History of the metre
The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision then in use, they also preferred a decimal system to ease their calculations. With the French Revolution (1789) came a desire to replace many features of the Ancien Régime, including the traditional units of measure. As a base unit of length, many scientists had favoured the seconds pendulum (a pendulum with a half-period of one second) one century earlier, but this was rejected as it had been discovered that this length varied from place to place with local gravity. A new unit of length, the metre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth flattening of 1/334. For practical purposes however, the standard metre was made available in the form of a platinum bar held in Paris. This in turn was replaced in 1889 at the initiative of the International Geodetic Association by thirty platinum-iridium bars kept across the globe. The comparison of the new prototypes of the metre with each other and with the Committee metre (French: Mètre des Archives) involved the development of specialized measuring equipment and the definition of a reproducible temperature scale. Progress in science finally allowed the definition of the metre to be dematerialized; thus in 1960 a new definition based on a specific number of wavelengths of light from a specific transition in krypton-86 allowed the standard to be universally available by measurement. In 1983 this was updated to a length defined in terms of the speed of light; this definition was reworded in 2019: The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs. During the mid nineteenth century the metre gained adoption worldwide, particularly in scientific usage, and it was officially established as an international measurement unit by the Metre Convention of 1875. Where older traditional length measures are still used, they are now defined in terms of the metre – for example the yard has since 1959 officially been defined as exactly 0.9144 metre. (Wikipedia).
