Unit of length




Reference value of length



A ruler, depicting two customary units of length, the centimetre and the inch


A unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension. The most common units in modern use are U.S. customary units in the United States and metric units elsewhere. 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.[1][2][3]




Contents






  • 1 Metric system


    • 1.1 SI


    • 1.2 Non-SI




  • 2 Imperial/US


  • 3 Marine


  • 4 Aviation


  • 5 Surveying


  • 6 Science


    • 6.1 Astronomy


    • 6.2 Physics




  • 7 Archaic


  • 8 Informal


  • 9 Other


  • 10 See also


  • 11 References


  • 12 Further reading





Metric system


Main article: Metric system


SI


Main article: International System of Units

See also: Orders of magnitude (length)

The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of ​1⁄299792458 seconds."[4] It is approximately equal to 6999999987839999999♠1.0936 yd. Other units are derived from the metre by adding prefixes from the table below:
































































































Standard prefixes for the SI units of measure

  • v

  • t

  • e
Multiples
 Prefix name

deca
hecto
kilo
mega
giga
tera
peta
exa
zetta
yotta
Prefix symbol
da
 h
 k
 M
 G
 T
 P
 E
 Z
 Y
Factor
 100
 101
 102
 103
 106
 109
 1012
 1015
 1018
 1021
 1024
 
Submultiples
 Prefix name
          
deci
centi
milli
micro
nano
pico
femto
atto
zepto
yocto
Prefix symbol
d
 c
 m
 μ
 n
 p
 f
 a
 z
 y
Factor
 100
 10−1
 10−2
 10−3
 10−6
 10−9
 10−12
 10−15
 10−18
 10−21
 10−24

For example, a kilometre is 7003100000000000000♠1000 m.



Non-SI


In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or ​1⁄100 of a metre.
Other non-SI units are derived from decimal multiples of the metre.

































Name
 Symbol
 SI value
fermi
 fm
femtometre

ångström
 Å
 100 picometres
micron
 μm
 1 micrometre

Norwegian/Swedish mil or myriametre
10,000 metres

x unit
 xu
 0.1 picometre


Imperial/US


Main articles: Imperial units and United States customary units

The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly 6999914400000000000♠0.9144 m by international treaty in 1959.[2][5]


Common Imperial units and U.S. customary units of length include:[6]




  • thou or mil (​1⁄1000 of an inch)


  • line (​1⁄12 of an inch)


  • inch (6998254000000000000♠25.4 mm)


  • foot (12 inches, 0.3048 m)


  • yard (3 feet (0.91 m))

  • (terrestrial) mile 5,280 feet (1,610 m)

  • (land) league 3 miles (4,800 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 = 7003185200000000000♠1852 m)



Aviation


Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.[citation needed]



Surveying




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 7001201168000000000♠20.1168 m)


  • rod (also called pole or perch) (quarter of a chain, or 7000502920000000000♠5.0292 m)



Science



Astronomy


Main article: Astronomical system of units

Astronomical measure uses:




  • Earth radius R⊕ ≈ 6,371 km[7]


  • Lunar distance LD ≈ 7008384402000000000♠384402 km.[8] Average distance between the center of Earth and the center of the Moon.


  • astronomical unit au. Defined as 7011149597870700000♠149597870700 m.[9] Approximately the distance between the Earth and Sun.


  • light-year ly ≈ 7015946073047258080♠9460730472580.8 km. The distance that light travels in a vacuum in one Julian year.[10]


  • parsec pc ≈ 7016308567758146719♠30856775814671.9 km or about 7016308567400801506♠3.26156 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:





































































Atomic property
 Symbol
 Length, in metres
 Reference
The classical electron radius
re
6985281794028500000♠2.817940285(31)×10−15
 [11]
The Compton wavelength of the electron
λC
6988242631021500000♠2.426310215(18)×10−12
 [11]
The reduced Compton wavelength of the electron
λC
6987386159267640000♠386.15926764(18)×10−15
 [12]
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle
λx

The Bohr radius of the hydrogen atom (Atomic unit of length)
a0
6989529177208299999♠5.291772083(19)×10−11
 [11]
The reduced wavelength of hydrogen radiation
 1 / R∞
6992911267050550900♠9.112670505509(83)×10−8
 [11]
The Planck length
 𝓁P
6965161619900000000♠1.616199(97)×10−35
 [13]

Stoney unit of length
lS
6965138100000000000♠1.381×10−35
Quantum chromodynamics (QCD) unit of length
lQCD
6984210300000000000♠2.103×10−16
Natural units based on the electronvolt
 1 eV−1
6993196999999999999♠1.97×10−7


Archaic


See also: 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 and 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 metres in length)


  • Football field (generally around 110 metres in length)

  • Thickness of a human hair (around 80 micrometres)



Other


Horse racing and other equestrian activities keep alive:




  • furlong ≈ 0.125 miles (201 m)


  • horse length ≈ 8 feet (2.4 m)



See also



  • List of examples of lengths

  • List of unusual units of measurement § Length

  • Medieval weights and measures

  • Orders of magnitude (length)

  • System of measurement

  • Units of measurement



References





  1. ^ Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins. Springer. ISBN 9781852336820..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output .citation q{quotes:"""""""'""'"}.mw-parser-output .citation .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .citation .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-maint{display:none;color:#33aa33;margin-left:0.3em}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}


  2. ^ ab Hinkelman, Edward G.; Sibylla Putzi (2005). Dictionary Of International Trade: Handbook Of The Global Trade Community. World Trade Press. p. 245. ISBN 9781885073723.


  3. ^ Judson, Lewis Van Hagen (1960). Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue 233. U.S. Department of Commerce, National Bureau of Standards. pp. 3–4. Retrieved 16 October 2012.


  4. ^ "17th General Conference on Weights and Measures (1983), Resolution 1". Retrieved 2012-09-19.


  5. ^ Donald Fenna (26 October 2002). A dictionary of weights, measures, and units. Oxford University Press. pp. 130–1. ISBN 978-0-19-860522-5. Retrieved 8 January 2012.


  6. ^ Cardarelli 2003, pp. 29–30


  7. ^ Moritz, H. (March 2000). "Geodetic Reference System 1980". Journal of Geodesy. 74 (1): 128–133. Bibcode:2000JGeod..74..128.. doi:10.1007/s001900050278.


  8. ^ Battat, J. B. R.; Murphy, T. W.; Adelberger, E. G. (January 2009). "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. 121 (875): 29–40. Bibcode:2009PASP..121...29B. doi:10.1086/596748. JSTOR 10.1086/596748.


  9. ^ Geoff Brumfiel (14 Sep 2012). "The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number". Retrieved 14 Sep 2012.


  10. ^ The IAU and astronomical units, International Astronomical Union, retrieved 2008-07-05


  11. ^ abcd Quinn, T.J.; Leschiutta, S.; Tavella, P. (August 2000). Recent advances in metrology and fundamental constants. Amsterdam ; Washington, DC: IOS Press, 2001. Proceedings of the International School of Physics "Enrico Fermi". pp. 142–143.


  12. ^ "Compton wavelength over 2 pi". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012.


  13. ^ "Planck length". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012.




Further reading



  • Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN 9780312370268.








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