Lights out : city lights are erasing the stars, wasting energy, and hurting wildlife. Find out what's being done to darken the night

A century ago, stargazers could look up and see a black sky speckled with 2,500 or so twinkling stars. They could easily make out the broad cluster of stars called the Milky Way galaxy, home to our solar system. Today, only 1 in 10 Americans can see this vision of the universe. And on a typical evening, most city dwellers glimpse just a few dozen stars. An eerie, orange sky blots out the rest.

The switch from dark nights to glowing skies began during the early 1900s, when Thomas Alva Edison invented a long-lasting light bulb that was bright enough to replace dim gas and kerosene lamps. Power lines began to snake their way across the U.S., electrifying the nation. At the same time, the country's population skyrocketed from 76 million in 1900 to over 300 million today. The result: More people living in sprawling cities and suburbs are slowly turning night into day.

Thirty years ago, astronomers were the first to express alarm about the artificial light that was obstructing the stars and planets that they study. Now, ecologists, or scientists that study animals and their environment, are also worried about the effects of this so-called light pollution.

NIGHT LIGHTS

Light pollution is caused by excess light beamed up into the sky by office buildings, street lamps, headlights, billboards, houses, and other sources.

The artificial light reflects off low clouds, or moisture and dust in the air, creating "sky glow," says Travis Longcore, an ecologist with the Urban Wildlands Group, a conservation organization in Los Angeles, California. Roughly one third of all lighting in the U.S. illuminates the sky rather than its intended target. Not only is this light wasted, but it is also damaging to the environment. According to the U.S. Department of Energy, coal-burning power plants generate 51 percent of the country's electricity. These power plants pollute the air with sulfur dioxide (a component of harmful acid rain), carbon dioxide (a heat-trapping greenhouse gas that is causing Earth's average temperature to rise), and nitrogen oxides (gases that create smog).

Wasted lighting also costs the U.S. approximately $10 billion per year in energy expenses, according to the International Dark-Sky Association (IDA), an environmental group in Tucson, Arizona.

FATAL FLIGHTS

Biologists have shown that our lit-up world also takes a toll on wildlife. Light pollution disrupts the way some animals behave. "Over millions of years, animals have come to depend on seasonal, monthly, and daily patterns of light and dark," says Longcore. "Only recently have humans lit up the night on a global scale."

Artificial lighting takes the largest toll on birds. Approximately 450 species, including most songbirds and many shorebirds, take to the wing at night for their twice-yearly migrations across North America. Like a captain at sea, they use the constellations and the moon for navigation. "When birds fly through brightly lit areas, they become disoriented," says Michael Mesure, executive director of the Fatal Light Awareness Program (FLAP), a Toronto-based environmental organization. As a result, the birds often crash into brilliantly lit buildings or broadcast towers, and die.

Sometimes whole flocks perish. According to Mesure, 50,000 birds died at Robins Air Force Base in Georgia over two nights in 1954 when they were drawn to upward-pointing spotlights. Disoriented by the bright lights, the birds flew straight into the ground. And in 1981, over 10,00O birds slammed into floodlit smokestacks at Canada's Hydrox Generating Plant near Kingston, Ontario.

ANIMALS AT RISK

Artificial lights take their toll on other nocturnal animals too. Sea turtles, which come ashore at night to lay their eggs, are very sensitive to light. Few females will venture onto lit-up beaches, like those along Florida's coastline, to nest. And newly hatched turtles need a dark night sky to orient themselves toward the sea, but artificial lights from beachfront properties lure them away. Biologists have found hatchlings wandering aimlessly on beaches, city streets, and in hotel parking lots. Once daylight arrives, the lost turtles are vulnerable to daytime predators and risk getting hit by cars as people travel to work and school.

Another study found that artificial lighting causes problems for salamanders. These nocturnal amphibians rely on the darkness of night to protect themselves from hungry predators. When exposed to artificial lighting, they remain hidden underneath leaves, waiting for later, darker hours before emerging for a meal. That might not sound like such a big deal, but "the later they come out, the less food they may be able to eat," says Sharon Wise, an ecologist at Utica College in New York, who was involved in the study.

Wise's colleague, Bryant Buchanan, had a similarly alarming finding: He discovered that artificial lights at night make nocturnal frogs stop calling to one another. If the males aren't calling, the females can't find mates to breed with, and the animals disappear.