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Symbol: m / Base data: length, SI base unit
Length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second
Currently based on the speed of light
The unit was first used in 1799. It measures the length of the meridian, and was calculated as the forty millionth part of the meridian. The international standard metre was created from this, and this remained the standard for a long time. The results acquired by comparing the length of the international standard metre and the speed of light are now used as the standard.
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