![Light pollution europe](https://upload.wikimedia.org/wikipedia/commons/e/e3/Light_pollution_europe.jpg)
Skyglow (or sky glow) is the diffuse luminance of the
night sky
The night sky is the nighttime appearance of astronomical object, 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.
Natural light sources in a ...
, apart from discrete light sources such as the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
and visible individual
stars. It is a commonly noticed aspect of
light pollution
Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
. While usually referring to luminance arising from artificial
lighting
Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylig ...
, skyglow may also involve any
scattered light seen at night, including natural ones like
starlight,
zodiacal light
The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction ...
, and
airglow
Airglow (also called nightglow) 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 diffu ...
.
In the context of light pollution, skyglow arises from the use of artificial light sources, including electrical (or rarely
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
) lighting used for illumination and advertisement and from
gas flare
A gas flare, alternatively known as a flare stack, flare boom, ground flare, or flare pit is a gas combustion device used in places such as petroleum refineries, chemical plants and natural gas processing plants, oil or gas extraction sites ha ...
s. 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
A developed country (or industrialized country, high-income country, more economically developed country (MEDC), advanced country) is a sovereign state that has a high quality of life, developed economy and advanced technological infrastruct ...
.
Causes
![suburban night sky](https://upload.wikimedia.org/wikipedia/commons/d/d4/Suburban_night_sky.jpg)
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
Signage is the design or use of signs and symbols to communicate a message. A signage also means signs ''collectively'' or being considered as a group. The term ''signage'' is documented to have been popularized in 1975 to 1980.
Signs are any ...
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 ), 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.
In most communities, direct upward emission averages about 10–15%.
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. In remote areas, snow brightens the sky, but clouds make the sky darker.
Mechanism
There are two kinds of light scattering that lead to sky glow: scattering from molecules such as N
2 and O
2 (called
Rayleigh scattering
Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of th ...
), 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
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
: the effects approximately balance when the observation point is near the light source.
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 ).
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. All-sky maps of skyglow brightness are produced with professional-grade imaging cameras with
CCD detectors and using stars as calibration sources.
Amateur astronomers have used the
Bortle Dark-Sky Scale
The Bortle scale (also known as the Bottle 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 li ...
to approximately quantify skyglow ever since it was published in ''
Sky & Telescope
''Sky & Telescope'' (''S&T'') 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;
*reviews of astronomic ...
'' 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. Amateurs also increasingly use
Sky Quality Meters (SQM) that nominally measure in astronomical photometric units of visual (
Johnson
Johnson is a surname of Anglo-Norman origin meaning "Son of John". It is the second most common in the United States and 154th most common in the world. As a common family name in Scotland, Johnson is occasionally a variation of ''Johnston'', a ...
V) magnitudes per square arcsecond.
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
:
which is known as "Walker's Law."
Walker's Law has been verified by observation
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 50 km) 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 with 4100K CCT LED Lighting](https://upload.wikimedia.org/wikipedia/commons/c/c9/Sky_Glow_with_4100K_CCT_LED_Lighting.png)
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
Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of th ...
of short wavelengths (see ).
When observing the night sky, even from moderately light polluted areas, the eye becomes nearly or completely dark-adapted or
scotopic
In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek ''skotos'', meaning "darkness", and ''-opia'', meaning "a condition of sight". In the human eye, ...
. 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
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visua ...
eye. Predominantly because of this effect, white light sources such as
metal halide
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, ...
,
fluorescent
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, ...
, or white
LED can produce as much as 3.3 times the visual sky glow brightness of the currently most-common
high-pressure sodium
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.
Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are ...
lamp, and up to eight times the brightness of low-pressure sodium or amber
Aluminium gallium indium phosphide
Aluminium gallium indium phosphide (, also AlInGaP, InGaAlP, GaInP, etc.) is a semiconductor material that provides a platform for the development of novel multi-junction photovoltaics and optoelectronic devices, as it spans a direct bandgap fro ...
LED.
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.
The effect of
Rayleigh scattering
Rayleigh scattering ( ), named after the 19th-century British physicist Lord Rayleigh (John William Strutt), is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of th ...
on skyglow impacts of differing light source spectra is very 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, or focuses exclusively on white LED light sources, or focuses concerns narrowly on the blue portion (<500 nm) of the spectrum. 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.
Improved efficiency (efficiency in distributing light onto the target area – such as the roadway – with diminished "waste" falling outside of the target area and more uniform distribution patterns) can allow designers to lower lighting amounts. 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
![Effect of light pollution on sky polarization](https://upload.wikimedia.org/wikipedia/commons/b/bc/Effect_of_light_pollution_on_sky_polarization.jpg)
Skyglow, and more generally
light pollution
Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
, 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
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...
s, because it reduces
contrast in the
night sky
The night sky is the nighttime appearance of astronomical object, 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.
Natural light sources in a ...
to the extent where it may become impossible to see all but the
brightest stars.
Many nocturnal organisms are believed to navigate using the
polarization signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
of
scattered moonlight. Because skyglow is mostly unpolarized, it can swamp the weaker signal from the moon, making this type of navigation impossible. 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.
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
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
. Fainter sights like the
zodiacal light
The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction ...
and
Andromeda Galaxy
The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gal ...
are nearly impossible to discern even with telescopes.
Effects on the ecosystem
The effects of sky glow in relation to the ecosystem have observed to be detrimental to a variety of different organisms. The lives of plants and animals alike (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.
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
Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...
,
commercialization
Commercialization or commercialisation is the process of introducing a new product or production method into commerce—making it available on the market. The term often connotes especially entry into the mass market (as opposed to entry into ...
, and
consumerism
Consumerism is a social and economic order that encourages the acquisition of goods and services in ever-increasing amounts. With the Industrial Revolution, but particularly in the 20th century, mass production led to overproduction—the su ...
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.
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, reproduction, migration, and communication. These changes can even result in the death of some species such as certain
migratory birds
Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by ...
, sea creatures, and nocturnal predators.
Besides the effect on animals, crops and trees are also very susceptible to destruction. The constant exposure to light has an impact of the
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
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.
See also
*
Light pollution
Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...
*
SKYGLOW
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 ...
*
Urbanization
Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...
*
Polarized light pollution
*
Over-illumination
Overillumination is the presence of lighting intensity higher than that which is appropriate for a specific activity. Overillumination was commonly ignored between 1950 and 1995, especially in office and retail environments.M.D. Simpson, ''A fl ...
*
Dark-sky movement
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, ...
*
International Dark-Sky Association (IDA)
*
Campaign for Dark Skies
The Commission for Dark Skies (CfDS) (formerly the Campaign for Dark Skies; the name was changed on March 29, 2015) is the United Kingdom's largest anti-light pollution, light-pollution campaign group forming part of the international dark-sky m ...
(CfDS)
Notes
References
{{Reflist, 30em
External links
List of peer reviewed research papers about sky glow(
CfDS) (examples of skyglow in the
UK)
Skyglow across the Great Lakes(examples of skyglow in the
US)
Filtering Skyglow(from
CCD camera
A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...
s)
Towns and Skyglow(
UK skyglow image collection)
Loss of the Nightan Android app for estimating skyglow by measuring naked eye limiting magnitude
Dark Sky Meteran iPhone app for measuring skyglow luminance
LED light pollution: Can we save energy and save the night?SPIE Newsroom article on reducing skyglow
Light pollution
Light sources
Night