Sky Brightness - Relative Contributions

Relative Contributions

The following table gives the relative and absolute contributions to night sky brightness at zenith on a perfectly dark night at middle latitudes without moonlight and in the absence of any light pollution.

(The S₁₀ unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec-2).

The total sky brightness in zenith is therefore ~220 S₁₀ or 21.9 mag/arcsec² in the V-band. Note that the contributions from Airglow and Zodiacal light vary with the time of year, the solar cycle, and the observer's latitude roughly as follows:

where S is the solar 10.7 cm flux in MJy, and various sinusoidally between 0.8 and 2.0 with the 11-year solar cycle, yielding an upper contribution of ~270 S₁₀ at solar maximum.

The intensity of zodiacal light depends mainly on the observer's ecliptic latitude, which is geographical latitude +- 23.5° (the inclination of the ecliptic plane), and varies as

where β is the ecliptic latitude and is smaller than 60°, in other cases the contribution is that given in the table.

In extreme cases natural zenith sky brightness can be as high as ~21.0 mag/arcsec², roughly twice as bright as nominal conditions.