1997: Scientists Pinpoint Moon’s Distance

.c The Associated Press

By A.J. DICKERSON

DETROIT (AP) – In a feat one astronomer describes as a “scientific slam-
dunk,” scientists have pinpointed how many inches separate the Earth and its
moon – 15 billion, give or take an inch.

Scientists used mirrors on the moon and telescopes that fired lasers from
Earth to come up with what University of Michigan astronomer Richard Teske
called one of the most accurate scientific experiments ever undertaken.

“Determining the moon’s distance to within one inch is an extraordinary
accomplishment,” he said. “These exquisite measurements – a kind of
scientific slam-dunk – are being used to test Albert Einstein’s theory of
gravity.”

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, he said 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 said.

The measurements have “been growing more and more accurate and more and more
interesting,” Teske said. 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 one of an unmanned Soviet missions. 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,” said Maria Zuber, a professor
of geophysics and planetary science at Massachusetts Institute of Technology.

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.

The 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 said. 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,” Teske said. “Perhaps by then nobody will much care
whether Albert Einstein was right.”

AP-NY-12-17-97 0549EST

Copyright 1997 The Associated Press. The information contained in the AP
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