ASTRONOMY Scientists try to determine exact distance to the moon

Scientists at New Mexico's Apache Point Observatory swear they're not being obsessive.
Just because they're building a 239,000-mile-long measuring stick to gauge the distance from Earth to the moon to within a fraction of an inch doesn't mean they've gone minutiae happy, said Kurt Anderson, site director and an astronomy professor at New Mexico State University.
"The whole point of this is not because we're obsessive," Anderson said with a laugh. "Making a measurement this accurate will let us test various things in physics, including Einstein's theory of general relativity, with a much higher degree of accuracy."
This fall, the scientists will begin a five-year series of measurements, not with a yardstick but by firing a specially designed laser through the facility's 3.5-meter telescope.
They will aim it at one of four cutting-board-size reflective mirrors left on the moon by Apollo Mission astronauts in the 1970s and time how long it takes for the light to get there and return to Earth.
How it works
"When we send a beam from the telescope, it spreads out from 3.5 meters to about two miles by the time it gets to the moon," said Bruce Gillespie, the site operations manager. "Some of that light hits the reflectors on the moon and comes back to Earth. When it returns, it's spread over five miles. All that takes about 2.5 seconds to complete."
By knowing the exact amount of time it takes the light to travel, scientists can calculate the precise distance between the two bodies.
"Of course, we also have to get a precise measurement of how far the observatory is from the center of the Earth," Anderson said. "We have to measure that to better than a millimeter accuracy."

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