Sky Brightness

Sky brightness is the visual perception of the sky and how it scatters and diffuses light, and the night sky is never completely dark because of several natural and artificial sources of illumination. During the day, direct sunlight scattering dominates it completely, while during twilight the picture is more complex because it depends on which atmospheric layers the low sun still lights from below the horizon. At night, natural sky brightness comes mostly from airglow, contributing about 65 percent, zodiacal light from interplanetary dust at about 27 percent, and scattered starlight at about 7 percent, while light pollution is an ever-growing source in urban areas, increasing at about 9.6 percent a year as of 2023 and making city skies 5 to 50 times brighter than a natural sky.