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Originally Posted by cweekly
another note bene: this is also related to the reason sunsets are so pretty: the red and orange hues are caused by the lengthening of ths sun's rays as the sunlight travels further, it's in effect being stretched, and longer wavelengths are more red, while shorter ones are more blue. (this "redshifting" is an example of the doppler effect, which also explains why car engines sound higher pitched as they race towards you then suddenly lower as they recede)
that's the gist anyway (from memory),someone else may post a more technically precise answer
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Sorry xoxoxoBruce, but his explanation of the colors at sunset is completely off. The reds and oranges you see in the sunset have nothing whatsoever to do with red shifting or stretching of wavelengths. For that to happen we'd have to be moving a significant percentage of the speed of light away from the sun.
But the real answer to why you get the reds and oranges at sunset is the same reason the sky is blue. Go back and read the quote that Undertoad posted with the original picture again:
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Green flashes are created by variations in refraction near the horizon. The refractive layer causes sunlight to be weakly dispersed into the constituent colours of red, yellow, green, blue and violet. Violet and blue light are normally scattered in the Earth's atmosphere, with the result that the last portion of the dispersed light to be observed as the Sun sets is green.
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See, when it's scattered, all the blue end of the spectrum gets scattered, and the red end isn't. Near sunset the light has to go through so much atmosphere that most of the blue end of the spectrum has scattered out (which makes the sky look blue), and all that's left is the reds and oranges (and some green, which is what the IotD is about).
(I was going to explain why blue scatters and red doesn't, but I figured the more I write, the less people are going to take the time to read it)