Sunlight is secretly every colour mixed together. Blue light is the bounciest, so it ricochets off the air in every direction until the whole sky glows blue.
Most people think the sky is blue because it reflects the blue of the sea. In fact the sky has no sea to copy; air molecules simply scatter short blue wavelengths far more than red, so blue arrives from every direction.
What's actually happening
Start with the surprise: sunlight is not yellow. It is every colour at once, blended so thoroughly that we call the blend "white". You only see the ingredients when something pulls them apart: a prism, a raindrop, or, as it turns out, the entire atmosphere.
Air molecules are thousands of times smaller than a wave of visible light, and tiny obstacles like that are picky about what they deflect. A light wave passing a molecule shakes its electrons, and the molecule re-radiates the wave in a new direction. Short, choppy blue waves shake those electrons far more violently than long, lazy red ones, about five to six times more. So as sunlight crosses the sky, blue light keeps getting knocked off course, bouncing molecule to molecule, until it finally reaches your eye from every direction at once. Look anywhere but at the sun and what you see is that rerouted blue.
A fair follow-up: violet scatters even more than blue, so why isn't the sky violet? Two reasons. The sun emits less violet than blue in the first place, and your eyes are much less sensitive to it. The sky is technically violet-ish; your hardware votes blue.
The whole sky glows blue because air scatters short wavelengths roughly five times more than long ones — the colour is light rerouted, not reflected.
- 1Fill a tall glass with water and stir in a few drops of milk: that is your atmosphere.
- 2Shine a phone torch through it from the side, in a dark room. Looking at the glass from the front, the liquid glows faintly blue: short wavelengths scattering toward you.
- 3Now look at the torch through the glass. The beam itself looks orange; the blue has been scattered out of it, exactly like a sunset.
Common questions
Two reasons. The sun emits less violet than blue to begin with, and human eyes are far less sensitive to violet. The sky is technically violet-ish, but your eyes vote blue.
Larger particles such as dust and water droplets scatter all colours roughly equally, rather than favouring blue. That even scattering from pollution and humidity turns the sky a milky white instead of deep blue.
The Moon has no atmosphere, so there are no molecules to scatter sunlight. With no scattering there is no blue, which is why astronauts stood in full sunlight under a pitch-black sky.