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"Bortle" redirects here. For the asteroid, see
4673 Bortle .
The Bortle scale is a nine-level numeric scale that measures the night sky 's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution . John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help amateur astronomers evaluate the darkness of an observing site, and secondarily, to compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies. It gives several criteria for each level beyond naked-eye limiting magnitude (NELM).[ 1] The accuracy and utility of the scale have been questioned in recent research.[ 2]
The table below summarizes Bortle's descriptions of the classes.
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
See also
In this 10-second exposure photo, facing south toward Sagittarius , light pollution obscures the stars and faintly visible Milky Way in the suburban night sky over Southern California .
References
External links