Bortle scale

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. Some classes can have very drastic differences from the one next to it, e.g, Bortle 4 - 5.

Class Title NELM Approx.
SQM[3]
mag/arcsec2		 Description Color
1 Excellent
dark-sky site		 7.6–8.0 21.7–22.0
  • the zodiacal light is visible and colorful
  • the gegenschein is visible
  • the zodiacal band is visible
  • airglow is readily visible
  • the Scorpius and Sagittarius regions of the Milky Way cast obvious shadows
  • many constellations, particularly fainter ones, are barely recognizable amid the large number of stars
  • many Messier and globular clusters are naked-eye objects
  • M33 (the Triangulum Galaxy) is a direct vision naked-eye object
  • limiting magnitude with 12.5 in (32 cm) reflector is 17.5 (with effort)
  • Venus and Jupiter affect dark adaptation
 Black
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
 Gray
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
 Blue
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
 Green/Yellow
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
 Orange
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
 Red
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 nearly or totally 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
 Red
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
 White
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
 White

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
  1. Bortle, John E. (February 2001). "Gauging Light Pollution: The Bortle Dark-Sky Scale". Sky & Telescope. Sky Publishing Corporation. Retrieved 2020-05-29.
  2. Crumey, Andrew (2014). "Human Contrast Threshold and Astronomical Visibility". Monthly Notices of the Royal Astronomical Society. 442 (3): 2600–2619. arXiv:1405.4209. Bibcode:2014MNRAS.442.2600C. doi:10.1093/mnras/stu992.
  3. "Dark Skies Awareness".
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