for National Geographic News
As light pollution worsens across the United States, astronomers are urging citizens to protect their night skies.
"Light has little utility when it shines up into the sky and does nobody good whatsoever," said William Brown, an astronomy professor at Colorado State University in Pueblo (Colorado map).
Brown and his students use telescopes to search the sky for asteroids that could potentially crash into Earth.
"It's a very difficult process, and it just makes it far more difficult if there's excess light in the sky," he said. "These asteroids are incredibly dimthey only reflect a tiny, tiny bit of light from the sun."
So last year Brown helped enact a dark-sky ordinance in Pueblo County, which requires all outdoor light fixtures to shine downward only.
The ordinance prohibits light streaming up into the night sky, where it interferes with astronomers attempting to get a clear view.
"There are a lot of fixtures that do that," he said. "[Shining lights upward is] a terrible waste of energy and a terrible waste of a dark sky."
According to the Tucson, Arizona-based International Dark Sky Association, an estimated 1,000 cities and towns in the U.S. have enacted some form of dark-sky ordinance.
But as the population grows and cities get bigger, the problem of light pollution is getting worse, not better, Brown says.
"In quite a few places that were excellent [for stargazing] it's getting more and more difficult," he said. (Related feature: Explore an interactive star chart.)
David Crawford is the executive director of the International Dark Sky Association. He says dark skies are good for human health and safety as well as for people's pocketbooks.
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