1988

Light Pollution

To our ancestors, the night sky was a source of reverence, inspiration, and wonder. On a clear, moonless night it was possible even from the cities to see thousands of stars with the unaided eye, including the grand, sweeping arch of the Milky Way. But the advent of modern civilization, and especially the growth of major cities and urban centers and their widespread use of electricity for artificial illumination, have significantly changed our relationship with the night sky. Rather than thousands of stars, most people in industrialized countries can now usually see only hundreds of stars on a clear night; residents of major cities might be lucky to see just 10 or 20 stars and a slew of airplanes, but certainly not the Milky Way. The night sky has lost its wonder for most people, becoming instead a dull, faintly glowing, and featureless part of the background.

The culprit in this nocturnal cosmic dulling is light pollution, the alteration of natural outdoor light levels by artificial light sources. Light pollution obscures fainter stars for people living in cities or suburbs, interferes with astronomical observations of faint sources, and can even have an adverse effect on the health of nocturnal ecosystems. It’s also economically inefficient—the point of lighting a home or building at night is to light the home or building, not to spend money and kilowatts lighting up the night sky.

In recognition of the growing global problem of light pollution, in 1988 a group of concerned citizens formed an organization known as the International Dark-Sky Association (IDA), with the mission “to preserve and protect the nighttime environment and our heritage of dark skies through quality outdoor lighting.” IDA now has about five thousand members worldwide who work with city and local governments, businesses, and astronomers to raise awareness about the value of dark skies and to help implement lighting solutions that are more energy efficient and economical and that lead to less light pollution.

Despite some notable successes establishing ordinances and building codes that are decreasing light pollution, the effect on astronomy continues to limit the utility of major observatories near large cities (such as the Mount Wilson Observatory, perched above Los Angeles). New telescopes are now usually built in remote deserts or on isolated dark mountaintops to escape the night sky’s growing glow.

A map of artificial night sky brightness for part of the Western Hemisphere, from the US Defense Meteorological Satellite Program. The reddest places, mostly in the eastern and western United States, are where light pollution makes the night sky nearly 10 times brighter than the natural sky.