Metre
The metre is the SI base unit of length. It is defined by fixing the speed of light in vacuum at exactly 299,792,458 metres per second.
Speed of light: The speed at which electromagnetic radiation propagates through a vacuum, exactly 299,792,458 metres per second. Its fixed numerical value defines the metre.
French Academy of Sciences: A French learned society established in 1666 that advised the state and advanced scientific research. Its commission helped establish the original metric system and metre.
International System of Units: The internationally agreed system of measurement units, abbreviated SI. The metre is one of its seven base units.
Ranging: The determination of distance to an object using a signal or measurement instrument. Modern ranging methods apply the metre through precisely timed light or radio signals.
International Prototype Kilogram: A platinum-iridium cylinder that served as the international standard of mass until 2019. Unlike the metre's current definition, it was an artifact-based standard for mass.
Cesium standard: A frequency standard based on the unperturbed transition frequency of cesium-133 atoms. The second's atomic definition supplies the time unit used to realize the metre.
Meridian arc: A segment of a meridian, a line running from the North Pole to the South Pole. The first metre was intended as one ten-millionth of the distance from the equator to the North Pole along a meridian.
Length: A physical quantity describing the extent of an object or the separation between points. The metre is the SI unit used to express this quantity.
Geodesy: The science of measuring Earth's shape, orientation, gravity field, and changing position. Geodetic measurements express Earth's dimensions and shifts in metres.
Pendulum clock: A clock that measures time using the regular motion of a swinging pendulum. Early proposals tied length to pendulum motion, unlike today's light-based definition.