Over the next year, Tom Murphy will try to pin down an exact distance to the Moon. By the time he figures it out, the distance will have changed. In truth, there will never be an exact distance to the Moon. Yet that hasn't stopped NASA from funding Murphy's effort.
The distance from the center of Earth to the center of the Moon is constantly changing. It varies by some 12 percent during each month. A passing asteroid can tug at the Moon and alter its position in relation to Earth, albeit ever so slightly. And over time, the Moon is creeping away from us, moving about 1 inch a year, on average, into space, said Murphy, a researcher at the University of Washington.
Still, there are some agreed-upon parameters. The Earth-Moon distance is, on average, roughly 238,900 miles (384,402 km). And astronomers know this to within about 1 inch (2 cm) of accuracy. Murphy wants to get the accuracy down to the thickness of a paper clip.
Anyway, all these statistics are not the point.
"The distance by itself isn't very interesting," Murphy said in a telephone interview. "But what we care about is the fact that the distance between Earth and the Moon can tell us a lot about the gravitational influences of the bodies."
Murphy's real motivation is to test Einstein's General Theory of Relativity, which is based on an assumption that gravity affects a feather and a bowling ball in the same manner. You cannot test this in your home, because air keeps the feather aloft. But if Einstein was right, then the Sun should be generating the same acceleration of Earth as for the Moon.
"It's very complicated," Murphy admits, "because they're going around each other, and that's not even a circular pattern."
For the next five years Murphy and colleagues will shoot lasers at the Moon, bounce them off five reflectors left by Apollo astronauts, then use the Apache Point telescope in New Mexico to study how long it takes the pulses to return. In a year, he figures he'll have some useable data.
"This probes at the fundamental characteristics of gravity, which is the most important force in our daily lives," Murphy said. "But it's actually the weakest of forces and the one we know the least about."
Working with Murphy are professors Christopher Stubbs and Eric Adelberger, and Jana Strasburg, a graduate student.
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