R and I sky brightnesses vary randomly by several tenths of a mag with variations in the OH airglow. The V and R sky brightness include a contribution of about 0.1 mag due to NaD light pollution. Light pollution is negligible in other bands, and it is decreasing. The sky is 0.4 mag brighter at solar maximum. Recent minima were in 1986.8, 1996.5.
Variation is approximately sinusoidal with time.
The sky is 0.3 mag brighter at airmass 1.5. Stars fainter than apparent magnitude 20 contribute negligibly to the total. For further information on optical sky brightness, see the ING sky-brightness web page .