In a feat one astronomer describes as a "scientific slam-dunk," scientists have pinpointed how many inches separate Earth and its moon - about 15 billion.
Scientists used mirrors on the moon and telescopes that fired lasers from Earth to come up with what University of Michigan astronomer Richard Teske calls one of the most accurate scientific experiments ever undertaken. Mr. Teske says it measured the distance at 15,134,310,000 inches, almost 239,000 miles.
"These exquisite measurements - a kind of scientific slam-dunk - are being used to test Albert Einstein's theory of gravity," he says.
The work, published in several journals, is done by physicists with an interest in Einstein's theory of relativity and what it has to tell about gravity, Teske said on Tuesday.
"Right now all is well with Einstein. Relativity's predictions seem to be correct, reinforcing physicists' beliefs that the theory is the best description we have for how nature operates," Teske says.
The measurements have "been growing more and more accurate and more and more interesting," Teske says. The international group of scientists is based at the Institute for Astronomical and Physical Geodesy in Munich, Germany.
The effort used mirrors placed on the moon's Earth-facing hemisphere by Apollo astronauts and by an unmanned Soviet mission.
Telescopes used for beaming laser light at the moon are located in Texas, Hawaii, France, Germany, and Australia.
Laser radiation beamed to the moon from one of the telescopes on Earth bounces off one of the lunar mirrors, returning to the same telescope about 2.6 seconds later.
Distance is measured by marking the time it takes laser pulses to make the round trip, a technique called laser ranging.
"Lasers are very accurate measuring devices," says Maria Zuber, a professor of geophysics and planetary science at Massachusetts Institute of Technology in Cambridge.
GRAVITY experiments also take into account the fact that the moon's orbit is gradually moving farther and farther away because of Earth's ever-so-slightly slowing rotation.
Studies in Arizona suggest the moon has moved about 2,100 miles farther away over the past 900 million years. Earth's slowing rotation is why an extra second is inserted almost every year, making the last minute of Dec. 31 last for 61 seconds, Teske says. After many millions of years, the length of the day-night cycle will approach one month.
"Both cycles will finally fall into step in the dim, far future when the earth will keep the same face toward the moon just as the moon now keeps the same face toward us," says Teske. "Perhaps by then nobody will much care whether Albert Einstein was right."