Bortle scale

The Bortle dark-sky scale is a nine-level numeric scale developed by amateur astronomer John E. Bortle to measure the night sky's brightness and the observability of celestial objects. Published in Sky & Telescope magazine in February 2001, it helps skywatchers evaluate sites by characterizing the interference from light pollution, ranging from Class 1 (darkest skies) to Class 9 (inner-city skies). The scale describes visibility of celestial objects and light sources, corresponding to naked-eye limiting magnitude and sky quality meter measurements.