light the same way as it does above your head.

    22-diyawana-oya-and-sorabora.jpg
  • During a total sun eclipse, the air turns dark, and you can see the stars. I was not prepared for this when I saw the eclipse of August 1999 and was totally astonished that more or less from one second to the next, I could see the stars, and Venus and Mercury high above my head near the (eclipsed) sun. Normally, you never see Venus and Mercury high above, but only close to the horizon.

    Example:

    eclipse03.jpg
Edited by Eise

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