Metre
Base SI unit of length, defined by light in vacuum.
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The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Its definition has evolved significantly since its inception. Originally conceived in 1791 by the French National Assembly, the metre was defined as one ten-millionth of the distance from the equator to the North Pole along a great circle passing through Paris, representing one quarter of the Earth's polar circumference.
This definition was replaced in 1799 with a prototype metre bar, a physical standard that was itself updated in 1889. A further redefinition in 1960 tied the metre to a specific number of wavelengths of a particular emission line from krypton-86. The current definition, adopted in 1983 and slightly refined in 2002 to clarify that the metre measures proper length, is based on the speed of light. Since the 2019 revision of the SI, the metre has been defined as the length of the path travelled by light in vacuum during a specific fraction of a second, where that second is itself defined by the hyperfine transition frequency of caesium.
From 1983 until 2019, the definition was formally the length of the path travelled by light in vacuum in a fraction of a second, before being rephrased to incorporate the caesium-based definition of the second. The word "metre" traces its etymology to the Greek verb for "measure" and the noun for "a measure," rooted in the Proto-Indo-European meh₁- 'to measure'. This range of meanings appears in Latin, French, and English, where the spelling "metre" is standard for the metric unit in all English-speaking nations except the United States and the Philippines, which use "meter." The use of the word "metre" for the French unit in English dates back at least to 1797.
Lore & Background
The metre is the fundamental unit of length within the International System of Units (SI). Its appearance is that of a standard linear measure, defined not by a physical object but by a universal constant: since 2019, it has been the distance light travels in a vacuum during a specific fraction of a second, with that second itself defined by the hyperfine transition frequency of caesium. The metre’s range is vast, extending from subatomic scales using SI prefixes like the nanometre (formerly millimicron) to astronomical distances typically expressed in kilometres, astronomical units, light-years, or parsecs rather than megametres. Its habitat is global, serving as the base unit for length in all scientific and most everyday contexts worldwide, though the spelling varies: “metre” is standard in all English-speaking nations except the United States and the Philippines, which use “meter.” Defining characteristics include its evolution from an Earth-based standard—originally conceived in 1791 as one ten-millionth of the quarter of Earth’s polar circumference measured through Paris—to a prototype bar in 1799, later refined in 1889, then redefined in 1960 using a specific krypton-86 emission line.
The current definition, adopted in 1983 and clarified in 2002 to specify proper length, was rephrased after the 2019 SI revision to explicitly tie the metre to the caesium-defined second. Practical equivalents include being exactly 1/0.9144 yards and exactly 1/0.0254 inches, with a simple mnemonic that one metre is nearly 3 feet 3⅜ inches, overestimating by 0.125 mm. Historically, the metre relates to ancient units: the Egyptian cubit was about 0.5 m, the Scottish ell was 0.941 m, the English ell 1.143 m, the Parisian toise was standardised at exactly 2 m in the mesures usuelles system, the Russian verst was 1.0668 km, and the Swedish mil was 10.688 km before metric conversion set it to 10 km.
Reader's Guide
The metre is the foundational unit of length in the SI system, used globally for science, commerce, and daily measurement. Its definition has evolved from a geodetic fraction of Earth's circumference to a prototype bar, then to atomic wavelengths, and finally to a constant of nature—the speed of light.
This progression reflects increasing precision and universality, culminating in a definition that ties the metre to fundamental physical constants. The metre's spelling varies: 'metre' is standard in most English-speaking nations, while 'meter' is used in the United States and the Philippines. SI prefixes allow decimal multiples and submultiples, though common usage often favors km, cm, and mm over less familiar units like dam or hm.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Metre (CC BY-SA 4.0).
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