Sky quality · the Bortle scale
How much sky can you spare?
Every night is graded by one simple dial: how much light the sky holds. Slide across the nine Bortle classes and watch the 32 objects in our catalog fade one by one as the glow takes over — then slide back to class 1 and see the sky as dark places do.
Test your sky
Reading the scale
The Bortle scale runs from 1 — a pristine dark sky to 9 — the inner city. It is felt, not measured: you judge it by what the naked eye can do. Is the Milky Way casting shadows, or is it invisible? Can you count the Pleiades, or is it a smudge? Does Andromeda glow at all? Each step below drops the naked-eye limiting magnitude by half a star — and stars, galaxies, and nebulae pay that cost directly.
Two benchmarks make the scale concrete. If the glow from your nearest town sits on the horizon as a colored haze, you are at class 5 or worse. If M33 — the faintest galaxy in our Local Group, at 5.7 magnitudes — is visible, you are at class 4 or darker. And under a class 1 or 2 sky, even M100, the faintest entry in our catalog at 9.3, answers clearly in 10×50 binoculars — something that is simply impossible past class 6.