unit of length

{{excerpt|History of measurement#Units of length}}

{{main article|Metric system}}

{{main article|International System of Units}}

{{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."{{cite web |url=http://www.bipm.org/en/CGPM/db/17/1/ |title=17th General Conference on Weights and Measures (1983), Resolution 1. |access-date=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}}.

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.

{{main article|Imperial units|United States customary units|English units#length}}

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.{{cite book|author=Donald Fenna|title=A dictionary of weights, measures, and units|url=https://archive.org/details/dictionaryofweig0000fenn|url-access=registration|access-date=8 January 2012|date=2002|publisher=Oxford University Press|isbn=978-0198605225|pages=[https://archive.org/details/dictionaryofweig0000fenn/page/130 130]–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, or 1760 yards 1609.344 m)
• (land) league {{convert|3|mile|m}}

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}})

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

File:Determination of the rute and the feet in Frankfurt.png

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}})

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.{{Cite book|last=Wilks, Kevin Joseph.|url=https://www.worldcat.org/oclc/27702954|title=Metrication in Australia : a review of the effectiveness of policies and procedures in Australia's conversion to the metric system|date=1992|publisher=Australian Govt. Pub. Service|others=Australia. Department of Industry, Technology, and Commerce.|isbn=0-644-24860-2|location=Canberra|pages=94|oclc=27702954}}{{Cite web|url=http://themetricmaven.com/wp-content/uploads/2012/03/Metrication-in-Australia-built-2013-06-24.pdf|title=Metrication in Australia}}

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.

{{main article|Astronomical system of units}}

Astronomical measure uses:

• Earth radius {{Earth radius}} ≈ 6,371 km{{cite journal |last=Moritz |first=H. |date=March 2000 |title=Geodetic Reference System 1980 |journal=Journal of Geodesy |volume=74 |issue=1 |pages=128–133 |doi=10.1007/s001900050278 |bibcode = 2000JGeod..74..128. |s2cid=195290884 }}
• Lunar distance LD ≈ {{val|384402|u=km}}.{{cite journal|last1=Battat|first1=J. B. R.|last2=Murphy|first2=T. W.|last3=Adelberger|first3=E. G.|title=The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range|journal=Astronomical Society of the Pacific|date=January 2009|volume=121|issue=875|pages=29–40|doi=10.1086/596748|jstor=10.1086/596748|bibcode = 2009PASP..121...29B |doi-access=free}} Average distance between the center of Earth and the center of the Moon.
• astronomical unit au. Defined as {{val|149597870700|u=m}}.{{cite web

| url = http://www.nature.com/news/the-astronomical-unit-gets-fixed-1.11416

| title = The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number.

| author = Geoff Brumfiel

| date = 14 Sep 2012

| access-date = 14 Sep 2012

}} Approximately the distance between the Earth and Sun.

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

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:

{{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)

{{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)
• American football field (100 yards in length)
• Thickness of a human hair (around 80 micrometers)

Horse racing and other equestrian activities keep alive:

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

• {{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

{{Reflist}}

• {{cite book|last=Whitelaw|first=Ian|title=A Measure of All Things: The Story of Man and Measurement|year=2007|publisher=Macmillan|isbn=9780312370268}}

{{Systems of measurement}}

{{Units of length used in Astronomy}}

{{Authority control}}