The darkness of the sky directly above Kitt Peak "has stayed remarkably constant" over the past two decades - a phenomenon that two researchers attribute to local lighting ordinances.
Kathryn F. Neugent and Philip Massey of Lowell Observatory compared light measurements they made over the past year to readings taken by Massey at Kitt Peak in 1988 and 1999.
They conclude that the brightness of the night sky at zenith is about the same as it was 20 years ago. The view toward Tucson has deteriorated significantly in that time but has stabilized in the past 10 years.
"This suggests that the strengthened lighting ordinances Tucson and Pima County established in the early 2000s have been quite effective," they say in a paper to be published in the Proceedings of the Astronomical Society of the Pacific.
The paper's overall conclusion - that the light quality of the night sky at zenith over Kitt Peak has not deteriorated in two decades - is "good news," said Christian Luginbuhl, of the U.S. Naval Observatory in Flagstaff.
But Luginbuhl questioned the finding that the view toward Tucson might have improved in the past decade. The paper's authors state they took into account the decreased solar activity during the time of their measurements. Luginbuhl said, however, that their correction was not precise enough to then conclude that the sky toward Tucson had darkened over the past 10 years by less than 0.1 magnitude (about 10 percent).
Massey said the paper concedes that the slight darkening is within his calculations' margin of error, but said he was "pleasantly surprised" at the overall results.
"The first time we reduced the data, it looked like Kitt Peak had gotten much, much brighter and I said to Kathryn 'Well, we just closed the place.' "
The team quickly discovered an error in those early calculations.
The bottom line, said Massey, is that the sky overhead is no worse than it was 20 years ago, even though the sky glow from Tucson is greater. And this occurred while Pima County added about 375,000 in population.
"We're really appreciative of the support we've been getting," said astronomer Buell Jannuzi, the former director of Kitt Peak who is cited in the paper for his "education and advocacy" about dark skies.
"There is still a lot to do as the lighting technology improves," Jannuzi said. "I'm encouraged because we continue to get support from businesses in the community."
Lighting engineer Chris Monrad said local ordinances and improvements in lighting products should help keep the skies dark in the future.
"It's basically just the application of properly engineered lighting systems that put the light where it's supposed to be - on the parking lot or the sidewalk and not shooting up into the sky."
A good example, Monrad said, is the plan to replace the lights at the University of Arizona's Wildcat Stadium, for which he is a consultant.
"We definitely put more light on the field and saved more than 50,000 watts in energy in doing it. It will bring it up to NCAA best-practice standards and reduce off-site light trespass and glare by 75 percent," he said.
Luginbuhl said more can be done.
He questioned the efficacy of recent changes to the lighting code used by Tucson and Pima County. The code allows commercial enterprises to use up to 450,000 lumens per acre - a restriction he called "no cap at all."
In Flagstaff, by comparison, the limits are 50,000 to 100,000 lumens.
"That doesn't mean the lighting codes in Tucson aren't having an effect," he said.
Luginbuhl said most of the improvement probably comes from ordinances that require lights be shielded to keep them from dispersing light into the atmosphere.
The bottom line for Kitt Peak, said Elizabeth Alvarez, assistant to the director, is that "we have worked for decades to preserve this asset and we are preserving it. This is wonderful news. Thank you, Tucson."
Contact reporter Tom Beal at email@example.com or 573-4158.