Air pollution matters. The white haze in a blue sky consists of microscopic water droplets that have condensed on tiny solid particles, primarily sulfate dust from distant factories and power plants. These particles are the precursors of acid rain. Sulfur emissions in the United States peaked in 1970 and have since been reduced by about 30 percent. Whether these reductions will continue or backslide is an open question. But the Clean Air Act of 1970 has meant darker skies in the 1990s than we otherwise would have had.

A windy cold front sweeping through a city can clear out local pollution, leaving the night marvelously dark. The windiest city and suburban nights are often the darkest. A passing rainstorm or blizzard can also leave an unusually dark night in its wake.

A case based on money, energy, and good-looking surroundings will get you farther than one based only on astronomy when appealing for light-pollution control. Light sent into the sky wastes money. It adds to noxious power-plant emissions, dependence on foreign oil, and all the other problems created by energy profligacy. It creates an annoying, garish nighttime environment. Those are the points that will carry the most influence with the public and elected officials.

Full-cutoff fixtures that illuminate the ground efficiently with a smaller bulb can save electricity so fast that the cost of retrofitting is typically recovered in three years. After that the savings are free and clear. The city of San Diego, for example, is now saving about $3 million a year in this manner.

Full-cutoff lighting looks more pleasant. By reducing glare it improves nighttime visibility, so that motorists, for instance, can see pedestrians and objects more clearly. Many full-cutoff fixtures are becoming available in various styles. The best ones rely entirely on reflection above the bulb rather than refraction by a plastic cover hanging below it. They provide not only less waste and glare but smoother, more uniform illumination. When people see what well-designed lighting looks like at night, they want it for their own area.

When light pollution is reduced, everybody wins. This conceptual breakthrough was what led David Crawford of Kitt Peak National Observatory and his amateur colleague Tim Hunter to found the International Dark Sky Association (IDA) in 1988. Kitt Peak's successful efforts against light pollution from neighboring Tucson, Arizona, convinced Crawford and Hunter that astronomers have common cause with everyone else. Before then, most astronomers had assumed that "good lighting" was exactly what they didn't want, and that they faced a hopeless battle against the rest of the world.