<P> The mean semi-major axis has a value of 384,402 km (238,856 mi). The time - averaged distance between Earth and Moon centers is 385,000.6 km (239,228.3 mi). The actual distance varies over the course of the orbit of the Moon, from 356,500 km (221,500 mi) at the perigee to 406,700 km (252,700 mi) at apogee, resulting in a differential range of 50,200 km (31,200 mi). </P> <P> Lunar distance is commonly used to express the distance to near - Earth object encounters . Lunar distance is also an important astronomical datum; the precision of this measurement to a few parts in a trillion has useful implications for testing gravitational theories such as general relativity, and for refining other astronomical values such as Earth mass, Earth radius, and Earth's rotation . The measurement is also useful in characterizing the lunar radius, the mass of the Sun and the distance to the Sun . </P> <P> Millimeter - precision measurements of the lunar distance are made by measuring the time taken for light to travel between LIDAR stations on the Earth and retroreflectors placed on the Moon . The Moon is spiraling away from the Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging Experiment . By coincidence, the diameter of corner cubes in retroreflectors on the Moon is also 6998380000000000000 ♠ 3.8 cm . </P> <Table> Lunar distance expressed in selected units <Tr> <Th> Unit </Th> <Th> Mean value </Th> <Th> Uncertainty </Th> <Th> Ref </Th> </Tr> <Tr> <Td> meter </Td> <Td> 7008384402000000000 ♠ 3.844 02 × 10 </Td> <Td> 6997110000000000000 ♠ 1.1 mm </Td> <Td> </Td> </Tr> <Tr> <Td> kilometer </Td> <Td> 7005384402000000000 ♠ 384 402 </Td> <Td> 6997110000000000000 ♠ 1.1 mm </Td> <Td> </Td> </Tr> <Tr> <Td> mile </Td> <Td> 238,856 </Td> <Td> 0.043 in </Td> <Td> </Td> </Tr> <Tr> <Td> Earth radius </Td> <Td> 60.32 </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> AU </Td> <Td> 1 / 388.6 = 6997256999999999999 ♠ 0.002 57 </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> light - second </Td> <Td> 7000128200000000000 ♠ 1.282 s </Td> <Td> 6989375000000000000 ♠ 37.5 ps </Td> <Td> </Td> </Tr> </Table>

How was the distance to the moon calculated