In Context

Though light at night has brought indisputable benefits, its excessive and unregulated use has been problematic.

In 2007, the United Nations estimated that for the first time in history the world had a predominantly urban population: 51 percent urban versus 49 percent rural.

Light pollution is a direct outgrowth of industrialization. While at least 2,500 individual stars should be visible in an unpolluted night sky, in a typical suburb only 200 to 300 stars are visible, and in a city, that number is often fewer than a dozen. According to astronomer Andrew Fraknoi, it is estimated that as many as 80 percent of people in the world have never seen the Milky Way.

In the film The City Dark, filmmaker Ian Cheney describes light pollution as light reflecting off moisture and dust in the air, creating a glow in the sky. The most common cause of light pollution is over-illumination — streetlights and outdoor security lights that spill outward, billboard and landscape lights directed upward and businesses like convenience stores and gas stations operating with excessive light output. Though light at night has brought indisputable benefits, its excessive and unregulated use has been problematic. According to the Institute for Local Self-Reliance, each night almost one third of the light used outdoors escapes into the night sky, where, rather than providing useful illumination, it causes glare, sky glow and other types of light pollution. According to estimates from the International Dark-Sky Association, each year in the United States more than $1 billion dollars are spent to generate this wasted light.

While there has been no global-scale data on the distribution and magnitude of artificial sky brightness ("ecology of the night" is a fairly recent area of study), many amateur astronomers use the Bortle Dark-Sky Scale to rate the night sky. The scale ranges from class 1 — the darkest skies — to class 9 — skies typical of the inner city. Class 1 skies can be found in places like remote areas of Chile, Antarctica or the Australian outback. According to a comprehensive study of night sky quality conducted by the U.S. National Park Service, the Natural Bridges National Monument in southeastern Utah is the only Bortle class 2 location in the United States. New York City rates as class 9.

According to Jane Brox (the author of Brilliant: The Evolution of Artificial Light), the brightest spots on the map correspond to prosperity and urbanization rather than population density. The National Bureau of Economic Research released a report in 2011 proposing satellite images be created of light at night and used as an indicator for measuring GDP growth. As of 2012, both Western Europe and the Eastern seaboard of the United States are brighter than any place in China or India.

To find the darkest skies near you, visit the International Dark-Sky Association (IDA) at and access the Dark Sky Finder or the Clear Sky Chart, which shows sky conditions over the following 48 hours for specific observation sites in North America (over 3,500 currently listed).

Photo caption: Lighting store owner Larry Birnbaum shows off his collection of antique and modern lighting.   Credit: Wicked Delicate Films LLC.

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