Bortle scale of night-sky brightness
| Class | Description | NELM | Typical location |
|---|---|---|---|
| 1 | Excellent dark sky Milky Way bright with structure; Zodiacal light, Gegenschein, and airglow visible. M33 naked-eye direct vision. Andromeda obvious. | 7.6-8.0 | Remote deserts, mountain peaks far from cities (Atacama, Mauna Kea outskirts, parts of the Sahara). |
| 2 | Typical truly dark site Airglow visible near horizon. Milky Way structure clear. M33 averted vision easy. Light domes from cities visible on horizon. | 7.1-7.5 | National parks far from population (Big Bend Texas, Tasmania, parts of Iceland). |
| 3 | Rural sky Milky Way obvious with some structure. Light pollution domes evident near horizon. M31 obvious; M33 visible with averted vision. | 6.6-7.0 | Rural communities, light pollution near horizon from nearest town. |
| 4 | Rural / suburban transition Milky Way visible overhead but no detail. Light domes extend 45° up. Clouds bright against dark patches of sky. | 6.1-6.5 | Edge of suburbia, far from urban core. |
| 5 | Suburban sky Milky Way barely visible overhead. Light domes blanket the horizon. Clouds clearly brighter than sky. | 5.6-6.0 | Typical suburban housing. |
| 6 | Bright suburban sky Milky Way not visible. Sky has visible gray cast. M31 only with averted vision under good conditions. | 5.1-5.5 | Dense suburbs. Most US/EU suburban areas. |
| 7 | Suburban / urban transition Sky bright gray-white. Only the brightest deep-sky objects (M31, M42) visible in binoculars. | 4.6-5.0 | Outer urban areas, very large suburbs. |
| 8 | City sky Sky glows whitish-orange. You can read by it. Only Moon, planets and brightest 50 stars visible. | 4.1-4.5 | City centers of most large cities. |
| 9 | Inner-city sky Sky brightly lit. Only the Moon, planets, and brightest few dozen stars visible. No constellations as we know them. | < 4.0 | Manhattan, central Tokyo, central London. |
NELM and what it means
NELM (naked-eye limiting magnitude) is the magnitude of the faintest star a typical observer can see directly overhead, with dark-adapted vision, away from any glare. It is the simplest single number that describes how dark your sky is.
Finding a dark site
The most-used tool is the world light pollution atlas at lightpollutionmap.info, which colors every region from white (Bortle 9) through yellow, green, blue and black (Bortle 1). For deep-sky observation, aim for Bortle 4 or darker. Driving 1-2 hours from most US/EU cities reaches Bortle 4-5; reaching Bortle 1-2 usually requires a multi-hour trip or a national park stay.
What changes with class
Bright objects like the Moon, planets and double stars look essentially the same from Bortle 1 and Bortle 9. The difference is everything else: nebulae, galaxies, the Milky Way structure, the count of stars in any given field. Andromeda is naked-eye obvious from Bortle 4 and below; in Bortle 7+ you need binoculars to find it at all.