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Broadly speaking, the original definition of the metre was |>> [[[ "one ten-millionth of the distance _ from the North Pole to the Equator _ on the meridian running through Paris." ]]] <<| This was adopted in competition with the definition of |>> [[[ "the pendulum length giving _ a half period of one second" ]]] <<| The definitions are remarkably similar because of the coincidence that EQN:\pi^2 is very nearly equal to /g,/ acceleration due to gravity. However, both definitions have problems. You can read more about them in the Wikipedia article quoted below. More recently, the metre has been redefined as: |>> [[[ "the distance travelled by light in vacuum during _ a time interval of 1/299 792 458 of a second." ]]] <<| Thus the speed of light is now, by definition, 299 792 458 m/s. ---- Further reading: * http://en.wikipedia.org/wiki/Metre