Why Sunsets Are Red

Daytime sunlight is white or yellow, yet when the sun goes down the sky turns red.

The sun itself does not change. So why does its color shift?

Whether by the sea, on a mountain, or in a city, the same thing happens every evening.

Maybe the sun itself turns red near sunset.

Like a fire cooling down and shifting color.

The color of sunlight does not change. What changes is the distance that light travels through the atmosphere.

Sunlight is white light, a mixture of every wavelength from red to violet. As it passes through Earth’s atmosphere, air molecules scatter shorter wavelengths more strongly than longer ones. This is called [Rayleigh scattering].

Short wavelength (blue) = strong scattering. Long wavelength (red) = weak scattering.

During the day, the sun is high overhead, and light takes a short path through the atmosphere. Over that short path, blue scatters and spreads across the whole sky. That is why the daytime sky looks blue.

Near sunset, the sun sits low on the horizon. Light enters the atmosphere at a steep angle, so the path through air becomes far longer (tens of times longer than at noon). Along that long path, blue is scattered away almost entirely. What survives is the weakly-scattered red and orange. Only that color reaches our eyes.

The sun itself is unchanged. Sunset is the result of which colors got filtered out along the way.

On the left is the light path at different sun angles. Lower the sun-altitude slider and it shifts from noon (overhead, short path) → afternoon → sunset (near the horizon, a path tens of times longer). Use the path toggle to emphasize how far the light travels through the air.

On the right is the scattering intensity by wavelength. Across the red-to-violet spectrum, shorter wavelengths scatter more strongly, and as the path grows longer the blue scatters away first until only red remains. Use the planet toggle to compare Earth (blue day, red sunset) with Mars (pink day, blue sunset, the opposite).

Use the sun-altitude slider to move from noon (short path, blue sky) to sunset (a path tens of times longer, red sky), use the wavelength toggle to see shorter wavelengths scatter first, and use the planet toggle to compare Earth with Mars (opposite colors).

Haze and dustOn hazy or polluted days, more particles scatter more wavelengths, so sunsets become deeper and redder.

Wildfire smokeMakes sunsets even more intensely red for the same reason.

Martian sunsetsThe opposite of Earth. Mars dust scatters red and lets blue through, so the Martian sky is pinkish at noon and blue at sunset.

A perfectly ordinary sunset is actually light that has traveled tens of times farther through air than usual.