Why Is the Sky Blue but the Sunset Orange?

Lie on your back in a park on a clear afternoon and the sky above you is a deep, even blue, from one edge of the view to the other. Stay until evening and the same sky, with the same air in it, turns gold, then orange, then pink near the horizon. Nothing has been painted and nothing has been switched. The sky has simply changed which colours it sends your way.

The story starts with sunlight, which looks white but is really every colour of the rainbow travelling together. Each colour travels as a wave of a different length. Red light has the longest waves we can see, and blue and violet have the shortest. On its way down through the atmosphere, that mixed beam meets the countless tiny gas molecules that make up the air.

Those molecules are far too small to block light, but they do knock it off course, and they are much better at knocking the short waves than the long ones. Blue light is bounced around in every direction, scattered and scattered again, until it seems to come from the whole sky at once. Wherever you look, away from the sun, some of that scattered blue is heading for your eyes. Red and yellow light mostly carry straight on.

At sunset, the light has a much longer journey. With the sun low on the horizon, its light crosses far more air before it reaches you, and by the time it arrives most of the blue has been scattered away to the sides. What is left is the long-wave light: the reds, oranges and yellows. Dust and tiny particles in the air can deepen the effect, which is why some evenings put on a far brighter show than others.

There is a puzzle hidden in all this. Violet waves are even shorter than blue ones and are scattered even more, so why is the sky not violet? Three things work together. The sun sends out less violet than blue in the first place. Violet is also scattered out of the sunlight faster on its way down. And our eyes are simply more sensitive to blue, so the mixture of colours coming from the sky looks blue to us, even though some violet is in there too.

How do we know it works like this? The effect is called Rayleigh scattering, after Lord Rayleigh, the physicist who described it. Its key rule is that the scattering gets much stronger as the waves get shorter, and that single rule explains both the blue afternoon and the red evening. It also predicts a different result on a planet with different air, and that is exactly what we see.

On Mars, the thin atmosphere is full of fine dust, which handles light differently from the gas molecules of our air. The result is a daytime sky with a butterscotch tint, and sunsets with a cool blue glow around the sun, the reverse of ours. So next time you lie on the grass under a blue sky, you are watching countless air molecules sort sunlight by colour, while on the planet next door the same sunlight gets sorted the other way round.