Metre

The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of Template:Sfrac of a second, where the second is defined by a hyperfine transition frequency of caesium.<ref name="SIBrochure9thEd"> Template:Citation</ref>

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's polar circumference is approximately Template:Val.

In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and 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 vacuum in Template:Sfrac of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency Template:Math. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures Template:Val, a change of about 200 parts per million from the original value of exactly Template:Val, which also includes improvements in the accuracy of measuring the circumference.

Spelling

Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>The most recent official brochure about the International System of Units (SI), written in French by the {{#invoke:Lang|lang}}, International Bureau of Weights and Measures (BIPM) uses the spelling metre; an English translation, included to make the SI standard more widely accessible also uses the spelling metre (BIPM, 2006, p. 130ff). However, in 2008 the U.S. English translation published by the U.S. National Institute of Standards and Technology (NIST) chose to use the spelling meter in accordance with the United States Government Printing Office Style Manual. The Metric Conversion Act of 1975 gives the Secretary of Commerce of the US the responsibility of interpreting or modifying the SI for use in the US. The Secretary of Commerce delegated this authority to the Director of the National Institute of Standards and Technology (Turner). In 2008, NIST published the US version (Taylor and Thompson, 2008a) of the English text of the eighth edition of the BIPM publication {{#invoke:Lang|lang}} (SI) (BIPM, 2006). In the NIST publication, the spellings "meter", "liter" and "deka" are used rather than "metre", "litre" and "deca" as in the original BIPM English text (Taylor and Thompson (2008a), p. iii). The Director of the NIST officially recognised this publication, together with Taylor and Thompson (2008b), as the "legal interpretation" of the SI for the United States (Turner). Thus, the spelling metre is referred to as the "international spelling"; the spelling meter, as the "American spelling".</ref><ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and the Philippines<ref>The Philippines uses English as an official language and this largely follows American English since the country became a colony of the United States. While the law that converted the country to use the metric system uses metre (Batas Pambansa Blg. 8) following the SI spelling, in actual practice, meter is used in government and everyday commerce, as evidenced by laws (kilometer, Republic Act No. 7160), Supreme Court decisions (meter, G.R. No. 185240), and national standards (centimeter, PNS/BAFS 181:2016).</ref> which use meter.

Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English.<ref>Template:Cite encyclopedia, s.v. ammeter, meter, parking meter, speedometer.</ref> The suffix "-meter" has the same Greek origin as the unit of length.<ref> Template:Cite encyclopedia, s.v. meter.</ref><ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Etymology

The etymological roots of metre can be traced to the Greek verb {{#invoke:Lang|lang}} (Template:Transliteration) ((I) measure, count or compare)<ref>Template:LSJ.</ref> and noun {{#invoke:Lang|lang}} (Template:Transliteration) (a measure),<ref>Template:LSJ.</ref> which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin ({{#invoke:Lang|lang}}), French ({{#invoke:Lang|lang}}), English and other languages. The Greek word is derived from the Proto-Indo-European root *meh₁- 'to measure'. The motto {{#invoke:Lang|lang}} (Template:Transliteration) in the seal of the International Bureau of Weights and Measures (BIPM), was approved by Adolphe Hirsch on 11 July 1875 and may be translated as "Keep the measure", thus calls for both measurement and moderation.<ref name=":6">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The use of the word metre (for the French unit {{#invoke:Lang|lang}}) in English began at least as early as 1797.<ref name="Oxford">Oxford English Dictionary, Clarendon Press 2nd ed. 1989, vol. IX p. 697 col. 3.</ref>

SI prefixed forms of metre

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, astronomical units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), rather than in Mm or larger multiples; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.

The terms micron and millimicron have been used instead of micrometre (μm) and nanometre (nm), respectively, but this practice is discouraged.<ref>Taylor & Thompson 2003, p. 11.</ref>

Equivalents in other units

Metric unit expressed in non-SI units					Non-SI unit expressed in metric units
1 metre	≈	1.0936	yard		1 yard	=	0.9144	metre
1 metre	≈	39.370	inches		1 inch	=	0.0254	metre
1 centimetre	≈	Template:Val	inch		1 inch	=	2.54	centimetres
1 millimetre	≈	Template:Val	inch		1 inch	=	25.4	millimetres
1 metre	=	1010	ångström		1 ångström	=	10−10	metre
1 nanometre	=	10	ångström		1 ångström	=	100	picometres

Within this table, "inch" and "yard" mean "international inch" and "international yard"<ref>Astin & Karo 1959.</ref> respectively, though approximate conversions in the left column hold for both international and survey units.

"≈" means "is approximately equal to";

"=" means "is exactly equal to".

One metre is exactly equivalent to Template:Sfrac inches and to Template:Sfrac yards.

A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet Template:Frac inches. This gives an overestimate of 0.125 mm.

The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm).<ref>Arnold Dieter (1991). Building in Egypt: pharaonic stone masonry. Oxford: Oxford University Press. Template:ISBN. p.251.</ref> Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite book</ref> The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly Template:Frac toise.<ref name=H&H>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Russian verst was 1.0668 km.Template:Sfn The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.<ref>Template:Cite encyclopedia</ref>

See also

• ISO 1Template:Spaced ndashstandard reference temperature for length measurements
• Metric prefix
• Metrication

References

• Template:AnchorTemplate:Cite book
• Template:AnchorAstin, A. V. & Karo, H. Arnold, (1959), Refinement of values for the yard and the pound, Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59–5442, Filed, 30 June 1959)
• Template:AnchorTemplate:Cite book
• Template:Cite book
• Template:Cite EB1911
• Template:AnchorTemplate:Cite book
• Template:AnchorTemplate:Cite book
• Template:Cite book
• Template:AnchorHistorical context of the SI: Meter. Retrieved 26 May 2010.
• Template:AnchorNational Institute of Standards and Technology. (27 June 2011). NIST-F1 Cesium Fountain Atomic Clock. Author.
• Template:AnchorNational Physical Laboratory. (25 March 2010). Iodine-Stabilised Lasers. Author.
• Template:Anchor{{#invoke:citation/CS1|citation

|CitationClass=web }}

• Template:AnchorRepublic of the Philippines. (2 December 1978). Batas Pambansa Blg. 8: An Act Defining the Metric System and its Units, Providing for its Implementation and for Other Purposes. Author.
• Template:AnchorRepublic of the Philippines. (10 October 1991). Republic Act No. 7160: The Local Government Code of the Philippines. Author.
• Template:Anchor Supreme Court of the Philippines (Second Division). (20 January 2010). G.R. No. 185240. Author.
• Template:AnchorTaylor, B.N. and Thompson, A. (Eds.). (2008a). The International System of Units (SI). United States version of the English text of the eighth edition (2006) of the International Bureau of Weights and Measures publication Le Système International d' Unités (SI) (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
• Template:AnchorTaylor, B.N. and Thompson, A. (2008b). Guide for the Use of the International System of Units (Special Publication 811). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 23 August 2008.
• Template:AnchorTurner, J. (deputy director of the National Institute of Standards and Technology). (16 May 2008). "Interpretation of the International System of Units (the Metric System of Measurement) for the United States". Federal Register Vol. 73, No. 96, p.Template:Nbsp28432–28433.
• Template:AnchorZagar, B.G. (1999). Laser interferometer displacement sensors in J.G. Webster (ed.). The Measurement, Instrumentation, and Sensors Handbook. CRC Press. Template:ISBN.