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Subject: Scientists to measure moon distance to nearest millimetre Author: Ken [ Next Thread | Previous Thread | Next Message | Previous Message ] Date Posted: 06:30:06 01/20/02 Sun Scientists to measure moon distance to nearest millimetre US scientists are to use an optical "tape measure" 239,000 miles long to find the distance to the moon to the nearest millimetre. They plan to employ a telescope, a laser beam and reflectors left by several lunar missions. It will provide the most accurate measure yet obtained of the Earth's distance from the moon. Astronomers have long known that the centre of the moon is about 238,700 miles from the centre of the Earth. In the early 1970s, the distance was measured to within about 25 centimetres. Technological advances since the mid-1980s have now sharply reduced that margin to about two centimetres. A team at the University of Washington in Seattle hopes to reduce that uncertainty. They will attach a laser to the 3.5 metre telescope at Apache Point, New Mexico, capable of shooting a one billion watt pulse of light at the moon. A number of "laser bullets" will be fired at one of five "retroreflectors" - banks of 100 to 300 reflective prisms left on the moon by three Apollo missions and two unmanned Soviet missions. By bouncing the light pulse off the reflectors and measuring how long it takes to make the round trip back to Earth, the distance can be calculated. Despite the power of the laser, the tightly focused beam will be two kilometres in diameter when it hits the moon. By the time the light makes it back to Earth it will have expanded to about 15 kilometres, or 9.3 miles. Story filed: 06:12 Sunday 20th January 2002 [ Next Thread | Previous Thread | Next Message | Previous Message ]