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The surface brightness of the night sky, denoted as
Sa, is given in the commonly used astronomical units of magnitudes per square arcsecond (mag arcsec−2). It is a derivative of the magnitude scale (mag) defining the visual impact of the star's brightness as a point light source. Magnitude scale is a logarithmic, relative and reverse scale, in which a star of magnitude 0 is 100 times brighter than a star of magnitude 5. The mag arcsec−2 scale determines the surface brightness of diffuse astronomical objects, such as nebulae, galaxies, comets, or just a background sky. The derived SI unit of luminance, or perceived brightness, is the candela per square metre (cd m−2), which is a measure of light emitted per unit area. It is possible to approximately convert mag arcsec−2 into cd m−2 using the following formula (Crawford 1997; Ściężor et al. 2010b; Kyba et al. 2011):
[cdm−2]=10.8×104×10[−0.4(magarcsec−2)].
(1)
Because of the very low surface brightness of the night sky, the commonly used unit is millicandela per square metre (mcd m−2). In order to maintain compliance with other publications that deal with this problem, as well as with readings from SQMs, I mainly use the scale (mag arcsec−2), also giving the appropriate values in the scale (mcd m−2).