# Bortle scale

Also called: Bortle class.

A 1–9 scale of night-sky darkness, from 1 (pristine, the Milky Way casts shadows) to 9 (inner city).

John Bortle published the scale in 2001 to help observers describe their skies. Class 1–2 skies show the zodiacal light, airglow and the Milky Way in rich detail; class 4 is rural-to-suburban, where the Milky Way still shows but is washed out near the horizon; class 6–7 suburbs show only its brightest parts, if any; class 8–9 city skies show a few dozen stars.

**In AstroPilot:** Your location's Bortle class (B1–B9) appears next to its name; every visibility rating depends on it.

**Related:** [Light pollution](https://astropilot.space/glossary/light-pollution.md), [SQM](https://astropilot.space/glossary/sqm.md), [Limiting magnitude](https://astropilot.space/glossary/limiting-magnitude.md), [Milky Way](https://astropilot.space/glossary/milky-way.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/bortle-scale