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{{Short description|Reference value of length}}{{Use American English|date=December 2023}}{{More citations needed|date=October 2012}}(File:Measurement_unit.jpg |thumb|right|A ruler, depicting two customary units of length, the centimeter and the inch)A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.BOOK, Cardarelli, François, Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins,archive.org/details/encyclopaediaofs0000card, registration, 2003, Springer, 9781852336820, BOOK, Hinkelman, Edward G., Sibylla Putzi, Dictionary Of International Trade: Handbook Of The Global Trade Community, 2005, World Trade Press, 245, 9781885073723, BOOK, Judson, Lewis Van Hagen, Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue,233, 1960, 3–4, U.S. Department of Commerce, National Bureau of Standards,books.google.com/books?id=Mialg1senzMC&q=Units+of+Weight+and+Measure+%28United+States+Customary+and+Metric%29, 16 October 2012,

Metric system

SI

{{See also|Orders of magnitude (length)}}The base unit in the International System of Units (SI) is the meter, defined as “the length of the path travelled by light in vacuum during a time interval of {{frac|299792458}} seconds.“WEB,www.bipm.org/en/CGPM/db/17/1/, 17th General Conference on Weights and Measures (1983), Resolution 1., 2012-09-19, It is approximately equal to {{val|1.0936|u=yards}}. Other SI units are derived from the meter by adding prefixes, as in millimeter or kilometer, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometer is {{val|1000|u=meters}}.

Non-SI

In the centimeter–gram–second system of units, the basic unit of length is the centimeter, or {{frac|100}} of a meter.Other non-SI units are derived from decimal multiples of the meter.{| class=“wikitable” style="margin-left: 20px;” background-color: #ffffff“! Name! Symbol! SI value align=“left“| fermi

fm| femtometer

| ångström | Å| 100 picometers| micron| μm

Micrometre>micrometer

| Norwegian/Swedish mil or myriameter| | 10,000 meters| x unit | xu| 0.1 picometer

Imperial/U.S.

The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly {{val|0.9144|u=m}} by international treaty in 1959.BOOK, Donald Fenna, A dictionary of weights, measures, and units,archive.org/details/dictionaryofweig0000fenn, registration, 8 January 2012, 2002, Oxford University Press, 978-0198605225, 130–131, Common imperial units and U.S. customary units of length include:{{harvnb|Cardarelli|2003|pp=29–30}}

• thou or mil ({{frac|1000}} of an inch)
• inch ({{val|25.4|u=mm}})
• foot (12 inches, 0.3048 m)
• yard (3 feet, 0.9144 m)
• (terrestrial) mile (5280 feet, 1609.344 m)
• (land) league {{convert|3|mile|m}}

Marine

In addition, the following are used by sailors:

• fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
• nautical mile (one minute of arc of latitude = {{val|1852|u=m}})

Aviation

Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.{{citation needed|date=March 2016}}

Surveying

(File:Determination of the rute and the feet in Frankfurt.png|thumb|right|Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service)Surveyors in the United States continue to use:

• chain (22 yards, or {{val|20.1168|u=m}})
• rod (also called pole or perch) (quarter of a chain, 5{{Frac|1|2}} yards, or {{val|5.0292|u=m}})

Building trades

The Australian building trades adopted the metric system in 1966 and the units used for measurement of length are meters (m) and millimeters (mm). Centimeters (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half meters is usually recorded as 2500 mm or 2.5 m; it would be considered non-standard to record this length as 250 cm.BOOK, Wilks, Kevin Joseph.,www.worldcat.org/oclc/27702954, Metrication in Australia : a review of the effectiveness of policies and procedures in Australia’s conversion to the metric system, 1992, Australian Govt. Pub. Service, Australia. Department of Industry, Technology, and Commerce., 0-644-24860-2, Canberra, 94, 27702954, WEB,themetricmaven.com/wp-content/uploads/2012/03/Metrication-in-Australia-built-2013-06-24.pdf, Metrication in Australia,

Surveyor’s trade

American surveyors use a decimal-based system of measurement devised by Edmund Gunter in 1620. The base unit is Gunter’s chain of {{convert|66|ft|m}} which is subdivided into 4 rods, each of 16.5 ft or 100 links of 0.66 feet. A link is abbreviated “lk”, and links “lks”, in old deeds and land surveys done for the government.

Science

Astronomy

Astronomical measure uses:

• Earth radius {{Earth radius}} ≈ 6,371 kmJOURNAL, Moritz, H., March 2000, Geodetic Reference System 1980, Journal of Geodesy, 74, 1, 128–133, 10.1007/s001900050278, 2000JGeod..74..128., 195290884,
• Lunar distance LD â‰ˆ {{val|384402|u=km}}.JOURNAL, Battat, J. B. R., Murphy, T. W., Adelberger, E. G., The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range, Astronomical Society of the Pacific, January 2009, 121, 875, 29–40, 10.1086/596748, 10.1086/596748, 2009PASP..121...29B, free, Average distance between the center of Earth and the center of the Moon.
• astronomical unit au. Defined as {{val|149597870700|u=m}}.WEB

, Approximately the distance between the Earth and Sun.

• light-year ly â‰ˆ {{val|9460730472580.8|u=km}}. The distance that light travels in a vacuum in one Julian year.{{citation| url =www.iau.org/public_press/themes/measuring/ | title = The IAU and astronomical units | publisher = International Astronomical Union | access-date=2008-07-05}}
• parsec pc â‰ˆ {{val|30856775814671.9|u=km}} or about {{val|3.26156|u=ly}}
• Hubble length 14.4 billion light-years or 4.55 gigaparsecs

Physics

In atomic physics, sub-atomic physics, and cosmology, the preferred unit of length is often related to a chosen fundamental physical constant, or combination thereof. This is often a characteristic radius or wavelength of a particle. Some common natural units of length are included in this table:{| class=“wikitable” style="margin-left: 20px;” background-color: #ffffff“! Atomic property! Symbol! Length, in meters! Reference align=“left“| The classical electron radius

re	2.817940285	e=-15}}	QUINN>FIRST1=T.J.	FIRST2=S.	FIRST3=P.	DATE=AUGUST 2000	LOCATION=AMSTERDAM; WASHINGTON, DC	ISBN=9781586031671,books.google.com/books?id=WE22Fez60EcC&q=bohr+ruler&pg=PA142,

| The Compton wavelength of the electron

λC	2.426310215	e=-12}}

| The reduced Compton wavelength of the electron

{{strikethrough|λ}}C	3.8615926764	e=-13}}	NIST >TITLE=COMPTON WAVELENGTH OVER 2 PI	WORK=THE NIST REFERENCE ON CONSTANTS, UNITS, AND UNCERTAINTY, 15 October 2012,

| The Compton wavelength (or reduced Compton wavelength) of any fundamental particle

{{strikethrough|λ}}x| |

| The Bohr radius of the hydrogen atom (Atomic unit of length)

a0	5.291772083	e=-11}}

| The reduced wavelength of hydrogen radiation

1 / R∞	9.112670505509	e=-8}}

| The Planck length

𝓁P	1.616199	e=-35}}	NIST >TITLE=PLANCK LENGTH	WORK=THE NIST REFERENCE ON CONSTANTS, UNITS, AND UNCERTAINTY, 15 October 2012,

Stoney units>Stoney unit of length	lS	1.381|e=-35}}

Quantum chromodynamics>QCD) unit of length	lQCD	2.103|e=-16}}

| Natural units based on the electronvolt

1 eV−1	1.97|e=-7}}

Archaic

{{See also|English units#Length|label 1=English units of length}}Archaic units of distance include:

• cana
• cubit
• rope
• league
• li (China)
• pace (the “double pace” of about 5 feet used in Ancient Rome)
• verst (Russia)

Informal

{{See also|List of humorous units of measurement|List of unusual units of measurement}}In everyday conversation, and in informal literature, it is common to see lengths measured in units of objects of which everyone knows the approximate width. Common examples are:

• Double-decker bus (9.5–11 meters in length)
• Football field (100 yards in length)
• Thickness of a human hair (around 80 micrometers)

Other

Horse racing and other equestrian activities keep alive:

• furlong = {{convert|1/8|mi|m|3}}
• horse length â‰ˆ {{convert|8|ft|m}}

See also

• {{Section link|List of conversion factors|Length}}
• List of examples of lengths
• {{Section link|List of unusual units of measurement|Length}}
• Medieval weights and measures
• Orders of magnitude (length)
• System of measurement
• Units of measurement

References

{{Reflist}}

Further reading

• BOOK, Whitelaw, Ian, A Measure of All Things: The Story of Man and Measurement, 2007, Macmillan, 9780312370268,

{{Systems of measurement}}{{Units of length used in Astronomy}}{{Authority control}}