Context: This article explains reasons for sky glow even after sunset.
Why sky glows even after sunset:
- The sun may have set, but it hasn't yet from the perspective of the upper atmosphere.
- The sun still shines on the atmosphere, but at a sharp angle beneath the horizon. During this time, we see the sky glow because molecules in the air scatter this sunlight in different directions, including towards us. This is also why the evening sky appears red at the horizon.
- Both this effect and the sky appearing blue during the daytime are the results of Rayleigh scattering (the scattering of sunlight by particles in the air that are much smaller than the light's wavelength).
- As the sun continues to set, a smaller amount of sunlight strikes the upper atmosphere. Nightfall truly begins when the sun is at least around 18 degrees below the horizon.
- The arrival of daylight happens through a similar process, in reverse. When the sun is at least 18 degrees below the horizon in the east, the sky starts to turn reddish again near the horizon. When the sun's position crosses the horizon (as seen by the observer), daylight breaks out, and the day begins.
- The sky can appear to glow even after the sun has dipped 18 degrees below the horizon due to other sources of light. These include the scattering of starlight by the atmosphere and — increasingly — light pollution.