When plans are drawn for preserving urban environments, they often account for important systems like watersheds, transportation corridors, and infrastructure. In Tucson, and throughout southern Arizona, the health of cities is also measured in light pollution — with a singular goal of maintaining dark skies.

Of the 43 observatories, university departments, and institutions associated with astronomy and space exploration in Arizona — accounting for a statewide impact of $252 million in 2006 alone, according to a new report — 31 are located in southern Arizona. Because of these facilities, Tucson is known as Optics Valley, a legacy begun in 1916 when the University of Arizona, in central Tucson, established the region’s first major observatory. Since then, the university has become “a world leader in space-borne exploration of the solar system,” according to its UANews.org website. For example, its planetary sciences department and Lunar and Planetary Laboratory “do more space science research by far than any other U.S. university.”

Kitt Peak National Observatory. Photo courtesy NOAO/AURA/NSF.

These, and the university’s Steward Observatory Mirror Laboratory, are critical for Optics Valley. But the dark skies initiative has the specific goal of benefiting the region’s observatories and telescopes, including Kitt Peak National Observatory, Mt. Graham International Observatory, the Multiple Mirror Telescope Observatory, and the Fred Lawrence Whipple Observatory on Mt. Hopkins.

Concerns over the impact of light pollution on local observatories led to passage of the City of Tucson/Pima County Outdoor Lighting Code (PDF) in 1994. The inaugural code’s purpose was “to provide standards for outdoor lighting so that its use does not unreasonably interfere with astronomical observations.” Fourteen years later, the code continues to be a model for dark sky ordinances nationwide, though the stated intent, with the 2006 amendment, has broadened: “The purpose of this code is to preserve the relationship of the residents of Tucson/Pima County to their unique desert environment through protection of access to the dark night sky. Intended outcomes include continuing support of astronomical activity and minimizing wasted energy….”

The world atlas of the artificial night sky brightness. Graphic courtesy Royal Astronomical Society.

With rapid development in southern Arizona, light pollution continues to be a concern. That’s one reason the International Dark-Sky Association set up shop in Tucson, though its geographic scope is much broader, as it “seeks to preserve dark skies worldwide for the benefit of society by promoting good outdoor lighting practices and educating the public on the rewards of preserving the stars.”

Though the ordinance is in place, the future of dark skies over urban (and urbanizing) environments remains — if you’ll excuse the pun — hazy.