The Bortle Dark-Sky Scale is a nine-level numeric measure of the night sky brightness of a particular location. It quantifies the observability of astronomical objects and the interference caused by light pollution and skyglow. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Light pollution, also known as photopollution or luminous pollution, is excess or obtrusive Light created mainly by Humans Among other effects Skyglow (or sky glow) is a kind of Light pollution, visible by the "glowing" effect seen in the skies over many cities and towns as a dome of light 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. Sky & Telescope (S&T is an American monthly Magazine covering all aspects of Amateur astronomy, including current events in The scale ranges from class 1, the darkest skies available on Earth, through class 9, inner city skies. [1]
The table below summarizes Bortle's descriptions of the classes. The colors are from The World Atlas of Artificial Night Sky Brightness, and they're provided as a convenience to the reader. The correlation between the colors and Bortle Classes is approximate at best.
| Class | Title | Color | Naked eye limiting magnitude | Description |
|---|---|---|---|---|
| 1 | Excellent dark sky site | black | 7. Limiting magnitude is an astronomical term which describes the faintest Apparent magnitude detectable or detected by a given instrument 6 – 8. 0 | Zodiacal light, gegenschein, zodiacal band visible; M33 direct vision naked-eye object; Scorpius and Sagittarius regions of the Milky Way cast obvious shadows on the ground; Airglow is readily visible; Jupiter and Venus affect dark adaptation; surroundings basically invisible. The 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 Gegenschein ( very roughy like GAY-guhn-shine German for "counter shine" is a faint brightening of the night sky in the region of the Antisolar The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a Spiral galaxy approximately 3 million light-years away in the The naked eye is a Figure of speech referring to human Visual perception that is unaided by enhancing equipment such as a Telescope or Scorpius ( Latin for Scorpion, symbol, Unicode ♏ is one of the Constellations of the Zodiac. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The airglow is the very weak emission of light by the Earth's atmosphere; as a result the night sky is never completely dark The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University In ocular physiology adaptation is the ability of the Eye to adjust to various levels of darkness and light |
| 2 | Typical truly dark site | gray | 7. 1 – 7. 5 | Airglow 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 globular clusters still distinct naked-eye objects. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. |
| 3 | Rural sky | blue | 6. 6 – 7. 0 | Some light pollution evident at the horizon; clouds illuminated near horizon, dark overhead; Milky Way still appears complex; M15, M4, M5, M22 distinct naked-eye objects; M33 easily visible with averted vision; zodiacal light striking in spring and autumn, color still visible; nearer surroundings vaguely visible. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation Messier 4 (also known as M4 or NGC 6121) is a Globular cluster in the Constellation Scorpius. Globular Cluster M5 (also known as Messier Object 5 or NGC 5904) is a Globular cluster in the Constellation Serpens. Messier 22 (also known as M22 or NGC 6656) is an elliptical Globular cluster in the Constellation Sagittarius near the Galactic Averted vision is a controversial technique for viewing faint objects visually |
| 4 | Rural/suburban transition | green | 6. 1 – 6. 5 | Light pollution domes visible in various directions over the horizon; zodiacal light is still visible, but not even halfway extending to the zenith 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. In broad terms the zenith is the direction pointing directly above a particular location ( Perpendicular, Orthogonal) |
| yellow | ||||
| 5 | Suburban sky | orange | 5. 6 – 6. 0 | 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 | red | 5. 1 – 5. 5 | 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, M31 is modestly apparent to the unaided eye. The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a Spiral galaxy approximately 3 million light-years away in the Binocular telescopes, or binoculars (also known as field glasses are two identical or Mirror - symmetrical telescopes mounted side-by-side and The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda |
| 7 | Suburban/urban transition | red | 5. 0 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 telescopes the brightest Messier objects are only ghosts of their true selves. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The Beehive Cluster (also known as Praesepe (Latin for"manger" M44 or NGC 2632) is an Open cluster in theconstellation A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. |
| 8 | City sky | white | 4. 5 at best | Sky glows white or orange--you 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 patterns may be weak or completely invisible. The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The Beehive Cluster (also known as Praesepe (Latin for"manger" M44 or NGC 2632) is an Open cluster in theconstellation In common usage a constellation is a group of celestial bodies that are connected together in some arrangement typically stars to form a visible figure or picture |
| 9 | Inner City sky | white | 4. 0 at best | Sky is brilliantly lit with many stars forming constellations invisible and many weaker constellations invisible; aside from Pleiades, no Messier object is visible to the naked eye; only objects to provide fairly pleasant views are the Moon, the Planets and a few of the brightest star clusters. The Pleiades (ˈpliːədiːz or /ˈplaɪədiːz/ also known as M 45, the '''Seven Sisters''', Seven Stars, SED, Matariki A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Star clusters are groups of Stars which are gravitationally bound |
References
- Northern Virginia Astronomy Club light pollution page
- The Bortle Dark-Sky Scale
- The World Atlas of Artificial Night Sky Brightness color coded
Footnotes
- ^ Bortle, John E. , The Bortle Dark-Sky Scale, retrieved 2007-09-08