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  • Moon's Measure Is Old Stuff

Photonics Spectra
Feb 1998
Kevin Robinson

AUSTIN, Texas -- When a reporter from the London Daily Telegraph and, later, another from the British Broadcasting Co. phoned Peter Shelus, an astronomer at the McDonald Observatory, to ask about his research, the scientist was somewhat baffled. The reporters wanted to know about Shelus' feat -- measuring the distance between the Earth and the moon to an accuracy of 1 to 3 cm. Shelus said, "My first reaction was, 'Where have you been? We have been doing this for 15 years.' "
Laser lunar ranging measures the distance from the Earth to the moon by timing the round-trip of a laser pulse as it travels between the Earth and a reflector on the moon's surface. According to Shelus, the first experiment was done in 1969 after the Apollo 11 astronauts set up the reflectors. The experiments have continued since. In the mid-1980s, Shelus' group upgraded its laser to a frequency-doubled Nd:YAG, enabling it to make the measurements with a 1- to 3-cm accuracy.
The confusion apparently stems from an inaccurate article by The Associated Press that highlighted the measurement as an important new step for science. This article evidently found its way to the World Wide Web, where it spurred the British reporters.
In fact, the usefulness of Shelus' work comes from his accumulation of nearly 30 years of data. His group makes measurements about 20 days out of every month. Subtle variations among all these measurements allow him and other scientists to test theories, including Einstein's Theory of General Relativity. "A single measurement is virtually useless," he explained. "We never work with a single measurement." In addition, he said he rarely translates the pulse time into physical distance because of the many variables that can introduce inaccuracies.
Shelus said he hopes the notoriety will help him keep his funding because the more data they collect over time, the more accurate their calculations are. The project, he said, has applications beyond pure physics, possibly providing clues about the global changes wrought by the interactions of the two bodies.
"The accurate measurement is a spectacular thing," he said. "But we've been doing it since the mid-'80s."

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