Moon measure.

Ever wonder exactly how far away the moon is from Earth?

Last December the media buzzed with the news of a "slam dunk" experiment. Scientists had pinpointed the exact distance from Earth to moon: 15 billion inches, give or take an inch. Guess what? That's old news--but still pretty cool.

"We've been doing this sort of thing for about 30 years now," says astronomer Peter Shelus of the McDonald Observatory in Texas. Astronomers regularly measure the moon's distance using a technique called laser ranging. They fire laser light from telescopes toward the moon. There, mirrors placed by astronauts (in the late 1960s) catch the light beams and reflect them back to Earth. By timing exactly how long light takes to reach the moon and bounce back home--about 2.6 seconds--scientists can calculate the distance to within an inch.

Why do astronomers care how many inches separate Earth and the moon? For one thing, the moon is drifting farther from Earth. That's because friction, caused by the planet's oceans sloshing against the atmosphere, is slowing Earth's rotation around its own axis. Think of an ice skater twirling on ice. By pulling in her arms, a skater's spin speeds up. As she slows down, her arms extend out. The same thing happens with the Earth and moon. The slower the Earth rotates, the farther away the moon moves--about 4 cm (1.6 in.) per year. So long, moon!