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.
Related terms
- SQM
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.
- 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.
- Limiting magnitude
The brightness of the faintest stars you can see. With the naked eye it ranges from 3 or 4 in a city to 6.5 or more under a very dark sky. A telescope of aperture D (in mm) pushes it further by about 5 log₁₀(D ÷ 7) magnitudes: a 200 mm telescope reaches roughly magnitude 14.
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