When Apollo 11 astronauts Neil Armstrong and Buzz Aldrin flew off from the surface of the moon 25 years ago, they left behind an array of instruments that scientists are still using to measure meticulously the motions of the Earth and its satellite.
Long after the Apollo team retired, younger scientists are learning with precision just how far the Earth and moon are separated.
They know how fast the moon rotates and how much the Earth wobbles as it spins on its axis.
Many instruments have stopped working, but clusters of reflecting devices, which require no power, remain. Earth-bound researchers use telescopes to aim laser beams at them and measure the time the beam takes to bounce from the reflectors and back to electronic detectors on Earth. Travel time, then, can be converted to distance.
Because the distance between the Earth and moon varies widely during the moon's elliptical orbit and because of tiny tidal forces on both, the scientists have determined that the average distance between the Earth and the moon is 385,000 kilometers. That is 239,227.45 miles. And the moon itself is steadily moving away from Earth, the laser ranging experiment has shown. Over the past 25 years, it has receded at a yearly rate of 3.8 centimeters, or 1.496 inches, the scientists report, and the motion is permanent.