Tuesday, June 8, 2010; HE03
Time was, the stars in the sky epitomized the very concept of countlessness. "Innumerable as the stars of night," wrote Milton. If the poet's contemporaries had tried enumerate the twinkling beacons above, they might have been able to make out 5,000 or more with the naked eye on a clear, moonless night. Today, a stargazing city-dweller would be lucky to identify a few dozen distinct points of light overhead, even under optimal meteorological conditions. And just one in three Americans can see the Milky Way from where they live.
What happened to the stars? They got polluted. Polluted by light.
It's not the stars themselves that have vanished, but rather the inky-black backdrop against which they used to be visible. Artificial light, cast upward from our cities and roads, has washed out the natural darkness. It has obscured the obscura. It has made the night false.
Here's what happens every second of every night: Billions of photons -- those "quantum particles" of light you might remember from physics class -- depart from earthbound light sources and shoot in all directions, including into the sky. There, they can bounce off dust and moisture in the air and change course, sometimes heading back down to Earth, but they may land miles from where they started.
To a ground-based observer at a photon-landing site -- and pretty much every spot on Earth is a landing site for artificially produced photons -- the sky itself appears to glow with the reflected light. The dustier or more humid the air, the greater is the potential for this form of light pollution, called sky glow. On overcast nights, the clouds are bathed in our discarded light, their underbellies acting like massive reflectors.
Even on clear nights, sky glow impairs star-viewing. Its brightness keeps our eyes from adjusting to the darkness and thus inhibits their ability to pick out faint light. A bright moon can have the same effect on our night vision. In addition, sky glow forces us to peer at the heavens through a veil of light, reducing the contrast that helps the brain identify faint stars.
Sky glow isn't the only kind of light pollution. "Light trespass" illuminates something that's not intended to be lit. Street lamps and porch lights are supposed to brighten roadways, sidewalks and doorways, but when they shine through the windows of your bedroom, they contribute to America's sleeplessness, and help the sales of blackout shades.
Light pollution comes from some sources that may seem innocuous, even comforting. Security floodlights, store and restaurant signs, and sleeping office buildings all cast light into the night sky. More conspicuously, so do billboards, baseball stadiums and car dealerships.
We may think of these lights as essential for safety or commerce. But often they're not. Why leave the lights on in an umpteen-story office building? That practice not only showers the sky with excess photons, it also wastes electricity and disorients night-flying birds, which have crashed fatally into illuminated structures in a phenomenon known as tower kill.
There's also evidence that all that artificial light may contribute to health problems by disrupting the body's circadian rhythms. Researchers in Israel, for example, have found that people living in brightly lit communities were more likely to develop breast cancer than those residing in darker parts of the country.
Very few lights are actually intended to shine into the sky. Most that shine upward -- or even sideways -- do so more or less accidentally. (There are legitimate safety reasons for telecommunication towers, skyscrapers, runways and bridges to be visible from above. But that's about it.) Consider that neighbor's poorly aimed porch light, the one piercing the curtains of your bedroom window. The Joneses are not trying to illuminate your boudoir (presumably), but they've done so just the same.
Light-pollution watchdogs such as the International Dark-Sky Association promote the use of light fixtures designed to limit that sort of problem. Fixtures that properly shield the bulbs inside, for example, can keep light aimed where it's needed and prevent it from trespassing elsewhere.
Maybe you've seen that Earth-at-night photo of our globe, a composite of hundreds of satellite images melded into one. On it, the world's oceans are dark, and underdeveloped continents mostly a dim gray. One can easily pick out major cities -- each is a brilliant point of light, a sort of anti-star -- and America's thickly settled Eastern Seaboard is like a Milky Way in reverse.
It's a beautiful image, until one pauses to reflect on why so much artificial light reached the orbiting cameras that recorded it. Every contributing photon had been wastefully sent into the sky. The image itself is both a product and a symptom of the global scale of light pollution.
Even in the most pristine corners of the United States, light pollution is nearly impossible to escape. In Great Basin National Park in Nevada, for example, the southern horizon is permanently illuminated at night by the sky-reflected glow of Las Vegas, which is about 200 miles away and lies below the horizon. According to research led by Chad Moore of the National Park Service, light from Sin City reaches more than half a dozen other national parks as well.
But it's still worth seeking out wild places where sky glow is minimal, if only because you might also encounter natural kinds of glow. Two to look for: the gegenschein and zodiacal light.
These peculiar phenomena occur because the orbits of the planets lie in a thin disk of interstellar dust. The sun's light reflects off that dust, just as artificial light reflects off dust in the air. Long after sunset, and hours before dawn, this reflected solar light faintly illuminates portions of the sky. At times, if the rest of the sky is dark enough, it paints a pale oval at the heavenly point directly opposite the sun's position. This is the gegenschein, German for "counter-shine."
The zodiacal light, on the other hand, is created by space dust closer to the horizon, which can also deflect the hidden sun's beams. That creates a thin pillar of light that follows the sun's daytime path. This so-called zodiacal light is sometimes called false dawn.
Better to see a false dawn, I say, than a false night.
Harder is general manager of health and science at U.S. News & World Report.