Kitt Peak in southern Arizona is blessed with two of the attributes astronomers most value in a telescope site, high elevation and low humidity, both of which reduce the atmospheric distortion of starlight before it reaches a ground-based telescope peering into the night sky. Add in generally clear weather, and it is no wonder that more than two dozen telescopes now dot the 2,100-meter summit. The National Optical Astronomy Observatory has operated the Kitt Peak National Observatory there since 1958, and several university consortia and research groups have built telescopes on the mountain as well.

A lot has changed in the Southwest since 1958, however. Between 1960 and 2000, the population of Tucson, just 70 kilometers northeast of Kitt Peak as the crow flies, swelled from 213,000 to 487,000. In the same time span, Tucson's land area nearly tripled.

But as the city sprawled outward, Kitt Peak has remained relatively dark, at least over the past two decades or so, according to a study in the October issue of the Publications of the Astronomical Society of the Pacific. Kathryn Neugent and Philip Massey of Lowell Observatory in Flagstaff, Ariz., measured the brightness of the night sky over Kitt Peak in 2009 and 2010 and compared the results to similar readings taken by Massey and his colleagues in 1988 and 1999.

Taking into account variations in the sun's irradiance at different phases of the solar cycle, Neugent and Massey found that the night sky directly above Kitt Peak is only slightly brighter than it was in 1988, and that the brightness has remained stable since 1999. In fact, looking closer to the horizon toward Tucson, Neugent and Massey found that the sky had actually darkened a bit since 1999.

The researchers credit local lighting ordinances with reining in light pollution in the area even as populations—and the urban centers themselves—have swelled. The city of Tucson and surrounding Pima County have outdoor lighting codes designed to save energy and protect the region's dark skies. (Astronomy is big business in and around Tucson, which is home to one of the country's leading astronomy departments, at the University of Arizona.)

The codes cap the total lumens per acre, require that billboard lights be aimed downward rather than skyward, and set nighttime curfews for illuminated outdoor signs, among other measures. Those codes, Neugent and Massey report, "have effectively stopped the impact of household, commercial and outdoor lights on the night sky."

Night sky above Kitt Peak: NOAO