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.
| Cause | Surface brightness (S10) | Percentage |
|---|---|---|
| Airglow | 145 | 65 |
| Zodiacal light | 60 | 27 |
| Scattered starlight | ~15 | 7 |
(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.
Read more about this topic: Sky Brightness
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