Metre
The metre (or meter in American spelling; symbol: m) is the base unit of length in the International System of Units (SI).
| Metre | |
|---|---|
 Seal of the International Bureau of Weights and Measures (BIPM) – Use measure (Greek: ΜΕΤΡΩ ΧΡΩ) | |
| 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 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. In 1799, the metre was redefined in terms of a prototype metre bar. The actual bar used was changed in 1889. 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 ΔνCs.