Metre

The metre (symbol: m) is the base unit of length in the International System of Units (SI). It is the most widely used unit for measuring distance, height, and size worldwide, and serves as the basis for derived units such as area (square metre) and volume (cubic metre).

The metre is part of a decimal system, meaning that smaller and larger units are calculated in powers of ten. For example, one metre equals 100 centimetres or 1000 millimetres, while 1000 metres equal one kilometre.

History

The metre was created in late 18th-century France during the Revolution, with the aim of replacing the various units of measurement used in different regions. The original definition was based on Earth's dimensions: the metre was set as one ten-millionth (1/10,000,000) of the distance from the equator to the North Pole along a specific meridian. A prototype bar was subsequently made and kept in France, from which other countries took copies.

Modern definition

Because the prototype bars changed and deteriorated, the definition was replaced with one based on natural physical properties. Initially it was measured using the wavelength of a specific light. Today the metre is defined in terms of the speed of light in a vacuum: light travels at a constant speed, and the metre is the distance light travels in a precisely defined fraction of a second. Consequently, the metre's measurement is tied to the definition of the second.

Use and subdivisions

Common subdivisions include the nanometre, micrometre, millimetre, centimetre, and kilometre. Physics and engineering use very small subdivisions, while geography and astronomy use larger ones. Many countries have made the metre the official unit for commerce and construction; some English-speaking countries still use units such as the foot and mile.