The brightness of the night sky in the absence of twilight and moonlight, measured in areas of sky devoid of discernible stars. It arises from the airglow, and from the zodiacal light and gegenschein, all of which vary with solar activity. There is a further component from the many stars and galaxies in the sky which are too faint to be seen individually but produce a significant background brightness. Sky brightness is measured in magnitudes per square arc second. The best mountain-top sites have a zenith sky brightness in the V band of around 21.8 magnitudes per square arc second, while 18.1 is typical for light-polluted sites. A lower sky brightness allows the detection of fainter objects. Telescopes in orbit are also affected by sky brightness, but not so greatly.
Subjects: Astronomy and Astrophysics.