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Zoom Space, a wonderful site in our kid's directory, Yahooligans!, offers the answer to your question in a nutshell: You may already know that sunlight is made up of all the colors of the rainbow: red, orange, yellow, green, blue, and violet. You probably also know that sunlight has to pass through our atmosphere before it reaches our eyes. The gas molecules in the atmosphere break up, or "scatter," the sunlight into its many parts. But they scatter some parts more effectively than others. Different colors of light have different energies, or wavelengths. Red light has a long wavelength and a lower energy; blue light has a short wavelength
and a higher energy. The gas molecules in the atmosphere scatter the higher-energy blue wavelengths better than the red wavelengths. So the sky looks blue. This also explains the brilliant colors of the sunset. Sunlight at dusk or dawn has to travel through an especially thick layer of atmosphere before it reaches your eyes (it's at an angle). As a result, even more higher-energy light is scattered, leaving those beautiful reds, yellows, and oranges. For a more detailed explanation of this phenomenon, check out Matt McIrvin's friendly and informative personal page.
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