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 of a second, where the second is defined by a hyperfine transition frequency of caesium.

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 through Paris, setting as that quarter of the Earth's polar circumference.

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 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 . This series of amendments did not alter the size of the metre significantly – modern measurements of the Earth's polar circumference give a figure of .

Etymology and spelling

The etymological roots of metre can be traced to the Greek verb () ((I) measure, count or compare) and noun () (a measure), 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 (), French (), English (meter for measuring instruments, but metre or meter in poetry) and other languages. The Greek word is derived from the Proto-Indo-European root *meh₁- 'to measure'. In English, the use of the word metre (for the French unit ) began at least as early as 1797.

Metre is the standard spelling of the metric unit for length in all English-speaking nations except the United States and the Philippines, which use meter.

History of definition

SI prefixed forms of metre

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 (), light-years (), or parsecs (), 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.

Equivalents in other units

{| class=wikitable style="margin:0 auto;"

|-

! colspan="5" style="text-align:left;"|Metric unit<br/>expressed in non-SI units

! colspan="4" style="text-align:left;"|Non-SI unit<br/>expressed in metric units

|-

| 1&nbsp;metre ||≈ ||style="text-align:right;"|1.0936 ||yard||

| 1&nbsp;yard||= ||style="text-align:right;"|0.9144 ||metre

|-

| 1&nbsp;metre ||≈ ||style="text-align:right;"|39.370 ||inches||

| 1&nbsp;inch||= ||style="text-align:right;"|0.0254 ||metre

|-

| 1&nbsp;centimetre ||≈ ||style="text-align:right;"| ||inch||

| 1&nbsp;inch||= ||style="text-align:right;"|2.54 ||centimetres

|-

| 1&nbsp;millimetre ||≈ ||style="text-align:right;"| ||inch||

| 1&nbsp;inch||= ||style="text-align:right;"|25.4 ||millimetres

|-

| 1&nbsp;metre ||= ||style="text-align:right;"|10||ångström||

| 1&nbsp;ångström||= ||style="text-align:right;"|10 ||metre

|-

| 1&nbsp;nanometre ||= ||style="text-align:right;"|10||ångström||

| 1&nbsp;ångström||= ||style="text-align:right;"|100 ||picometres

|}

Within this table, "inch" and "yard" mean "international inch" and "international yard" 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 &nbsp;inches and to &nbsp;yards.

<!-- 1 metre ≈ 39.370 078 740 157 5 in / or 39.375 in = 1000.125 mm // what is this for? -->

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

The ancient Egyptian cubit was about 0.5&nbsp;m (surviving rods are 523–529&nbsp;mm). Scottish and English definitions of the ell (2&nbsp;cubits) were 941&nbsp;mm (0.941&nbsp;m) and 1143&nbsp;mm (1.143&nbsp;m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2&nbsp;m and was standardised at exactly 2&nbsp;m in the mesures usuelles system, such that 1&nbsp;m was exactly &nbsp;toise. The Russian verst was 1.0668&nbsp;km. The Swedish mil was 10.688&nbsp;km, but was changed to 10&nbsp;km when Sweden converted to metric units.

See also

  • ISO 1standard reference temperature for length measurements
  • Metric prefix
  • Metrication

Notes

References

Cited bibliography

  • Astin, A. V. & Karo, H. Arnold (1959). Refinement of values for the yard and the pound. Washington, D.C.: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59–5442, Filed, 30 June 1959)
  • Historical context of the SI: Meter. Retrieved 26 May 2010.
  • National Institute of Standards and Technology (27 June 2011). NIST-F1 Cesium Fountain Atomic Clock. Author.
  • National Physical Laboratory (25 March 2010). Iodine-Stabilised Lasers. Author.
  • Republic 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.
  • Republic of the Philippines. (10 October 1991). Republic Act No. 7160: The Local Government Code of the Philippines. Author.
  • Supreme Court of the Philippines (Second Division) (20 January 2010). G.R. No. 185240. Author.
  • Taylor, 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.
  • Taylor, 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.
  • Turner, 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.28432–28433.
  • Zagar, B.G. (1999). Laser interferometer displacement sensors in J.G. Webster (ed.). The Measurement, Instrumentation, and Sensors Handbook. CRC Press. .