City lights have long been a nuisance for astronomers. They create a haze of light that washes out faint stars and fogs photographic plates.
But with the explosive growth of cities, light pollution is no longer a nuisance. It is a crisis that threatens the future of humanity's oldest science, according to astronomers.
Many major observatories have been so severely affected by city lights that they can do only minimal astronomical research. Because the rapid growth of cities is expected to continue, some scientists predict the end of ground-based optical astronomy within two to three decades.
The situation of Kitt Peak National Observatory, in the Quinlan Mountains of southern Arizona, exemplifies the threat and possible ways to avert it. The observatory has felt the encroachment of neighboring Tucson's lights, and fought back.
''We are not going to allow (light pollution) to hurt the observatory,'' says William T. Robinson, lighting consultant at Kitt Peak. ''We must not allow that to happen.''
The problem lies in the mercury vapor streetlamps that emit a low-frequency light which is easily scattered by dust and hydrocarbon pollutants suspended in the air. The scattering spreads the light across the sky, interfering with astronomical observations.
Such streetlamps generate light by passing an electric current through a small quantity of mercury vapor. Although the lamps provide adequate illumination, most of their light lies in the ultraviolet and blue range of the spectrum, useless to the human eye and positively harmful to astronomy.
In addition to optical drawbacks, mercury vapor lamps are very expensive to operate.
''A couple decades ago, mercury vapor lamps were the best lights available,'' Mr. Robinson explains. ''They were inexpensive to buy, and although they were expensive to operate, no one was concerned about energy costs before 1973 (the year of the Arabian oil embargo).''
Now, however, energy bills are climbing steadily, squeezing municipal budgets as well as those of private consumers.
Working with Kitt Peak scientists, the city of Tucson found a solution to the double-edged problem of astronomically high energy bills and astronomical pollution: sodium lights.
Passing an electrical current through sodium gas instead of mercury vapor produces a pink or yellow-orange light, depending on the type of lamp used. Since the human eye evolved in the yellow light of the sun, our sight is more attuned to the higher-frequency light of the yellow range of the spectrum. In addition, energy costs of sodium lamps are considerably lower than for mercury lights.
With 19 telescopes at different sites under the auspices of Kitt Peak, and more than 100 astronomers in the area, Tucson is one of the world's largest astronomical centers. The city is proud of its international reputation as a scientific hub and has proved itself more than willing to help its astronomers.
In 1973 the city council passed a light pollution ordinance designed to reduce the problem by requiring mercury vapor lamps to be provided with special shields to direct light downward onto the streets instead of allowing the light to spread into the sky. The ordinance also encouraged installation of sodium lamps when the old mercury vapor lights failed.
Working with Robinson, Tucson has begun a more aggressive campaign to replace existing mercury vapor lights with sodium lamps. Two years ago the city began its Comprehensive Roadway Illumination Project, a five-year plan that takes into consideration traffic safety, benefit-cost ratios, public reaction, and utility bills as well as the needs of astronomers.
City planners have a choice between two types of sodium lamps: low-pressure lights, in which the vacuum in the lighting bulb is very strong; and high-pressure sodium lights, in which the vacuum is weak.
Astronomers prefer low-pressure lamps, because the light produced is in a narrow bank of yellow light, easily filtered out or ignored. The high-pressure lamps generate a wider band of light in the pink range.
Richard Nassi, Tucson city traffic engineer in charge of outdoor lighting, points out that low-pressure sodium lights have disadvantages that might limit their use in a city.
The strong yellow color generated by the lights tends to wash out other colors, he explains, making the lights less desirable for commercial businesses. In addition, the light obscures traffic lights, which could cause problems at intersections.
Low-pressure sodium lights, however, are adequate for normal illumination and security needs, he adds. A test area around the University of Arizona campus is being planned to determine popular reaction to the fixtures.
High-pressure sodium lights are being placed along heavily traveled main traffic arteries, according to Isaac Shaffer, the high-voltage electrician supervisor for Tucson, who has worked extensively with the sodium lamps.
''We just converted 400 lights from mercury vapor to high-pressure sodium,'' he said. ''This will save about $15,500 a year in utility bills, enough to light 2,500 homes for a year.''
Costs for converting a three-mile stretch of Tucson's main traffic corridor are expected to run around $64,000, but savings on energy bills will pay for the conversion in three years, Mr. Shaffer adds.
According to Mr. Nassi, Tucson expects to convert all its main streets by 1985, as well as sports fields and tennis courts. Public reaction has been very favorable, he says.''
People are always calling my office to say how much they like the sodium lights and asking us to convert (the lights in) their neighborhoods,'' Nassi said.
Public support for the changes in streetlights has been an important consideration in the battle against light pollution.
Robinson has spent two years visiting each community in southern Arizona to convince local authorities of the benefits of sodium lights.
He feels that by working at the local level rather than having the state government impose a lighting code, there is greater support for the changes and more vigorous enforcement of the ordinances.
''It's really a matter of getting out and talking to the communities,'' he said. ''We try to appeal to everyone's natural inclination to save money and preserve the United States' scientific prestige.''
The argument has been remarkably successful. There are many people sincerely interested in astronomy, Robinson explains, and for those to whom astronomical problems are distant and abstract, there is the appeal to the pocketbook. Either way, astronomers benefit.
Robinson is ready to start another two-year campaign to bring sodium lights to the rest of the state. He plans to start with Maricopa County, Arizona's largest county and seat of the capital, Phoenix.
The Maricopa Association of Governments has already received initial proposals. It has formed a committee to study the planned lighting changes. Robinson will visit the county this fall to explain the proposals more fully.
Since its passage in 1973, Tucson's lighting ordinance has become a model for similar codes throughout the country.