Each night almost of a third of the light used out-of-doors escapes into the night sky where, instead of providing useful illumination, it causes glare, sky glow and other types of light pollution. About 2.500 individual stars should be visible to the human eye in an unpolluted night sky; but in a typical suburb only 200 to 300 stars are visible, and in a city, fewer than a dozen stars may shine through the artifically lit sky.
In addition to compromising the quality of the environment, light pollution amounts to an enormous amount of wasted energy. The International Dark-Sky Association estimates that each year in the United States, over $1 billion dollars is spent to generate this wasted light.
The most common causes of light pollution are streetlights that fail to deliver all of their light downward, outdoor security lights around buildings, billboards lit from below, landscape illumination directed upward, and businesses like convenience stores and gas stations that operate under extremely high levels of illumination.
Many of municipalities, counties and states have adopted, or are considering adopting, ordinances designed to limit light pollution. We have listed a few of the strategies that stand out. The International Dark-Sky Association is the primary on-line resource for light pollution information.