Artificial lighting that shines where it is neither needed nor wanted. Such inefficient lighting has many detrimental effects, including on human health, wildlife, the natural environment, and energy consumption. Light pollution is a significant problem for astronomers in or near urban areas. Light shining into the sky is reflected by particles and water vapour in the atmosphere (skyglow), overpowering the light from celestial objects and degrading the performance of telescopes. From many urban locations, only the brightest stars are now visible. The skyglow from a town or city can be visible over distances of 100 km or more. Increasing awareness of the energy wastage involved, coupled with improved designs of lighting, has led to some limitation in the spread of light pollution.

http://photojournal.jpl.nasa.gov/catalog/PIA02991