This article is about night sky luminance from artificial light. For the natural atmospheric phenomenon, see airglow. For the interplanetary phenomenon, see zodiacal light. For the wider environmental impacts of artificial light, see light pollution.
Mexico City at night, showing skyglowA map from 1996 to 1997 showing the extent of skyglow over Europe
Skyglow (or sky glow) is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to luminance arising from artificial lighting, skyglow may also involve any scattered light seen at night, including natural ones like starlight, zodiacal light, and airglow.[1][2]
In the context of light pollution, skyglow arises from the use of artificial light sources, including electrical (or rarely gas) lighting used for illumination and advertisement and from gas flares.[3] Light propagating into the atmosphere directly from upward-directed or incompletely shielded sources, or after reflection from the ground or other surfaces, is partially scattered back toward the ground, producing a diffuse glow that is visible from great distances. Skyglow from artificial lights is most often noticed as a glowing dome of light over cities and towns, yet is pervasive throughout the developed world.
Causes
In this 10-second exposure, facing south toward Sagittarius, three forms of light pollution are present: skyglow, glare, and light trespass.
Light used for all purposes in the outdoor environment contributes to skyglow, by sometimes avoidable aspects such as poor shielding of fixtures, and through at least partially unavoidable aspects such as unshielded signage and reflection from intentionally illuminated surfaces. Some of this light is then scattered in the atmosphere back toward the ground by molecules and aerosols (see §Mechanism), and (if present) clouds, causing skyglow.
Research indicates that when viewed from nearby, about half of skyglow arises from direct upward emissions, and half from reflected, though the ratio varies depending on details of lighting fixtures and usage, and distance of the observation point from the light source.[4][5] In most communities, direct upward emission averages about 10–15%.[4] Fully shielded lighting (with no light emitted directly upward) decreases skyglow by about half when viewed nearby, but by much greater factors when viewed from a distance.
Skyglow is significantly amplified by the presence of snow, and within and near urban areas when clouds are present.[6] In remote areas, snow brightens the sky, but clouds make the sky darker.
In remote areas on moonless nights clouds appear dark against the sky. In or near developed areas skyglow is strongly enhanced by clouds.
Mechanism
There are two kinds of light scattering that lead to sky glow: scattering from molecules such as N2 and O2 (called Rayleigh scattering), and that from aerosols, described by Mie theory. Rayleigh scattering is much stronger for short-wavelength (blue) light, while scattering from aerosols is less affected by wavelength. Rayleigh scattering makes the sky appear blue in the daytime; the more aerosols there are, the less blue or whiter the sky appears. In many areas, most particularly in urban areas, aerosol scattering dominates, due to the heavy aerosol loading caused by modern industrial activity, power generation, farming and transportation.
Despite the strong wavelength dependence of Rayleigh scattering, its effect on sky glow for real light sources is small. Though the shorter wavelengths suffer increased scattering, this increased scattering also gives rise to increased extinction: the effects approximately balance when the observation point is near the light source.[7]
For human visual perception of sky glow, generally the assumed context under discussions of sky glow, sources rich in shorter wavelengths produce brighter sky glow, but for a different reason (see §Dependence on light source).
Measurement
Professional astronomers and light pollution researchers use various measures of luminous or radiant intensity per unit area, such as magnitudes per square arcsecond, watts per square meter per steradian,(nano-)Lamberts, or (micro-)candela per square meter.[8] All-sky maps of skyglow brightness are produced with professional-grade imaging cameras with CCD detectors and using stars as calibration sources.[9][10] Amateur astronomers have used the Bortle Dark-Sky Scale to approximately quantify skyglow ever since it was published in Sky & Telescope magazine in February 2001.[11] 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. Amateurs also increasingly use Sky Quality Meters (SQM) that nominally measure in astronomical photometric units of visual (Johnson V) magnitudes per square arcsecond.[note 1]
A calibrated all-sky map in the vicinity of Ashurst Lake, Arizona, showing skyglow brightness, including artificial (Phoenix and Flagstaff, Arizona) and natural sources (airglow, Milky Way) are visible (U.S. National Park Service).
Dependence on distance from source
Sky glow brightness arising from artificial light sources falls steeply with distance from the light source, due to the geometric effects characterized by an inverse square law in combination with atmospheric absorption. An approximate relation is given by
Walker's Law has been verified by observation[13][9] to describe both the measurements of sky brightness at any given point or direction in the sky caused by a light source (such as a city), as well as to integrated measures such as the brightness of the "light dome" over a city, or the integrated brightness of the entire night sky. At very large distances (over about 50km) the brightness falls more rapidly, largely due to extinction and geometric effects caused by the curvature of the Earth.
Dependence on light source
Sky glow and stars visible with high-pressure sodium lighting – Calibrated model of Flagstaff, Arizona, US as viewed from 10 km.Sky glow and stars visible with 4100K CCT LED lighting – Calibrated model of Flagstaff, Arizona, US as viewed from 10 km.
Different light sources produce differing amounts of visual sky glow. The dominant effect arises from the Purkinje shift, and not as commonly claimed from Rayleigh scattering of short wavelengths (see §Mechanism).[7][15] When observing the night sky, even from moderately light polluted areas, the eye becomes nearly or completely dark-adapted or scotopic. The scotopic eye is much more sensitive to blue and green light, and much less sensitive to yellow and red light, than the light-adapted or photopic eye. Predominantly because of this effect, white light sources such as metal halide, fluorescent, or white LED can produce as much as 3.3 times the visual sky glow brightness of the currently most-common high-pressure sodium lamp, and up to eight times the brightness of low-pressure sodium or amber Aluminium gallium indium phosphide LED.
Skyglow brightness ratio (compared to low-pressure sodium) vs. distance for various lamp types.
Sky Glow brightness ratios for different lamp types[note 2]
In detail, the effects are complex, depending both on the distance from the source as well as the viewing direction in the night sky. But the basic results of recent research are unambiguous: assuming equal luminous flux (that is, equal amounts of visible light), and matched optical characteristics of the fixtures (particularly the amount of light allowed to radiate directly upward), white sources rich in shorter (blue and green) wavelengths produce dramatically greater sky glow than sources with little blue and green.[14] The effect of Rayleigh scattering on skyglow impacts of differing light source spectra is small.
Much discussion in the lighting industry and even by some dark-sky advocacy organizations (e.g. International Dark-Sky Association) of the sky glow consequences of replacing the currently prevalent high-pressure sodium roadway lighting systems with white LEDs neglects critical issues of human visual spectral sensitivity,[17] or focuses exclusively on white LED light sources, or focuses concerns narrowly on the blue portion (<500nm) of the spectrum.[18][19] All of these deficiencies lead to the incorrect conclusion that increases in sky glow brightness arising from the change in light source spectrum are minimal, or that light-pollution regulations that limit the CCT of white LEDs to so-called "warm white" (i.e. CCT <4000K or 3500K) will prevent sky glow increases.[14] Improved efficiency (efficiency in distributing light onto the target area – such as the roadway – with diminished "waste" falling outside of the target area[20] and more uniform distribution patterns[citation needed]) can allow designers to lower lighting amounts.[citation needed] But efficiency improvement sufficient to overcome sky glow doubling or tripling arising from a switch to even warm-white LED from high-pressure sodium (or a 4–8x increase compared to low-pressure sodium) has not been demonstrated.
Negative effects
Skyglow is mostly unpolarized, and its addition to moonlight results in a decreased polarization signal. Humans cannot perceive this pattern, but some arthropods can.
Skyglow, and more generally light pollution, 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[21] and other biological systems,[22] including humans. Skyglow is a prime problem for astronomers, because it reduces contrast in the night sky to the extent where it may become impossible to see all but the brightest stars.[note 4]
Many nocturnal organisms are believed to navigate using the polarizationsignal of scattered moonlight.[24] Because skyglow is mostly unpolarized, it can swamp the weaker signal from the moon, making this type of navigation impossible.[25] Close to global coastal megacities (e.g. Tokyo, Shanghai), the natural illumination cycles provided by the moon in the marine environment are considerably disrupted by light pollution, with only nights around the full moon providing greater radiances, and over a given month lunar dosages may be a factor of 6 less than light pollution dosage.[26]
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.[27] Fainter sights like the zodiacal light and Andromeda Galaxy are nearly impossible to discern even with telescopes.
Effects on the ecosystem
The effects of sky glow in relation to the ecosystem have been observed to be detrimental to a variety of organisms. The lives of plants and animals (especially those which are nocturnal) are affected as their natural environment becomes subjected to unnatural change. It can be assumed that the rate of human development technology exceeds the rate of non-human natural adaptability to their environment, therefore, organisms such as plants and animals are unable to keep up and can suffer as a consequence.[28]
Although sky glow can be the result of a natural occurrence, the presence of artificial sky glow has become a detrimental problem as urbanization continues to flourish. The effects of urbanization, commercialization, and consumerism are the result of human development; these developments in turn have ecological consequences. For example, lighted fishing fleets, offshore oil platforms, and cruise ships all bring the disruption of artificial night lighting to the world's oceans.[29]
As a whole, these effects derive from changes in orientation, disorientation, or misorientation, and attraction or repulsion from the altered light environment, which in turn may affect foraging, predator-prey dynamics,[30][31] reproduction,[32] migration, and communication. These changes can result in the death of some species such as certain migratory birds,[33] sea creatures,[34] and nocturnal predators.[35]
Besides the effect on animals, crops and trees are also susceptible to destruction. The constant exposure to light has an impact of the photosynthesis of a plant, as a plant needs a balance of both sun and darkness in order for it to survive. In turn, the effects of sky glow can affect production rates of agriculture, especially in farming areas that are close to large city centers.[citation needed]
↑ SQM meters have a notably different spectral response than the human eye, and even from the Johnson V response they nominally use. As a consequence SQM measures are not accurate for tracking visual impressions, particularly as spectral characteristics change from yellow sources such as HPS to white sources such as LED. Likewise, the difference between SQM measures and a true Johnson V measure is dependent on the skyglow spectrum and source(s) of artificial luminance.[12]
↑ Results for within cities or near the light source, based on work of Luginbuhl et al.[7] and Aubé et al.[15]
↑ 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). More accurately, by leaving large portions of the spectrum relatively unpolluted, the narrow-spectrum emission from low-pressure sodium lamps and to a lesser extent from amber direct emission Aluminium gallium indium phosphide LED allows more opportunity for astronomers to "work around" the resulting light pollution.[23] 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.
Related Research Articles
Light pollution is the presence of unwanted, inappropriate, or excessive artificial lighting. In a descriptive sense, the term light pollution refers to the effects of any poorly implemented lighting, during the day or night. Light pollution can be understood not only as a phenomenon resulting from a specific source or kind of pollution, but also as a contributor to the wider, collective impact of various sources of pollution.
Rayleigh scattering, named after the 19th-century British physicist Lord Rayleigh, is the predominantly elastic scattering of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scattering medium, the amount of scattering is inversely proportional to the fourth power of the wavelength, e.g., a blue color is scattered much more than a red color as light propagates through air.
The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere. It is also called sky radiation, the determinative process for changing the colors of the sky. Approximately 23% of direct incident radiation of total sunlight is removed from the direct solar beam by scattering into the atmosphere; of this amount about two-thirds ultimately reaches the earth as photon diffused skylight radiation.
A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.
A laser guide star is an artificial star image created for use in astronomical adaptive optics systems, which are employed in large telescopes in order to correct atmospheric distortion of light. Adaptive optics (AO) systems require a wavefront reference source of light called a guide star. Natural stars can serve as point sources for this purpose, but sufficiently bright stars are not available in all parts of the sky, which greatly limits the usefulness of natural guide star adaptive optics. Instead, one can create an artificial guide star by shining a laser into the atmosphere. Light from the beam is reflected by components in the upper atmosphere back into the telescope. This star can be positioned anywhere the telescope desires to point, opening up much greater amounts of the sky to adaptive optics.
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
The Tyndall effect is light scattering by particles in a colloid such as a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, so blue light is scattered much more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motorcycles, in particular two-stroke machines where the burnt engine oil provides these particles. The same effect can also be observed with tobacco smoke whose fine particles also preferentially scatter blue light.
Airglow is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed. This phenomenon originates with self-illuminated gases and has no relationship with Earth's magnetism or sunspot activity.
The dark-sky movement is a campaign to reduce light pollution. The advantages of reducing light pollution include an increased number of stars visible at night, reducing the effects of electric lighting on the environment, improving the well-being, health and safety of people and wildlife, and cutting down on energy usage. Earth Hour and National Dark-Sky Week are two examples of such efforts.
International Dark Sky Week is held during the week of the new moon in April, when people worldwide may turn off their lights to observe the beauty of the night sky without light pollution. This event was founded in 2003 by high school student Jennifer Barlow of Midlothian, Virginia. It has been endorsed by the International Dark-Sky Association, the American Astronomical Society, the Astronomical League, and Sky & Telescope.
Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night is easily visible. If light sources were removed from the night sky, only direct starlight would be visible.
Cosmic infrared background is infrared radiation caused by stellar dust.
Ocean turbidity is a measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification. Highly turbid ocean waters are those with many scattering particulates in them. In both highly absorbing and highly scattering waters, visibility into the water is reduced. Highly scattering (turbid) water still reflects much light, while highly absorbing water, such as a blackwater river or lake, is very dark. The scattering particles that cause the water to be turbid can be composed of many things, including sediments and phytoplankton.
An LED street light or road light is an integrated light-emitting diode (LED) light fixture that is used for street lighting.
North light is sunlight coming through a north-facing window. Because it does not come directly from the sun, it remains at a consistent angle and colour throughout the day and does not create sharp shadows. It is also cooler than direct sunlight due to the way the Earth's atmosphere scatters light via Rayleigh scattering.
Ecological light pollution is the effect of artificial light on individual organisms and on the structure of ecosystems as a whole.
Globe at Night is an international scientific research program that crowdsources measurements of light pollution in the night sky. At set time periods within each year, the project asks people to count the number of stars that they can see from their location and report it to the project's website. The coordinating researchers compile this information to produce a public, freely available map of global light pollution. By September 2011, almost 70,000 measurements had been made. The use of data collected by the public makes the program an example of citizen science. Globe at Night began as a NASA educational program in the US organized by the NOAO, and was expanded internationally during the 2009 International Year of Astronomy; it is an offshoot of the GLOBE Program, which focuses on school-based science education.
The spectral G-Index is a variable that was developed to quantify the amount of short wavelength light in a visible light source relative to its visible emission. The smaller the G-index, the more blue, violet, or ultraviolet light a lamp emits relative to its total output. It is used in order to select outdoor lamps that minimize skyglow and ecological light pollution. The G-index was originally proposed by David Galadí Enríquez, an astrophysicist at Calar Alto Observatory.
Light pollution is the presence of unwanted artificial light that brightens the night sky. Improperly shielded lights are the source of many of the issues regarding the light pollution in Hawai'i. Urban centers in the cities are often so bathed in light that over a hundred kilometers from the city's edge, the light pollution resulting from the glow is present. Fabio Falchi is quoted as stating that “light pollution is one of the most pervasive forms of environmental alteration” due to its destructive nature in both un- and protected areas such as national parks. Dark night skies are an important natural, cultural, scientific, educational, and economic resource for Hawai‘i.
References
↑ Roach, Franklin E. & Gordon, Janet L. (1973). The Light of the Night Sky. Dordrecht and Boston, Massachusetts: D. Reidel.
↑ Ashley, A.; Duriscoe, D.; Luginbuhl, C. (2017). "Measuring the color and brightness of artificial sky glow from cities using an all-sky imaging system calibrated with astronomical methods in the Johnson-Cousins B and V photometric systems". American Astronomical Society, AAS Meeting. 229: 236.20. Bibcode:2017AAS...22923620P.
↑ Bierman, A. (2012). "Will switching to LED outdoor lighting increase sky glow?". Lighting Research and Technology. 44 (4): 449–458. doi:10.1177/1477153512437147. S2CID110024170.
↑ Rich, C.; Longcore, T., eds. (2006). Ecological Consequences of Artificial Night Lighting. Washington; Covelo; London: Island Press.
↑ C.B. Luginbuhl (2001). R. J. Cohen; W. T. Sullivan III (eds.). "Why Astronomy Needs Low-Pressure Sodium Lighting". Preserving the Astronomical Sky, IAU Symposium No. 196. 196: 81–86. Bibcode:2001IAUS..196...81L.
↑ Warrant, Eric; Dacke, Marie (1 January 2010). "Visual Orientation and Navigation in Nocturnal Arthropods". Brain, Behavior and Evolution. 75 (3): 156–173. doi:10.1159/000314277. PMID20733292. S2CID22906227.
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