Why is the sky blue?

The sky is blue because air scatters blue light far more than red. Sunlight looks white, but it is really all the colours of the rainbow mixed together. When it enters the atmosphere, it bumps into the molecules of nitrogen and oxygen that make up the air, and they scatter it in every direction. Blue light travels in shorter, choppier waves than red light, and short waves get scattered much more. So wherever you look in the sky, away from the Sun, blue light is coming at you from all sides.

People puzzled over this for centuries. Around 1506, Leonardo da Vinci guessed that tiny particles in the air were scattering sunlight. In 1869 the physicist John Tyndall saw that light bouncing off very fine particles looked faintly blue, but he couldn't explain why blue won. Two years later, Lord Rayleigh cracked it with the maths: the amount of scattering rises steeply as the wavelength gets shorter, so blue is scattered much more than red. The effect still carries his name, Rayleigh scattering.

Your eyes finish the job. The scattered light is a mix that our eyes read as blue blended with white, which is why the sky looks pale blue rather than deep blue, and fades to almost white near the horizon, where the light has crossed much more air.

Change the air and you change the sky. On Mars, a thin atmosphere full of fine dust turns the daytime sky orange.