# Metre

## SI unit of length / From Wikipedia, the free encyclopedia

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The **metre** (or **meter** in American spelling; symbol: **m**) is the base unit of length in the International System of Units (SI).

**Quick facts: metre, General information, Unit system, Unit...**▼

metre | |
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General information | |

Unit system | SI |

Unit of | length |

Symbol | m[1] |

Conversions | |

1 m[2] in ... | ... is equal to ... |

SI units | |

Imperial/US units | |

Nautical units | ≈ 0.00053996 nmi |

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's polar circumference is approximately 40000 km.

In 1799, the metre was redefined in terms of a prototype metre bar, the bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in a vacuum in 1/299792458 of a second. After the 2019 redefinition of the SI base units, this definition was rephrased to include the definition of a second in terms of the caesium frequency Δ*ν*_{Cs}. This series of amendments did not alter the size of the metre significantly - today Earth's polar circumference measures 40007.863 km, a change of 0.02% from the original value of exactly 40000 km, which also includes improvements in the accuracy of measuring the circumference.

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