Planetary 2 Apollo Laser Ranging Experiments Yield Results Scientists who analyze data from the Lunar Laser Ranging Experiment have reported some watershed results from these long-term experiments, begun 25 years ago when the Apollo 11 astronauts deployed a reflector array in the Sea of Tranquillity. We also have 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. Each corner cube is 3.8 centimeters in diameter.
Corner cubes reflect a beam of light directly back toward the point of ori gin.
The first laser ranging retroreflector was positioned on the Moon in 1969 by the Apollo 11 astronauts. By beaming laser pulses at the reflector from Earth, scientists have been able to determine their roundtrip travel time that gives the distance bet ween the two bodies at any time to an accuracy of about 3 centimeters. At the Moon's surface the beam is roughly four miles wide. Even under good atmospheric viewing conditions, only one photon is returned very few seconds. From the ranging experiments, scientists know that the average distance between the centers of the Earth and the Moon is 385,000 kilometers, with an accuracy of better than one part in 10 billion.