Skyglow
Encyclopedia
Lighting
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight...
of the night sky
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,...
or parts of it. The most common cause of skyglow is artificial light that emits 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:...
, which accumulates into a vast glow that can be seen from miles away and from high in the sky. Skyglow from artificial lights is common throughout the world and can be observed over most cities and towns as a glowing dome of the populated area. Skyglow's light domes can be large, as in that over a city, or small, as in that over an over-illuminated
Over-illumination
Over-illumination is the presence of lighting intensity beyond that required for a specified activity. Over-illumination was commonly ignored between 1950 and 1995, especially in office and retail environments; only since then has the interior design community begun to reconsider this practice.The...
shopping center or a stadium.
Although often referring to artificial light, skyglow also includes natural sources of diffuse nighttime light like the 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...
, starlight, and 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...
emitted high in the upper atmosphere.:
Skyglow can also be caused by natural occurrences, such as the 1908 Tunguska event
Tunguska event
The Tunguska event, or Tunguska blast or Tunguska explosion, was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 7:14 a.m...
, in which a meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...
spanning a few meters in mean radius
Earth radius
Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km...
exploded 5–10 kilometers above the Podkamennaya Tunguska River
Podkamennaya Tunguska River
The Podkamennaya Tunguska is a river in Krasnoyarsk Krai, Russia; it is an eastern tributary of the Yenisei and has a length of . The name of the river comes from the fact that it flows under pebble fields without open water...
in the Krasnoyarsk Krai region of Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. The explosion is estimated to have had released more or less 15 megatons of energy, which is around 1,000 times as powerful as the atomic bomb
Atomic bombings of Hiroshima and Nagasaki
During the final stages of World War II in 1945, the United States conducted two atomic bombings against the cities of Hiroshima and Nagasaki in Japan, the first on August 6, 1945, and the second on August 9, 1945. These two events are the only use of nuclear weapons in war to date.For six months...
that exploded over Hiroshima, Japan in 1945 and about one third as powerful as the thermonuclear bomb Tsar Bomba
Tsar Bomba
Tsar Bomba is the nickname for the AN602 hydrogen bomb, the most powerful nuclear weapon ever detonated. It was also referred to as Kuz'kina Mat , in this usage meaning "something that has not been seen before"....
, the most powerful nuclear bomb ever detonated. The light emitted from the Tunguska explosion was so great that it created skyglow as far away as England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, where the population experienced a number of weeks of intermittent "bright nights" (a term that is now synonymous with skyglow).
Dependence on distance from source
For relatively small distances between the light source and observer, the intensity of the skyglow contribution from a single source of light is inversely proportional to the distance between the source and the observer (falls off as 1/r). This can be understood as follows: The illumination of any portion of atmosphere visible by the observer falls as 1/r2, but the path length of illuminated air along any given line of sight which is illuminated comparably to the brightest illumination grows linearly with distance; together, this gives a 1/r dependence. This is valid when the distance between the source and observer is smaller than the scale heightScale height
In various scientific contexts, a scale height is a distance over which a quantity decreases by a factor of e...
of the atmosphere. At distances much larger, where the path length of illuminated air is limited by the height of the atmosphere itself, this becomes an inverse-square (1/r2) dependence.
Negative effects
Skyglow, and more generally light pollutionLight 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:...
, has various negative effects: from aesthetic diminishment of the beauty of a star-filled sky, through energy and resources wasted in the production of excessive or uncontrolled lighting, to impacts on birds and other biological systems,
including humans. Skyglow is a prime problem for astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s, because it reduces contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
in the night sky
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,...
to the extent where it may become impossible to see all but the brightest stars. It is a widely held misunderstanding that professional astronomical observatories can "filter out" certain wavelengths of light (such as that produced by low-pressure sodium
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure...
). More accurately, by leaving large portions of the spectrum relatively unpolluted, the narrow-spectrum emission
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
from low-pressure sodium
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure...
lamps allows more opportunity for astronomers to "work around" the resulting light pollution. Even when such lighting is widely used, skyglow still interferes with astronomical research as well as everyone's ability to see a natural star-filled sky.
Due to skyglow, people who live in or near urban areas see thousands fewer stars than in an unpolluted sky, and commonly cannot see 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...
. Fainter sights like the 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...
and Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
are nearly impossible to discern even with telescopes.
Causes
The several causes of skyglow mainly differ in source. For example, public lighting provides a different form of light pollution than attention-grabbing strobe lamps, and these differ from commercial lighting installations. Light from electric lampsElectric light
Electric lights are a convenient and economic form of artificial lighting which provide increased comfort, safety and efficiency. Most electric lighting is powered by centrally-generated electric power, but lighting may also be powered by mobile or standby electric generators or battery systems...
shines directly upward into the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
from poorly shielded fixtures, and reflects from surfaces like the ground or streets into the sky. On clear nights, some of this light is then scattered in the atmosphere by molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s and aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
s back toward the ground, causing skyglow.
Near urban areas skyglow is made considerably worse when clouds are present, and when snow is on the ground. This is because both clouds and snow have a very high albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
, preventing the light from escaping Earth's atmosphere, or being absorbed on the ground. In pristine areas snow brightens the sky, but clouds make the sky much darker.
Mechanism
There are two causes of the light scattering that lead to airglow: scattering from molecules such as N2 and O2 (called Rayleigh scatteringRayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
), and that from aerosols, called Mie scattering
Mie theory
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...
. Rayleigh scattering is much stronger for short-wavelength (blue) light, while scattering from aerosols is little affected by wavelength. In most places, most particularly in urban areas, aerosol scattering dominates, due to the heavy aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
loading caused by modern industrial processes and transportation. Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
makes the sky appear blue in the daytime; the more aerosols there are, the less blue or whiter the sky appears. When the air is clear and relatively free of aerosols, blue or white light (for example from metal halide lamp
Metal halide lamp
Metal-halide lamps, a member of the high-intensity discharge family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source. By adding rare earth metal salts to the mercury vapor lamp, improved luminous efficacy and light color is obtained...
s, fluorescent lamp
Fluorescent lamp
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...
s, and white light-emitting diodes
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
) contributes significantly more to sky-glow than an equal amount of yellow light (for example from high- and low-pressure sodium vapor lamp
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure...
s). Another effect that makes skyglow from white light sources worse than from yellow arises from the Purkinje effect
Purkinje effect
The Purkinje effect is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.This effect introduces a difference in color contrast under different levels of...
, where the eye becomes more sensitive to bluer/whiter light when adapted to low light levels, as experienced under night time conditions. A simple metric for first effect is the Rayleigh Scatter Index, discussed in a brief article
and a 2003 presentation to both the International Dark-Sky Association
International Dark-Sky Association
The 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...
Conference and the Illuminating Engineering Society of North America
Illuminating Engineering Society of North America
The Illuminating Engineering Society of North America is a non-profit learned society that was founded in New York City on January 10, 1906. The IES's stated mission is to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into...
,
which indicates that high pressure sodium sources produce roughly one-third to one-half of the skyglow compared to the output of typical metal halide sources, based on the same amount of light entering the atmosphere and pure Rayleigh scattering. When the Purkinje effect
Purkinje effect
The Purkinje effect is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the color spectrum at low illumination levels.This effect introduces a difference in color contrast under different levels of...
is also considered the effect is magnified, to where yellow sources can produce as little as one-eighth the skyglow of an equivalent output white light source, particularly when the observer is located at some distance from the light pollution source, the sky is darker, and the eye more completely dark adapted.
Measurement
Astronomers have used the Bortle Dark-Sky ScaleBortle Dark-Sky Scale
The Bortle Dark-Sky Scale is a nine-level numeric scale that measures the night sky's and stars' brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution and skyglow. John E...
to measure skyglow ever since it was published in 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 in February 2001. The scale rates the darkness of the night sky inhibited by skyglow with nine classes and provides a detailed description of each position on the scale.
See also
- 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:...
- Polarized light pollutionPolarized light pollutionPolarization is a property of light waves that describes the orientation of their oscillations. Polarized light pollution is a subset of the various forms of light pollution referring specifically to polarized light....
- Over-illuminationOver-illuminationOver-illumination is the presence of lighting intensity beyond that required for a specified activity. Over-illumination was commonly ignored between 1950 and 1995, especially in office and retail environments; only since then has the interior design community begun to reconsider this practice.The...
- 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) - Campaign for Dark SkiesCampaign for Dark SkiesThe Campaign for Dark Skies is the United Kingdom's biggest anti-light-pollution campaign group.It is part of the British Astronomical Association and affiliated with the International Dark-Sky Association , and composed of a network of who try to improve lighting in their areas and advise...
(CfDS)
External links
- List of peer reviewed research papers about sky glow
- Skyglow: the effect of poor lighting (CfDSCfdsCFDS or cfds may mean*Contracts for difference*Campaign for Dark Skies *Centralized Fault Display System in avionics electronics...
) (examples of skyglow in the UKUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
) - Skyglow across the Great Lakes (examples of skyglow in the USUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
) - Filtering Skyglow (from CCDCharge-coupled deviceA charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
cameraCameraA camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s) - Towns and Skyglow (UKUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
skyglow image collection) - What Blue Skies Tell Us About Light Pollution (an article introducing the Rayleigh Scatter Index which evaluates the potential for contribution to skyglow by different light sources)
- Sources, Surfaces and Scatter (a presentation from 2003 to the IDA Conference and the IESNA Roadway Lighting Committee on atmospheric scatter and the potential for contribution to skyglow by different light sources)