Light pollution (also known as photopollution, luminous pollution) is excess or obtrusive light created by humans. Among other effects, it can obscure all but a few stars to city dwellers, render astronomical observatories useless, and disrupt ecosystems.

Unlike most forms of pollution, light pollution isn't persistent; turn the lights off, and the dark sky comes back immediately. The difficulty is in actually getting this to happen. Like other pollution, it is a side effect of industrial civilization: it comes from sources such as domestic lighting, offices, factories, streetlights, and lit sporting venues.

Light pollution is most severe in the highly industrialised, densely populated areas of the United States, Europe, and Japan, but even relatively small amounts of light can affect sensitive instruments. When a city grows near an observatory, so does the orange glow of light pollution, known as skyglow and quantified using the Bortle scale . Most major optical observatories are surrounded by zones many kilometres in diameter wherein light emissions are severely restricted. In 1980, San Jose, California, replaced all street lamps with low pressure sodium lamps, whose light is easier for nearby Lick Observatory to filter out. Similar programs are now in place in Arizona and Hawaii.

Light pollution is not just a concern for astronomers. Light shining into the eyes of pedestrians and drivers -- glare -- can reduce night vision for more than an hour after exposure. The deep shadows created by poorly directed home security lighting can give intruders more places to hide.

Some researchers believe that light pollution affects human, animal, and insect behaviors, disrupting ecosystems. Marianne Moore, who studies zooplankton at Wellesley College, believes that light pollution around lakes prevents fish from eating surface algae, helping cause algal blooms that can kill off the lakes' plants. It may affect ecosystems in other ways; for example, by altering the behavior of moths that pollinate night-blooming flowers. Many lepidopterists and entomologists (such as Kenneth Frank ) have documented that night-time light may interfere with moths' ability to navigate; Michael Mesure, founder of the Fatal Light Awareness Program , believes that it does the same to birds.

The Journal of the National Cancer Institute published two 2001 studies arguing that there is "an association between exposure to light at night and breast cancer risk." As always in new fields of scientific study, more research is needed.

Light pollution can be reduced by shielding street lamps so that they light the street below and not the sky above, and by turning off unneeded outdoor lights: for example, only lighting football stadiums when there are people inside. Lower power on night-time light will also help minimize glare.

Organisations

International Dark-Sky Association (IDA) Campaign for Dark Skies (CfDS)

See also

• light trespass
• skyglow
• glare
• lighting regulation
• lighting
• pollution
• light
• dark-sky movement
• visual pollution
• astronomy
• ecology

External links

• LightLynx List
• Open Directory Project: Light Pollution
• IDA Links and Web Resources
• International Dark Sky Association (IDA)
• Ecology of the night
• How many stars ...? A unique experiment!
• Citizens for Responsible Lighting
• Malta Astronomical Society Light Pollution Awareness Group
• Santa Clara Valley Lighting & Lick Observatory
• The Discover article relating light pollution to insects, birds, and breast cancer
• The night sky in the World
• Light Pollution Awareness Site (USA)
• New England Light Pollution Advisory Group
• DarkSky List Forum discussion group and resource library
• Campaign for Dark Skies (UK)
• The Urban Wildlands Group "Ecological Consequences of Artificial Night Lighting Conference
• SELENE New York