A ruler for the night
What is the Bortle scale?
The Bortle scale is a nine-level scale for describing how dark a night sky really is — not with instruments, but with your own eyes. It was created by amateur astronomer John E. Bortle and published in Sky & Telescope magazine in 2001, drawing on nearly five decades of observing from sites across North America.
Its genius is that it describes what you can actually see. Class 1 is a sky so dark the Milky Way casts shadows and the zodiacal light — sunlight scattered off dust between the planets — is unmistakable. Class 9 is an inner-city sky where the whole heavens glow, most constellations vanish, and only the Moon, the bright planets and a handful of stars survive. Every level in between is defined by landmarks any observer can check: Is the Milky Way visible? Is it detailed? Can you spot the Andromeda Galaxy or the Triangulum Galaxy with the naked eye? Do clouds look darker than the sky, or brighter?
Because it's anchored in things people can see, the scale is a wonderfully human way to talk about light pollution — and it's the language we use across Project Nightwatch to describe what our sensors measure.
Dark to bright
The nine classes, explained
Each class lists the sky-brightness range our sensors associate with it (in mag/arcsec² — remember, higher = darker) and the faintest stars typically visible to the naked eye (the limiting magnitude). Tap a class on the network dashboard to jump straight to it here.
From sensor to scale
How Project Nightwatch estimates the class
The Bortle scale was written for human eyes, so our Unihedron Sky Quality Meters can't measure it directly. Instead, each station records the brightness of the sky at the zenith in mag/arcsec², and we convert the night's best moon-free reading into an approximate Bortle class using widely accepted thresholds:
| Class | Name | Sky brightness | Naked-eye limit |
|---|
The conversion is deliberately approximate. Real-world Bortle classes blend sky brightness with transparency, altitude and an observer's own eyesight, so think of each badge on this site as a well-informed estimate of the class of sky you'd experience at that station on that night.
Following standard practice in light-pollution research, readings above 22.0 mag/arcsec² are excluded as likely dark-cloud artefacts. Class 9 is included for completeness — our network's darkest-to-brightest range spans Classes 1–8, and skies brighter than Class 8's threshold are all reported as Class 8.