Skyglow 580076 210698910 2008-05-07T00:38:31Z Scetoaux 817661 Reverted edits by [[Special:Contributions/69.9.206.246|69.9.206.246]] ([[User talk:69.9.206.246|talk]]) to last version by Keilana [[Image:México City at Night 2005.jpg|350px|thumb|right|[[Mexico City]] at night, with a brightly illuminated sky.]] '''Skyglow''' (or '''sky glow'''), is a kind of [[light pollution]], visible by the "glowing" effect seen in the skies over many cities and towns. Skyglow often refers to man-made light, but the term also includes natural sources of diffuse nighttime light, such as the [[zodiacal light]], light from faint stars, and natural [[air glow]] emitted high in the atmosphere<ref name="The Light of the Night Sky"> {{cite book | title = "The Light of the Night Sky" | author = F.E. Roach and Janet L. Gordon | publisher = D. Reidel (Dordrecht-Holland/Boston-USA) | date = 1973 }}</ref>. ==Negative effects== Skyglow, and more generally [[light pollution]], has many diverse 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 [[bird|birds]] (see [http://www.flap.org Fatal Light Awareness Program (FLAP)]) and other biological systems<ref name="ECANL Proceedings"> {{cite book | title="Ecological Consequences of Artificial Night Lighting" | author = C. Rich, T. Longcore (eds.) | publisher = Island Press (Washington; Covelo; London) | date = 2006 }}</ref>, including humans. Skyglow is a prime problem for [[astronomer|astronomers]], because it reduces [[contrast]] in the [[night sky]] to the extent where it may become impossible to see all but [[brightest stars|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 [[Sodium vapor lamp|low-pressure sodium]]) - it is more accurate to say that by leaving large portions of the spectrum relatively unpolluted, the narrow-spectrum [[emission spectrum|emission]] from [[Sodium vapor lamp|low-pressure sodium]] lamps allows more opportunity for astronomers to "work around" the resulting light pollution<ref name="IAU Symposium 196 article"> {{cite journal | title = "Why Astronomy Needs Low-Pressure Sodium Lighting" | author = C.B. Luginbuhl, in "Preserving the Astronomical Sky," IAU Symposium No. 196, eds. R. J. Cohen and W. T. Sullivan, III , pp. 81-86, 2001 | publisher = PASP, San Francisco, USA | date = 2001 }}</ref>. 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]]. Fainter sights like the [[Zodiacal light]] and the [[Andromeda Galaxy]] are nearly impossible to discern even with telescopes. ==Causes== There are several causes of sky glow. These causes mainly differ in source. For example, public lighting provides a different form of light pollution than attention-grabbing [[strobe|strobe lamps]], and these differ from commercial lighting installations. Light from [[electric light|electric lamps]] shines directly upward into the [[atmosphere]] from poorly shielded fixtures, and reflects from surfaces like the ground or streets into the sky. Some of this light is then scattered in the atmosphere by [[molecule|molecules]] and [[aerosol|aerosols]] back toward the ground, causing skyglow. ==Mechanism== There are two causes of the light scattering that lead to airglow: scattering from molecules such as N<sub>2</sub> and O<sub>2</sub> (called [[Rayleigh scattering]]), and that from [[aerosols]], called [[Mie theory|Mie scattering]]. 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]] loading caused by modern industrial processes and transportation. [[Rayleigh scattering]] 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 lamps]] and [[fluorescent lamp|fluorescent lamps]]) 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 lamps]]). Another effect that makes skyglow from white light sources worse than from yellow arises from the [[Purkinje effect]], 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<ref name="IDA/IESNA Article"> {{cite web | url = http://resodance.com/ali/bluskies.html | title = IDA/IESNA Article on Skyglow | accessdate = 2007-12-11 | author = | publisher = }}</ref> and a 2003 presentation to both the [[International Dark-Sky Association]] Conference and the [[Illuminating Engineering Society of North America]],<ref name="IDA/IESNA Powerpoint"> {{cite web | url = http://www.mindspring.com/~keithdm@earthlink.net/scatter.pps | title = IDA/IESNA Powerpoint on Skyglow | accessdate = 2007-12-11 | author = | publisher = }}</ref> 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]] 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. ==Measuring sky glow== Astronomers have used the [[Bortle Dark-Sky Scale]] to measure sky glow ever since it was published in ''[[Sky & Telescope]]'' magazine.<ref>{{cite news | last=Bortle | first=John E. | title=Observer's Log — Introducing the Bortle Dark-Sky Scale | date=[[February 2001]] | publisher=Sky & Telescope | url=http://skyandtelescope.com/printable/resources/darksky/article_84.asp}}</ref> The [[Bortle Dark-Sky Scale|Bortle Scale]] rates the darkness of the sky, inhibited by sky glow, on a scale of one to nine, providing a detailed description of each position on the scale. ==References== {{Reflist}} ==See also== *[[Campaign for Dark Skies]] (CfDS) ==External links== *[http://www.star.le.ac.uk/~dbl/cfds/skyglow.htm Skyglow: the effect of poor lighting] ([[CfDS]]) (examples of skyglow in the [[United Kingdom|UK]]) *[http://www.streetlights.us/ Skyglow across the Great Lakes] (examples of skyglow in the [[United States|US]]) *[http://home.earthlink.net/~sac8/SkyglowBits.htm Filtering Skyglow] (from [[Charge-coupled device|CCD]] [[camera]]s) *[http://www.star.le.ac.uk/~dbl/cfdsdisk/cfdsdisk/Towns%20and%20skyglow/ Towns and Skyglow] ([[United Kingdom|UK]] skyglow image collection) *[http://resodance.com/ali/bluskies.html 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) *[http://www.mindspring.com/~keithdm@earthlink.net/scatter.pps 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) [[Category:Light pollution]] [[pt:Sky glow]]