The Bortle scale: how dark is your sky?
The Bortle scale describes how bright the night sky is at a place, from class 1 (a pristine dark sky far from any city) to class 9 (an inner-city sky). It was proposed by the amateur astronomer John E. Bortle in 2001 and is now the usual way to say how much light pollution a site has.
The 9 classes at a glance
| Class | Sky | What you see | Faintest stars (naked eye) |
|---|---|---|---|
| 1 | Excellent dark sky | The Milky Way casts faint shadows; the zodiacal light is obvious | ≈ 7.6–8.0 |
| 2 | Typical truly dark site | Detailed Milky Way, clouds are black holes in the sky | ≈ 7.1–7.5 |
| 3 | Rural sky | Milky Way still rich; some glow on the horizon | ≈ 6.6–7.0 |
| 4 | Rural / suburban transition | Milky Way visible but without detail near the horizon | ≈ 6.1–6.5 |
| 5 | Suburban sky | Milky Way faint, washed out near the horizon | ≈ 5.6–6.0 |
| 6 | Bright suburban sky | Milky Way only overhead, if at all | ≈ 5.1–5.5 |
| 7 | Suburban / urban transition | Sky grey-orange, Milky Way invisible | ≈ 4.6–5.0 |
| 8 | City sky | Only bright constellations are recognisable | ≈ 4.1–4.5 |
| 9 | Inner-city sky | Moon, planets and a few bright stars | ≈ 4.0 at best |
What it means for observing
The Moon, planets and double stars are fine even from a city (classes 7–9). Bright star clusters need class 5–6 or darker. Galaxies, nebulae and the Milky Way really need class 4 or better — moving from class 7 to class 3 can show you ten times more stars.
How NightSkyCast estimates the class
We estimate the Bortle class for every point from NASA satellite images of night lights (VIIRS Black Marble): each lit area adds brightness that fades with distance. It is an estimate, not a measurement — for exact values astronomers use a sky quality meter (SQM).
How to find a darker sky
Open the sky map, turn on the light pollution layer and look for green and dark-blue areas 30–60 km from your town, away from the glow of big cities. Tap a spot to see its class and tonight’s forecast — a dark site is only useful on a clear, moonless night.