Friday, August 24, 2012

Lunar Distance (LD)

In astronomy, a lunar distance (LD) is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,400 kilometers. The actual distance varies over the course of the orbit of the moon, from 356,700 kilometres at the perigee and 406,300 kilometres at apogee.
High-precision measurements of the lunar distance are made by measuring the time taken for light to travel between LIDAR stations on Earth and retroreflectors placed on the Moon.
The Moon is spiraling away from Earth at an average rate of 3.8 cm per year, as detected by the Lunar Laser Ranging Experiment.The recession rate is considered anomalously high. By coincidence, the diameter of corner cubes in retroreflectors on the Moon is also 3.8 cm.
The first person to measure the distance to the Moon was the 2nd-century-BC astronomer and geographer Hipparchus, who used simple trigonometry. He was approximately 26,000 km off the actual distance, an error of about 6.8%.

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