A composite satellite photograph of the Earth at night.
Unlike most forms of pollution, light pollution isn't persistent: turn the lights off, and the dark sky comes back immediately. Like other pollution, though, it is a side effect of industrial civilization: it comes from sources such as domestic lighting, offices, factories, street lighting, 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 be important for sensitive applications - most major optical observatories have zones many kilometres in diameter severely restricting light emissions. In 1980, San Jose, California, replaced all street lamps with low pressure sodium lamps to reduce the light pollution for nearby Lick Observatory, as the light from these lamps is easier for the 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 can reduce visibility. It also reduces night vision, which takes an hour (or more) to return after exposure to bright lights. Light directed out at the viewer (away from a building) causes areas of deep shadow; much home security lighting possibly makes houses easier to be broken into, because intruders have more places to hide.
Some researchers believe that light pollution affects human, animal, and insect behaviors in other ways, disrupting ecosystems: for instance Marianne Moore (who studies zooplankton at Wellesley College) believes that light pollution around lakes prevents the surfacing of fish to eat algae from lake surfaces, helping cause algal blooms which then kills off the lake's plants. Many lepidopterists and entomologists (such as Kenneth Frank and Michael Mesuren) believe that night-time light may interfere with moths' navigational abilities, whereas Rod Crawford, founder of the Fatal Light Awareness Program, believes that it interferes with the navigational abilities of birds.
Night blooming flowers that have traditionally depended on moths for pollination may be impacted by night lighting, as there is no replacement pollinator that would not be impacted by the artificial light. This can lead to species decline of plants that are unable to reproduce, and change the long term ecology of an area.
The Journal of the National Cancer Institute has 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 saves energy and helps keep the night sky dark. Lower wattages on night-time light will also help minimize glares.
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