On the significance of the measure
magnitudes per square arcsecond:
The unit is most often used to describe the surface brightness of celestial objects like galaxies and nebulae. An overall or integrated visual magnitude is usual given for such objects. The Sculptor Galaxy NGC 253 has an overall visual magnitude of 7.1, being compared to a star of visual magnitude 7.1. However, for NGC 253 this light is spread over a large area and each square arcsecond of the surface of NGC 253 is a lot less bright then the light of the mag 7.1 star spread over a square arcsecond. In fact the average light given off by each square arcsecond of NGC 253 is equivalent to a magnitude 21.4 star. This is known as its surface brightness. Thus NGC 253 has a surface brightness of 21.4 mags per sq arcsecond. Alternatively, surface brightness can be given in mags per sq arcminute. For NGC 253, this is 12.5 (just take off 8.9).
Now, the skyglow can be measured as the surface brightness of the sky i.e. how bright each square arcsecond of the sky appears.
This is what a Sky Quality Meter measures.
Roughly speaking, the surface brightness of the sky is ...
17 centre of a major city
18 high density suburb around city centre
19 outer suburbs with lower density population
20 low density fringe suburbs, uninhabited areas adjacent
21 rural area with a town in the distance
22 isolated location. Darkest sky possible.
In my mountain suburb of Glenbrook, the skyglow is 20.2.
A half Moon (first or last quarter) increases the skyglow to 19.7.
The Full Moon brightens the sky to 18.
According to Tony Flanders, an object is detectable if its surface brightness is up to 3 magnitudes fainter than the skyglow.
However, the ability to detect an object depends on how sharp its edges are. If it fades away gradually towards the edges, it is harder to detect.
See ...
http://mysite.verizon.net/vze55p46/id18.html
Why can you see an object that is actually fainter than the skyglow?
The object's surface brightness and the surface brightness of the sky have an additive effect. However, there is a level at which a faint object's surface glow does not add enough to the skyglow to be distinguishable from it.
Even at the darkest possible site, there is a limit of detectability for any Earth-based telescope.
Regards, Rob