The Sky Quality Meter, a small device that measures the brightness of the sky background in magnitudes per square arcsecond. The higher the number, the darker the sky: about 21.7 to 22 under a perfect sky, 19 in the suburbs, 17 or less in a city. It allows sites to be compared objectively.
Related terms
- Bortle scale
A 1 to 9 scale describing the quality of a night sky, proposed by John Bortle in 2001. In class 1 the sky is so dark that the Milky Way looks textured and full of detail; class 9 is a city centre where only a few stars show. Typical countryside in France or the UK is class 4 or 5.
- Surface brightness
The brightness of an extended object spread over its area, in magnitudes per square arcsecond. A large magnitude 9 galaxy can be harder to see than a small magnitude 11 nebula. It is what matters when judging how visible a deep-sky object will be.
- Light pollution
Artificial lighting that brightens the night sky and drowns faint stars, nebulae and the Milky Way. A dome of light spreads for tens of kilometres around towns. It also harms wildlife and health; some towns now switch off street lights in the middle of the night.
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