Thursday, Oct. 08, 2009

Growing Apart

Was it something we said? While our two celestial bodies remain locked in orbit, the moon is slowly — very slowly — inching away from Earth, at a rate of about 3.8 cm a year. Right now the moon is more than 238,000 miles from Earth, but when it formed, it was just 14,000 miles away.

How do scientists know? The moon's distance is measured by bouncing laser beams off reflectors on the moon's surface that astronauts from the Apollo missions left behind. Scientists can measure the time it takes for the laser beams to travel there and back and calculate the distance with a high degree of accuracy. Eventually, the moon's distance will substantially weaken the oceans' tides and total eclipses of the sun won't be possible for observers on Earth, since the moon will have moved too far away. But that could still take another billion years.