Bortle refers to a scale introduced by John E. Bortle in 2001, measuring the level of light pollution in the sky from 1 (a truly dark-sky site) to 9 (the sky as seen from a very big city). Light pollution has a significant impact on what you can observe or photograph, because the stronger it is, the less objects you can see in the sky.
The Bortle Classes
- Class 1 → Excellent dark-sky site
- Class 2 → Typical truly dark site
- Class 3 → Rural sky
- Class 4 → Rural/suburban transition
- Class 5 → Suburban sky
- Class 6 → Bright suburban sky
- Class 7 → Suburban/urban transition
- Class 8 → City sky
- Class 9 → Inner-city sky
Light pollution maps make use of this colour-coded scale to display how light polluted a location is.