Understanding the Bortle Scale: A Guide to Sky Darkness

The Bortle Dark-Sky Scale is a nine-level system used by astronomers to measure the night sky’s brightness. Developed by John E. Bortle in 2001, it helps stargazers understand how much light pollution will interfere with their observations.

Class	Type	Sky Quality (SQM)	What You Can See
1	Excellent Dark Sky	21.8 – 22.0	The Milky Way casts shadows; M33 is a naked-eye object.
2	Truly Dark Site	21.6 – 21.8	Milky Way is highly structured; airglow is visible near the horizon.
3	Rural Sky	21.3 – 21.6	Some light pollution at the horizon; Milky Way still appears complex.
4	Rural/Suburban	20.4 – 21.3	Light domes visible; Milky Way lacks fine detail.
5	Suburban Sky	19.3 – 20.4	Milky Way is weak or invisible near the horizon; clouds are bright.
6	Bright Suburban	18.5 – 19.3	Zodiacal light is invisible; Milky Way only visible at the zenith.
7	Suburban/Urban	18.0 – 18.5	Milky Way is totally invisible; the sky has a grayish-white hue.
8	City Sky	17.5 – 18.0	Stars forming constellations are weak or invisible; you can read a map.
9	Inner-City Sky	< 17.5	Brilliantly lit sky; only the Moon and planets are visible.

Why SQM Matters

While the Bortle Scale is based on what the human eye sees, Sky Quality Meter (SQM) readings provide a scientific measurement in “magnitudes per square arcsecond.” A higher SQM number (like 21.9) means a darker sky, while a lower number (like 18.0) indicates heavy light pollution.