How did the definition of a metre change over time?
The origin of the metre as the SI unit of length dates back to 1790 as the length of a pendulum with a half-period of one second. In 1791, a definition of ten-millionth of the equator-north pole distance was accepted. On October 14, 1960, a metre was defined as 1650763.73 wavelengths of the radiation emitted by krypton-86 atom in vacuum.