Nib asked for
Why Sunsets Look Red
At sunset, sunlight crosses so much extra air that most of the blue is scattered out of the direct beam, leaving mostly red and orange to reach your eyes.
4 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1noon sun
Start with what you already know: air scatters blue light sideways far more than red. At midday, the sun is high and its light takes a fairly short path through the atmosphere. Only a modest share of the blue gets knocked out of the beam, so the sun looks nearly white, and the scattered blue fills the sky around it.
At midday, the short path through air removes only some of the blue.
a common mix-upThe sun itself changes color at sunset.
2long path
At sunset the sun sits near the horizon, so its light reaches you at a shallow angle. Instead of dropping straight down through the atmosphere, it has to travel a long slanted path, passing through many times more air before it gets to you.
Low sun means a much longer path through the atmosphere.
a common mix-upThe sun looks red because it is farther away at sunset.
Does the sun get farther away at sunset, or does the light just travel through more air?
Show the answer
The light travels through more air; the sun's distance is essentially unchanged.
3blue peels off
Along that long path, scattering happens again and again. Blue is scattered most strongly, so each stretch of air peels more blue out of the direct beam and sends it off in other directions. Green and yellow lose a good share too, while red, which scatters least, mostly survives.
Along a long path, blue is scattered out of the beam first and fastest.
a common mix-upRed light is added or created at sunset.
4blue subtracted
What reaches your eyes is the beam that is left: white light with much of its blue and green subtracted. What remains is weighted toward orange and red, so the sun and the sky near it glow in those colors. Nothing new is made; the colors were removed from the other end of the spectrum.
Sunset red is white light minus the blue that scattering took away.
If the atmosphere were twice as thick, what would a sunset look like?
Show the answer
Redder and dimmer, since even more blue and green would be scattered out of the beam.
A blue sky and a red sunset are the same effect: air scatters blue first, so you see blue in the sky overhead and red in the sun that's left.