How dark is dark
The Bortle scale has nine steps, and each one changes what you see in your eyepiece.
Naming sky brightness, and what each step actually costs you.
Nine steps, real consequences
John Bortle published his scale in 2001 as a practical observer's tool, and it remains the most widely used framework for naming the darkness of a sky. Its nine classes run from the blackest rural sites — where zodiacal light casts visible shadows and your dark-adapted eye sees airglow ripple across the Milky Way — down to the washed-out suburban or urban ceiling where only the moon and the planets survive with any dignity.
The losses at each step are not subtle. Between a Class 1 sky and a Class 4 (a rural site with some suburban light dome on the horizon) you typically surrender a full magnitude of limiting magnitude: objects your eye can hold at Class 1 become ghosts you cannot confirm at Class 4 and miss entirely at Class 6. That single magnitude step matters because surface brightness — the property that actually decides whether you see a galaxy or a nebula — falls off sharply as background glow rises to meet it. A large, dim object suffers more than a compact, bright one; face-on spirals are among the first casualties.
Sky quality meters give Bortle's intuitive scale a numerical spine. They measure magnitudes per square arcsecond: a Class 1 site might return 22 or above; a Class 6 suburban sky sits around 19 to 20. Each drop of roughly one magnitude per square arcsecond in the background cuts the faintest visible surface brightness by the same amount. The arithmetic is unforgiving.
That single magnitude step matters because surface brightness — the property that actually decides whether you see a galaxy or a nebula — falls off sharply as background glow rises to meet it.
Light pollution is not uniform in character either. A high-pressure sodium dome on the horizon reddens and blurs in a particular way; LED street lighting, now common, spreads a broader, harder glow that filters help less readily. Dark adaptation — already slow and fragile — is harder to protect under any artificial sky.
The practical lesson is narrow: knowing your Bortle class tells you not just what you might see, but what to stop hunting for. Chasing a Class 2 object from a Class 5 site is not a question of skill or aperture. It is a question of photons, and the sky is keeping them.