TUCSON, Ariz. - For astronomers here, keeping the sky dark is a matter of their survival. For business owners, keeping the sky lighted up means much the same thing.
Both sides are clashing over one of the nation's stiffest county ordinances intended to keep nighttime skies dark around a pair of observatories.
Nearby business owners are lobbying Pima County to modify the ordinance, which regulates outdoor lighting levels for new construction and extensive renovations, before it takes effect in late September.
Local businesses, including a chain of convenience stores, oppose the measure because they say it would jeopardize the safety of their customers and employees.
While hundreds of communities have had outdoor lighting codes for decades, the law is the first to establish a per-acre limit on the amount of light in a large metropolitan area.
The restrictions impose stiffer rural-area curbs on new development near the Whipple Observatory on Mount Hopkins and the National Optical Astronomical Observatory on Kitt Peak.
Pima County's code ranges from 300,000 lumens per acre for outdoor lighting at commercial sites - if the lights are shielded - to 150,000 in largely residential areas and 65,000 in rural areas.
Lumen is a measure of the amount of visible light from an electric light or lamp. A 100-watt light bulb, for instance, gives off 10,000 lumens.
''No one outside Arizona has imposed a lumens-per-acre limitation,'' said Chris Luginbuhl, an astronomer with the United States Naval Observatory in Flagstaff.
The law is only the third in the country to limit lumens per acre. Flagstaff, where two observatories are located, and Cottonwood allow up to 100,000 lumens per acre.
Southern Arizona is home to a number of large telescopes and is considered the nation's best mainland location for astronomical observations because of low levels of humidity and light pollution.
Yet Tucson's growth has already increased the amount of light scattered into the sky on Kitt Peak's eastern horizon by 10 percent to 20 percent over the last decade, said Kitt Peak director Richard Green.
Astronomers and scientists are among the ordinance's strongest supporters. They have watched as Tucson and surrounding communities grow, creating an expanding light field that competes with the stars overhead.
But Lawrence Hecker, an attorney representing Circle K stores, noted that the industry standard for lighting - and the level found at new outlets - is about 900,000 lumens, more than three times the new code's limit.
''We are in agreement with the intent and purpose of the proposed ordinance but feel that it goes too far and doesn't strike a reasonable balance with safety,'' he said.
Hecker said attracting customers to area stores and gas stations is a secondary consideration in the light debate. The motivation is safety, he said.
County Supervisor Mike Boyd said he voted against the ordinance Monday out of concern for ''the mom who's got to go out for the gallon of milk after hours.''
Some supporters of the ordinance argue businesses owners simply want more light to catch more people's attention.
''They're using a bunch of scare tactics,'' said Jim Singleton, chairman of the Tucson-Pima County-Marana outdoor lighting committee that recommended the ordinance.
Green believes opponents are sincere in citing safety concerns, but said that's where engineering can come into play. For instance, the new county sheriff's facility in Flagstaff used shielding to keep its lighting within 50,000 lumens per acre.
''I can't emphasize enough that this is not a conflict between astronomer darkness and safety,'' Green said. ''Lighting experts believed that nighttime activity can be safely supported at the lighting levels accepted in this new code.''
On the Net:
International Dark Sky Association: http://www.darksky.org
Tosco Corp. for Circle K: http://www.tosco.com
Pima County: http://www.co.pima.az.us
Whipple Observatory: http://cfa-www.harvard.edu/cfa/ep/flwo.html