As you ponder the clear, blue sky, you may wonder just what produces such a beautiful color. The sky is not haphazardly blue; its color is no accident of nature. There is a real scientific phenomenon behind the color of the sky. The sky is blue because of a process called Rayleigh scattering. This process involves the scattering of light off of molecules in the atmosphere.
When light moves through the atmosphere, most of its wavelengths are able to simply pass right through. This is particularly true of its longer wavelengths. Shorter wavelengths, however, are less capable of passing through and are instead absorbed by the gas molecules in the atmosphere. It is important to understand that gas molecules absorb all colors of light; some are simply more readily absorbed than others. The sky is blue because blue light is more readily absorbed, while other wavelengths pass through easily.
To understand why the sky is blue, you must consider what happens when blue light is absorbed by gas molecules in the atmosphere. When blue light is absorbed, it is scattered in various directions, radiating throughout the sky. Since it is scattered so far and wide, the sky is blue no matter where you are positioned and where you choose to look. Sunlight consists of a full range of colors. However, the sky is blue because the efficiency with which blue light is scattered allows it to dominate what you see when you look up.
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If you pay attention to the color near the horizon, you’ll notice the color there appears to be paler than the sky right above you. This is due to the fact that light, when located farther away, must pass through more air before it gets near you. Some of this distant blue light gets scattered in other directions and less of it actually reaches your line of sight. As such, although you see blue sky near the horizon, it appears pale or white.
Although the sky is blue from your position on the ground, it actually looks black from space or on the moon. Since there is no atmosphere in space, the light from the sun is not scattered and colored light doesn’t reach your eyes. Without our atmosphere, we would look up to see a black sky. Even a slightly thinner atmosphere would change our sky, making it appear a lighter blue.