Why the Sky Is Blue
Ask a young child to paint the sky, and she will reach for blue. But sunlight itself is not blue. Light from the sun looks white because it contains a mix of all the colors of the rainbow. You can see that mixture pulled apart when sunlight passes through a glass prism, or through raindrops after a storm. So where does the blue come from?
The answer lies in the air. Earth's atmosphere is made mostly of nitrogen and oxygen molecules, far too small to see. When sunlight enters the atmosphere, it runs into these molecules, and some of the light is scattered, or bounced off in new directions. But not all colors are scattered equally. Light travels in waves, and each color has a different wavelength. Red light has long waves; blue and violet light have much shorter ones. Tiny molecules scatter short waves far more strongly than long ones.
As a result, blue light is scattered all across the sky. Wherever you look overhead, some of that scattered blue light is heading toward your eyes, so the whole sky seems to glow blue. This effect is called Rayleigh scattering, after the British scientist Lord Rayleigh, who described it mathematically in the 1870s.
If short waves scatter most, why isn't the sky violet? Several reasons work together. Sunlight contains less violet than blue to begin with, some violet light is absorbed high in the atmosphere, and our eyes are much more sensitive to blue than to violet. The mix that reaches us looks sky blue.
Scattering also explains sunsets. When the sun is low on the horizon, its light must travel through much more air to reach you than it does at noon. Along the way, most of the blue is scattered out of the direct beam, leaving mainly the longer red, orange and yellow wavelengths. That is why the setting sun, and the clouds near it, can blaze with warm colors.
Other worlds offer a useful comparison. The Moon has almost no atmosphere, so there is almost nothing to scatter sunlight; even in daytime, the sky above the Moon looks black. On Mars, the thin air is full of fine reddish dust, which gives the daytime sky a butterscotch tint, and Martian sunsets, photographed by robotic rovers, actually look bluish. The color of a sky, it turns out, is a kind of report on what the air is made of.