Apollo 11 astronauts placed
a reflector array in the Sea of Tranquillity, on the Moon,
to begin lunar laser ranging. Additional retroreflectors
were placed on the moon by the Apollo 14 and 15 missions.
Also, a French-built retroreflector was carried aboard the
Soviet Lunakhod 2. These four are used regularly.
ranging involves sending a laser beam through an optical
telescope. The beam enters the telescope where the eyepiece
would be, and the transmitted beam is expanded to become
the diameter of the main mirror, then bounced off the surface
toward the reflector on the Moon.
the laser beam hits a reflector, scientists at the ranging
observatories use extremely sensitive filtering and amplification
equipment to detect the return signal, which is far too
weak to be seen with the human eye. Even under good atmospheric
viewing conditions, only one photon is received every few
has provided the most accurate technique available for measuring
the Moon's rotation and orbit. It has also determined that
the length of an Earth day has distinct small-scale variations.
Also observed are crustal plate drifts on Earth. Measurements
show that the Moon is moving away from the Earth at a rate
of about 3.8 centimeters per year. Laser ranging has also
made possible a wealth of new information about the dynamics
and structure of the Moon.
objective is to see if the Earth - with its iron core -
and the Moon - which is mostly silicate material - fall
around the Sun at the same rate. There is strong evidence
that the Moon has a liquid core, and laser ranging has allowed
us to determine with great accuracy the rate at which the
Moon is gradually receding from the Earth.
of the laser ranges has allowed us to verify the Equivalence
Principle - an assumption of Einstein's Theory - with an
accuracy of 2x10-13. Several tests of Einstein's theory
of General Relativity have also been made. The measurements
so far are in agreement, within their uncertainties, with
Einstein's theory. Also, no change in the gravitational
constant - a measurement of the force of gravity - has been
detected with the lunar laser data.
beaming laser pulses at the reflector from Earth, scientists
have been able to determine the round-trip travel time that
gives the distance between the observatory and retroreflector.
site at Observatoire de la Côte d'Azur, near Grasse
as an Apollo Experiment
11 30th anniversary