The Laser Ranging Retroreflector experiment is a setup on the Moon for very accurately measuring the distance between Earth and the Moon. The setup was installed during the Apollo 11, 14, and 15 missions (1969-1971). After 30 years, the Laser Ranging Retroreflector is still being used for data collection.

The setup of the Laser Ranging Retroreflector consists of four arrays of reflecting mirrors, designed to reflect an incoming light beam in the direction it came from. The reflectors are illuminated by lasers, aimed through telescopes on Earth. The reflected laser beam is then observed with the telescope. The distance between Earth and the Moon can be calculated by measuring the time it takes for the laser light pulses to make the round trip.

The accuracy of this method is phenomenal; the relative accuracy is better than one part in 10 billion. Visualize this as measuring the distance between Los Angeles and New York to 0.01" (0.25 mm). The average distance between the centers of the Earth and the Moon was measured to be 239 000 miles (385 000 kilometers). Measurements over the last 30 years have shown that the Moon is receding from the Earth at about 1.5" (3.8 cm) per year. The Laser Ranging Retroreflector has also been used to measure changes in the shape of the Earth, to monitor changes in the length of an Earth day, to accurately back-calculate the occurrence of solar eclipses to 1400 B.C., and to confirm Einstein's theory of relativity.

Although highly accurate, the measurement technique is very challenging. Due to the gravitational pull of the Earth, the Sun, the planets and even some asteroids, the reflectors appear as moving targets. Also, the reflectors are too small to observe directly from Earth. Compare this to rifle shooting at a moving dime, two miles away. Another complication is the divergence of the laser beam; on the Moon's surface, the laser beam is approximately one mile wide. The laser beam further diverges on its way back to Earth. Hence, the reflected signal is very weak; on the order of one photon per second.

Several telescopes around the world (Texas, Hawaii, France, Germany and Australia) are involved with Laser Ranging Retroreflector experiments.

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