![]() The retreat is the result of the ocean tides on Earth, which cause our planet’s rotation period to slowly increase. Lunar laser ranging’s first scientific contribution was to produce an accurate measurement of how quickly the Moon is moving away from Earth: 3.8 centimeters per year. France’s Observatoire de la Côte d’Azur, the other major lunar-ranging station, uses a smaller telescope but has begun ranging with an infrared laser, which is about 8 times more efficient than the standard green laser. Statistical analysis smooths out the differences in ranges between individual photons, producing a distance to the Moon with an accuracy of about 1 millimeter.ĪPOLLO ranges to the Moon about six times per month and targets all five of the retroreflectors during each session. No more than a few photons from each pulse return to the telescope, but the telescope fires thousands of laser bullets during each ranging session, allowing it to collect thousands of photons per session. The laser is fired in 100-picosecond pulses-“bullets of light” just 2 centimeters thick, says Murphy, who heads the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO). Photons are beamed toward the Moon through a telescope, such as the 3.5-meter telescope at Apache Point Observatory in New Mexico, the largest instrument ever to conduct lunar laser ranging. The Apollo 11 and 14 retroreflectors each contain one hundred 3.8-centimeter corner cubes, whereas the Apollo 15 array contains 300, so it produces the strongest return signal. The telescope, part of the University of Texas at Austin, conducted laser observations from 1969 to the mid-1980s, when laser ranging was moved to a smaller telescope. ![]() McDonald Observatory’s 2.7-meter telescope beams a laser toward the Moon. Those, in turn, have helped scientists determine the Moon’s recession rate, probe its interior structure, and test gravitational theory to some of the highest levels of precision yet obtained. A half century of these observations has provided precise measurements of the shape of the Moon’s orbit, wobbles in the Moon’s rotation, and other parameters. Scientists time the round-trip travel time of each pulse, allowing them to measure the Earth-Moon distance to within a millimeter. Known as a lunar laser ranging retroreflector, it bounces pulses of laser light back to their sources on Earth. The other was an aluminum frame filled with chunks of fused-silica glass that looked a bit like a high-tech egg crate.Īlong with similar devices left on the Moon by Apollo 14 and 15, the instrument is still working-the only Apollo surface experiment that continues to provide data. One was a solar-powered seismometer that collected 21 days of observations before expiring in late August. When Neil Armstrong and Edwin “Buzz” Aldrin blasted off the Moon on 21 July 1969, they left a couple of packages at Tranquility Base.
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