|MadSci Network: Astronomy|
Well, around here for the last month or two the sky has mostly been grey in the daytime. But I don't suppose that is really what you were asking.
The easy part of your question to answer is the "black at night" bit. Black is simply the colour we see when there is no light coming from the object we are looking at. So at night time, the sky looks black (nearly) because no light is coming from the sky. Light comes from the sun and the stars.
Why is it any different in the daytime? On the moon, it isn't. There, the sky looks black all the time, night or day. The difference is that the earth has a layer of air - the atmosphere.
But the atmosphere is colourless, not blue. So this still doesn't explain why the sky is blue. When sunlight hits the atmosphere, a part of it - a fairly small part - gets bent around, or "scattered". So part of a sunbeam that was headed somewhere to the West of you, gets scattered, and comes to you from the West, instead of where the sun is. And much the same thing happens with all of the other directions. So the whole sky gets bright, but not nearly as bright as the sun, with all of the small parts of sunbeams that were originally headed for somewhere else, and come to you from the various parts of the sky instead.
The last step to clear up the mystery is simply to tell you that the blue light gets scattered much more than the red. And so that is why the sky away from the sun looks blue. If you look at the sky closer to the sun, being very very careful NOT TO LOOK AT THE SUN, you will find that the sky there is a paler blue, or even white, because there is more of the red and yellow light in the sky-light when it does not have to bend so far. And around sunset, when the sunlight has to pass through a lot more air, the sun and the sky around it look orange or red, because all of the blue parts of the light have scattered away.
You will remember earlier that I said the sky is *NEARLY* black at night. Sometimes, when there is a very bright full moon, you can just make out that the night-time sky can also be a little bit blue because of scattering. And there is quite another effect that means that when you take a long exposure colour photograph of the night-time sky it comes out green! That is because there are some chemical reactions going on in the upper atmosphere about 80 kilometres high that give out a very faint green light!
But mostly, the sky is black at night and blue in the daytime when it is not hidden by clouds!
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