dipper@utastro.UUCP (Debbie Byrd) (07/18/85)
We're learning more about the Earth -- because of laser beams bounced off the moon. More -- after this. July 18 Lunar Laser Ranging In 1969, the first astronauts on the moon left behind equipment that still helps us learn about the Earth. The astronauts placed on the lunar surface a special kind of reflective panel -- designed to reflect light from a laser beam fired up from Earth. Sixteen years later, laser beams are still being bounced off several of these reflective panels placed in different locations on the moon. Each pulse of the laser sends millions of trillions of photons -- tiny particles of light -- toward one of the lunar reflectors. Two and a half seconds later a few of those photons return from their half-million mile journey. If all goes well one of them may be collected by the same Earth-based telescope initially used to transmit the outgoing laser beam. Measurement of the exact round-trip flight-time of the light gives the distance between the moon and the Earth at the time the laser fired. For fifteen years, until 1984, almost all of the the lunar laser ranging data had been collected by the 107-inch telescope at McDonald Observatory. Now there's a network of lunar laser ranging stations around the world -- in Hawaii, France and Australia. And since the moon's orbit is now so well known, it's possible to use the moon as a reference point in space -- so that laser beams bounced off the moon let us measure how much the ground is shifting around earthquake faults on Earth -- and also begin to measure from year to year the drift of Earth's continents. Script by Diana Hadley. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin