In most situations, light helps us see. But when it comes to looking at the night sky, light is actually a kind of pollution. It prevents our view of some of life's most striking sights: stars, planets, and even galaxies . "When I was a little boy, I loved the night sky. I remember looking up and the sky was filled with stars. I became an astronomer because I was amazed by their beauty," says Robert Gent, "Now in most big cities kids can't see the stars like I did." Normally, about 2,500 individual   stars are visible to the human eyes without using any special equipment. But because of light pollution, you actually see just 200 to 300 and fewer than a dozen from some cities. Only one in three Americans can see our own galaxy with the naked eyes. Those people live far away from the lights of big cities, office buildings, and shopping malls. Fortunately, there's an inexpensive and useful way. If we shine lights down at the ground instead of up into the sky, and use lower brightness levels, we can save big amounts of energy and keep the beauty of the night sky. Many cities and towns have passed laws limiting lights at night, making sure enough shine for safety without creating much light pollution. Light pollution affects more than our view of the heavens and can harm wildlife. Migrating birds  sometimes fly over cities and become confused by the brightness, flying in circles until they drop from _ Sea turtles need dark beaches for nesting and won't approach bright lights. Too much light at night may even affect human health. For all these reasons, researchers are working on ways to use lights only when and where they are truly needed. Everyone deserves to look up at the sky. Many cities and towns have passed laws limiting lights at night mainly to   _  .
save the electricity make sure of migrating birds' safety let you count the number of stars avoid much light pollution