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The blue color of our sky is caused by scattering of light in the atmosphere.
We owe some of our most spectacular atmospheric phenomena to various types of scattering: the blue of the sky, the red of the sunset, the white of clouds and, that epitome of rare occurrences, the blue moon. Most blue and green bird-feather colors involve scattering, as do many animal and some vegetable blues.
Why isn't the sky black?
Our sky is colored because our atmosphere makes blue light when sunlight passes through it. This phenomenon is called "scattering." It is similar to when sunbeams reveal themselves in the presence of dust, most spectacularly in cathedrals. The atmosphere causes the sky to be colored.
Moonrise with blue scattering caused by earth's atmosphere, photographed from the orbit of Space Shuttle Discovery in 1999.
Earthrise with black sky since there is no atmosphere, photographed from the orbit of Apollo 11 in 1969.
If there were no atmosphere, the sky would appear black, like the lunar sky in Apollo pictures taken from the moon. But even a black sky has some lightness... Even at night, the sky always has a faint color, called "skyglow" by astronomers. Atmospheres also cause skyglow from faint airglow in the upper atmosphere (a permanent, low-grade aurora), and starlight scattered in the atmosphere. Even if there were no atmosphere, faint skyglow is caused by sunlight reflected off interplanetary dust (zodiacal light), and background light from faint, unresolved stars and nebulosity.
