The Bortle dark-sky scale, explained
The Bortle scale rates night-sky darkness from Class 1 (a truly dark sky where the Milky Way casts shadows) to Class 9 (an inner-city sky where only the Moon, planets, and brightest stars show). This page explains each class — what you can actually see, the approximate naked-eye limiting magnitude, and how it maps to the VIIRS satellite radiance we measure for every town.
What the Bortle scale measures
The Bortle scale, introduced by amateur astronomer John Bortle in 2001, rates how dark a night sky is on a nine-step ladder — from Class 1, a truly dark wilderness sky where the Milky Way casts faint shadows, to Class 9, an inner-city sky where only the Moon, planets, and a few bright stars survive the glow. It's the language stargazers use to answer "how good is this site, really?"
Each town on this site carries an estimated Bortle band, measured from VIIRS 2024 satellite night-lights at the town center and mapped to the classes below. It's an honest satellite proxy for planning — the true darkest skies are usually a short drive out of any town, and a survey-grade reading needs a sky-quality meter on the ground. See the methodology for exactly how the mapping works.
The scale, class by class
- Bortle 1-2 Truly dark sky naked-eye limit ≈ mag 7.6–8.01–2 3 4 5 6 7 8 9
The Milky Way is bright enough to cast faint shadows; the zodiacal light and airglow are obvious, and dozens of deep-sky objects show to the naked eye. This is about as dark as skies get in the U.S.
- Bortle 3 Rural sky naked-eye limit ≈ mag 6.6–7.01–2 3 4 5 6 7 8 9
The Milky Way shows real structure overhead, the Andromeda and Triangulum galaxies are naked-eye, and light domes from distant towns sit low on the horizon. Excellent for stargazing and astrophotography.
- Bortle 4 Rural / suburban transition naked-eye limit ≈ mag 6.1–6.51–2 3 4 5 6 7 8 9
The Milky Way is still visible overhead but washes out toward the horizon, and light domes are prominent. The brighter nebulae and star clusters are within easy reach of binoculars.
- Bortle 5 Suburban sky naked-eye limit ≈ mag 5.6–6.01–2 3 4 5 6 7 8 9
Only faint traces of the Milky Way survive, and only near the zenith on the clearest nights. The Andromeda Galaxy is a challenge with the naked eye; a telescope still shows plenty.
- Bortle 6 Bright suburban sky naked-eye limit ≈ mag 5.1–5.51–2 3 4 5 6 7 8 9
The Milky Way is effectively invisible and the sky has a grayish glow near the horizon. Bright star clusters and the Moon and planets are the reliable targets; galaxies need a telescope.
- Bortle 7 Suburban / urban transition naked-eye limit ≈ mag 4.6–5.01–2 3 4 5 6 7 8 9
The whole sky glows grayish-white. Only the brighter stars, the Moon, the planets, and a handful of the showiest clusters cut through; deep-sky observing is difficult.
- Bortle 8 City sky naked-eye limit ≈ mag 4.1–4.51–2 3 4 5 6 7 8 9
The sky is bright enough to read by. Only the Moon, the planets, and the few brightest stars and clusters are visible — the constellations lose most of their fainter stars.
- Bortle 9 Inner-city sky naked-eye limit ≈ mag under 4.11–2 3 4 5 6 7 8 9
Only the Moon, the bright planets, and a scattering of the very brightest stars pierce the glow. For real stargazing you'll want to drive to a darker site.
Using it to plan a night
Darkness is only half the picture. Even a Bortle 2 site is useless under thick cloud or a bright, high Moon — so pair a town's rating with tonight's live conditions (the "tonight" panel on every town page) and the Moon phase. For deep-sky objects and meteor showers, aim for a dark site on a night near the new Moon with clear skies.