Why the sky is blue and sunsets are red: it's the same light

The blue sky and the red sunset are the same effect, seen from two angles. Air molecules scatter short, blue wavelengths far more than long, red ones: about six times more, thanks to a steep 1/wavelength⁴ law.

Look away from the Sun and you see that sideways-scattered blue. Look at the setting Sun and its light has crossed about 38 times more air than at noon, so the blue has been stripped out and only orange and red get through. Drag the Sun down and watch it happen.