Bortle Dark-Sky Scale
Encyclopedia
The Bortle Dark-Sky Scale is a nine-level numeric scale that measures the night sky
's and star
s' brightness
(naked-eye and stellar limiting magnitude
) of a particular location. It quantifies the astronomical
observability of celestial objects and the interference caused by light pollution
and skyglow
. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope
magazine to help amateur astronomers compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies.
The table below summarizes Bortle's descriptions of the classes. The colors are from the World Atlas of Artificial Night Sky Brightness, and are provided as a convenience to the reader. The correlation between the colors and Bortle classes is approximate at best.
Night sky
The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
's and star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s' brightness
Sky brightness
The fact that the sky is not completely dark at night can be easily observed. Were the sky absolutely dark, one would not be able to see the silhouette of an object against the sky....
(naked-eye and stellar limiting magnitude
Limiting magnitude
In astronomy, limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable or detected by a given instrument....
) of a particular location. It quantifies the astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
observability of celestial objects and the interference caused by light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
and skyglow
Skyglow
Skyglow is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky...
. John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope
Sky & Telescope
Sky & Telescope is a monthly American magazine covering all aspects of amateur astronomy, including the following:*current events in astronomy and space exploration;*events in the amateur astronomy community;...
magazine to help amateur astronomers compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies.
The table below summarizes Bortle's descriptions of the classes. The colors are from the World Atlas of Artificial Night Sky Brightness, and are provided as a convenience to the reader. The correlation between the colors and Bortle classes is approximate at best.
| Class | Title | Color key | Naked-eye limiting magnitude Limiting magnitude In astronomy, limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable or detected by a given instrument.... |
Stellar limiting magnitude (with 12.5" reflector Reflecting telescope A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from... ) |
Description |
|---|---|---|---|---|---|
| 1 | Excellent dark-sky site | 7.6–8.0 | 19 at best | Zodiacal light Zodiacal light Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders... , gegenschein Gegenschein The gegenschein is a faint brightening of the night sky in the region of the antisolar point.- Explanation :Like the zodiacal light, the gegenschein is sunlight reflected by interplanetary dust... , zodiacal band visible; M33 direct vision naked-eye Naked eye The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"... object; Scorpius Scorpius Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east... and Sagittarius Sagittarius (constellation) Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow... regions of the Milky Way Milky Way The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky... cast obvious shadows on the ground; airglow Airglow Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist... is readily visible; Jupiter Jupiter Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,... and Venus Venus Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows... affect dark adaptation; surroundings basically invisible. |
|
| 2 | Typical truly dark site | 7.1–7.5 | 17 at best | Airglow Airglow Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist... weakly visible near horizon; M33 easily seen with naked eye; highly structured summer Milky Way; distinctly yellowish zodiacal light bright enough to cast shadows at dusk and dawn; clouds only visible as dark holes; surroundings still only barely visible silhouetted against the sky; many Messier Messier object The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets... globular clusters still distinct naked-eye objects. |
|
| 3 | Rural sky | 6.6–7.0 | 16 at best | Some light pollution evident at the horizon; clouds illuminated near horizon, dark overhead; Milky Way still appears complex; M15 Messier 15 Messier 15 or M15 is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764... , M4 Messier 4 Messier 4 or M4 is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1746 and catalogued by Charles Messier in 1764. It was the first globular cluster in which individual stars were resolved.-Visibility:M4 is conspicuous in even the smallest of... , M5 Messier 5 Messier 5 or M5 is a globular cluster in the constellation Serpens. It was discovered by Gottfried Kirch in 1702... , and M22 Messier 22 Messier 22 is an elliptical globular cluster in the constellation Sagittarius, near the Galactic bulge region... distinct naked-eye objects; M33 easily visible with averted vision Averted vision Averted vision is a technique for viewing faint objects which uses peripheral vision. It involves not looking directly at the object, but looking a little off to the side, while continuing to concentrate on the object. This subject is discussed in the popular astronomy literature but only a few... ; zodiacal light striking in spring and autumn, color still visible; nearer surroundings vaguely visible. |
|
| 4 | Rural/suburban transition | 6.1–6.5 | 15.5 at best | Light pollution domes visible in various directions over the horizon; zodiacal light is still visible, but not even halfway extending to the zenith Zenith The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e... at dusk or dawn; Milky Way above the horizon still impressive, but lacks most of the finer details; M33 a difficult averted vision object, only visible when higher than 55°; clouds illuminated in the directions of the light sources, but still dark overhead; surroundings clearly visible, even at a distance. |
|
| 5 | Suburban sky | 5.6–6.0 | 15 at best | Only hints of zodiacal light are seen on the best nights in autumn and spring; Milky Way is very weak or invisible near the horizon and looks washed out overhead; light sources visible in most, if not all, directions; clouds are noticeably brighter than the sky. | |
| 6 | Bright suburban sky | 5.1–5.5 | 14.5 at best | Zodiacal light is invisible; Milky Way only visible near the zenith; sky within 35° from the horizon glows grayish white; clouds anywhere in the sky appear fairly bright; surroundings easily visible; M33 is impossible to see without at least binoculars Binoculars Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects... , M31 is modestly apparent to the unaided eye. |
|
| 7 | Suburban/urban transition or Full Moon | 4.6–5.0 | 14 at best | Entire sky has a grayish-white hue; strong light sources evident in all directions; Milky Way invisible; M31 and M44 may be glimpsed with the naked eye, but are very indistinct; clouds are brightly lit; even in moderate-sized telescope Telescope A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses... s the brightest Messier objects are only ghosts of their true selves. At a full moon night the sky is not better than this rating even at the darkest locations with the difference that the sky appears more blue than orangish white at otherwise dark locations. |
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| 8 | City sky | 4.1–4.5 | 13.5 at best | Sky glows white or orange—one can easily read; M31 and M44 are barely glimpsed by an experienced observer on good nights; even with telescope, only bright Messier objects can be detected; stars forming familiar constellation Constellation In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky.... patterns may be weak or completely invisible. |
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| 9 | Inner-city sky | 4.0 at best | 13 at best | Sky is brilliantly lit, with many stars forming constellations invisible and many weaker constellations invisible; aside from Pleiades Pleiades (star cluster) In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky... , no Messier object is visible to the naked eye; only objects to provide fairly pleasant views are the Moon Moon The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more... , the planet Planet A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,... s, and a few of the brightest star cluster Star cluster Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than... s. |
See also
- Sky & TelescopeSky & TelescopeSky & Telescope is a monthly American magazine covering all aspects of amateur astronomy, including the following:*current events in astronomy and space exploration;*events in the amateur astronomy community;...
(S&T) - Night skyNight skyThe term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
- Sky brightnessSky brightnessThe fact that the sky is not completely dark at night can be easily observed. Were the sky absolutely dark, one would not be able to see the silhouette of an object against the sky....
- Light pollutionLight pollutionLight pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
- SkyglowSkyglowSkyglow is the illumination of the night sky or parts of it. The most common cause of skyglow is artificial light that emits light pollution, which accumulates into a vast glow that can be seen from miles away and from high in the sky...
- Dark-sky movementDark-sky movementThe dark-sky movement is a campaign by people who want to reduce light pollution so people can see the stars, to reduce the effects of unnatural lighting on the environment, and to cut down on energy usage....
- International Dark-Sky AssociationInternational Dark-Sky AssociationThe International Dark-Sky Association is a U.S.-based non-profit organization incorporated in 1988 by founders Dr. David Crawford, a professional astronomer, and Dr. Timothy Hunter, a medical doctor/amateur astronomer...
(IDA) - Amateur astronomyAmateur astronomyAmateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
External links
by Sky & TelescopeSky & Telescope
Sky & Telescope is a monthly American magazine covering all aspects of amateur astronomy, including the following:*current events in astronomy and space exploration;*events in the amateur astronomy community;...