Why is the sky blue (and sunsets red)?
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Why is the sky blue (and sunsets red)?
The short answer
The sky is blue because sunlight is a mix of all colors, and the blue light bounces off the tiny bits of air much more than red light does. When you look at the sky away from the sun, you catch that bounced-around blue coming at you from every direction.
How it works
Sunlight looks white but is really all the colors mixed together. When it travels through the air, it hits the gas molecules that make up the air, and the bluer colors get bounced off in all directions far more easily than red. By day you look away from the sun and catch that scattered blue filling the whole sky. At sunset the sun is low, so its light has to cross a much longer path of air, the blue gets bounced away before it reaches you, and mostly red survives the trip, which is why the sun and sky turn orange and red.
What people get wrong
Many people think the blue sky and red sunsets are two separate, unrelated things, or that the sky is blue because it reflects the blue ocean. Really it is one single effect: blue light bounces off the air easily. A blue daytime sky and a red sunset are the same bouncing seen two ways, you catch the scattered blue by day and see what is left after the blue has bounced away at sunset.
The catch
If blue bounces so well, you might expect a purple sky, because violet light actually bounces even more than blue. But the sun sends out less violet, and our eyes are far more sensitive to blue, so blue wins. And this only works where there is air to bounce off, on the airless Moon there is nothing to scatter the light, so even in broad daylight its sky stays black.
Questions kids ask
Why isn't the sky purple if purple bounces even more?
Violet light does scatter more than blue, but the sun puts out less violet light and our eyes are much better at detecting blue, so the sky reads as blue to us.
Why is the sunset red instead of blue?
At sunset the sun is low, so its light crosses far more air to reach you. The blue has all bounced away along that long path, leaving mostly red and orange to come straight through.
Why is the sky on the Moon black even in daytime?
The Moon has almost no air, so there are no molecules to bounce sunlight around. With nothing to scatter the light, the sky stays black even while the sun is shining.
Is the sky blue because it reflects the ocean?
No. The sky is blue because air scatters blue sunlight in every direction, and this happens over deserts and land too. The ocean often looks blue partly because it reflects that blue sky back.
For grown-ups
This is Rayleigh scattering: air molecules scatter light in proportion to 1/wavelength to the fourth power, so short (blue) wavelengths scatter far more strongly than long (red) ones. By day you see this scattered short-wavelength light arriving from all over the sky; near sunset the path length through the atmosphere is much longer, so blue is scattered out of the direct beam and the transmitted light is dominated by red and orange. The sky appears blue rather than violet because sunlight contains less violet, the eye's cones are less sensitive to violet, and some violet is absorbed in the upper atmosphere.