Science Watch; Inch by Inch to the Moon
Published: December 23, 1997
In a feat one astronomer called a ''scientific slam-dunk,'' scientists have pinpointed how many inches separate 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 the number. Dr. Richard Teske, an astronomer at the University of Michigan, called the experiments one of the most accurate ever undertaken.
''Determining the Moon's distance to within one inch is an extraordinary accomplishment,'' he said last week. ''These exquisite measurements -- a kind of scientific slam-dunk -- are being used to test Albert Einstein's theory of gravity.''
He added: ''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.''
The effort used mirrors placed on the Moon by Apollo astronauts and by an unmanned Soviet flight. Telescopes used to beam laser light at the Moon are in Texas, Hawaii, France, Germany and Australia.
Distance is measured by marking the time it takes laser pulses to make the round trip, a technique called laser ranging. Laser radiation beamed to the Moon from a telescope on Earth bounces off one of the lunar mirrors, returning to the same telescope about 2.6 seconds later.
Gravity experiments also take into account the fact that the Moon's orbit is gradually moving farther away because of Earth's ever-so-slightly slowing rotation.
The Earth's slowing rotation is why an extra second is inserted almost every year, making the last minute of Dec. 31 last 61 seconds, Dr. 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,'' Dr. Teske said. ''Perhaps by then nobody will much care whether Albert Einstein was right.''