optics - Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?

Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?

Answer

The keywords here are Rayleigh scattering. See also diffuse sky radiation.

But much more simply, it has to do with the way that sunlight interacts with air molecules. Blue light is scattered more than red light, so during the day when we look at parts of the sky that are away from the sun, we see more blue than red. During sunset or sunrise, most of the light from the sun comes towards the earth at a sharp angle, so now the blue light is mostly scattered away, and we see mostly red light.