Class |
Title |
NELM |
Approx. SQM[3] mag/arcsec2 |
Description |
1 |
Excellent dark-sky site |
7.6–8.0 |
21.7–22.0 |
|
2 |
Typical truly dark site |
7.1–7.5 |
21.5–21.7 |
- the zodiacal light is distinctly yellowish and bright enough to cast shadows at dusk and dawn
- airglow may be weakly visible near horizon
- clouds are only visible as dark holes against the sky
- surroundings are barely visible silhouetted against the sky
- the summer Milky Way is highly structured
- many Messier objects and globular clusters are naked-eye objects
- M33 is easily seen with naked eye
- limiting magnitude with 12.5" reflector is 16.5
|
3 |
Rural sky |
6.6–7.0 |
21.3–21.5 |
- the zodiacal light is striking in spring and autumn, and color is still visible
- some light pollution evident at the horizon
- clouds are illuminated near the horizon, dark overhead
- nearer surroundings are vaguely visible
- the summer Milky Way still appears complex
- M15, M4, M5, and M22 are naked-eye objects
- M33 is easily visible with averted vision
- limiting magnitude with 12.5" reflector is 16
|
4 |
Rural/suburban transition |
6.1–6.5 |
20.4–21.3 |
- the zodiacal light is still visible, but does not extend halfway to the zenith at dusk or dawn
- light pollution domes visible in several directions
- clouds are illuminated in the directions of the light sources, dark overhead
- surroundings are clearly visible, even at a distance
- the Milky Way well above the horizon is still impressive, but lacks detail
- M33 is a difficult averted vision object, only visible when high in the sky
- limiting magnitude with 12.5" reflector is 15.5
|
5 |
Suburban sky |
5.6–6.0 |
19.1–20.4 |
- only hints of zodiacal light are seen on the best nights in autumn and spring
- light pollution is visible in most, if not all, directions
- clouds are noticeably brighter than the sky
- the Milky Way is very weak or invisible near the horizon, and looks washed out overhead
- when it is half moon (first/last quarter) in a dark location the sky appears like this, but with the difference that the sky appears dark blue
- limiting magnitude with 12.5" reflector is 15
|
6 |
Bright suburban sky |
5.1–5.5 |
18.0–19.1 |
- the zodiacal light is invisible
- light pollution makes the sky within 35° of the horizon glow grayish white
- clouds anywhere in the sky appear fairly bright
- even high clouds (cirrus) appear brighter than the sky background
- surroundings are easily visible
- the Milky Way is only visible near the zenith
- M33 is not visible, M31 is modestly apparent
- limiting magnitude with 12.5" reflector is 14.5
|
7 |
Suburban/urban transition |
4.6–5.0 |
- light pollution makes the entire sky light gray
- strong light sources are evident in all directions
- clouds are brightly lit
- the Milky Way is invisible
- M31 and M44 may be glimpsed, but with no detail
- through a telescope, the brightest Messier objects are pale ghosts of their true selves
- when it is full moon in a dark location the sky appears like this, but with the difference that the sky appears blue
- limiting magnitude with 12.5" reflector is 14
|
8 |
City sky |
4.1–4.5 |
<18.0 |
- the sky is light gray or orange – one can easily read
- stars forming familiar constellation patterns may be weak or invisible
- M31 and M44 are barely glimpsed by an experienced observer on good nights
- even with a telescope, only bright Messier objects can be detected
- limiting magnitude with 12.5" reflector is 13
|
9 |
Inner-city sky |
4.0 |
- The sky is brilliantly lit
- many stars forming constellations are invisible and many fainter constellations are invisible
- aside from the Pleiades, no Messier object is visible to the naked eye
- the only objects to observe are the Moon, the planets, and a few of the brightest star clusters
|