Unit of length

Reference value of length
File:Measurement unit.jpg
A ruler, depicting two customary units of length, the centimetre 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.[1][2][3]

Metric system

SI

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 1299792458 seconds."[4] It is approximately equal to 1.0936 yd. Other SI units are derived from the metre by adding prefixes, as in millimetre or kilometre, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometre is 1000 m.

Non-SI

In the centimetre–gram–second system of units, the basic unit of length is the centimetre, or 1100 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

The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly 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 (11000 of an inch)
  • inch (25.4 mm)
  • foot (12 inches, 0.3048 m)
  • yard (3 feet, 0.9144 m)
  • (terrestrial) mile (5280 feet, 1609.344 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 = 1852 m)

Aviation

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

Surveying

File:Determination of the rute and the feet in Frankfurt.png
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 20.1168 m)
  • rod (also called pole or perch) (quarter of a chain, 512 yards, or 5.0292 m)

Science

Astronomy

Astronomical measure uses:

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 2.817940285(31)×10−15 [11]
The Compton wavelength of the electron λC 2.426310215(18)×10−12 [11]
The reduced Compton wavelength of the electron λC 3.8615926764(18)×10−13 [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 5.291772083(19)×10−11 [11]
The reduced wavelength of hydrogen radiation 1 / R 9.112670505509(83)×10−8 [11]
The Planck length 𝓁P 1.616199(97)×10−35 [13]
Stoney unit of length lS 1.381×10−35
Quantum chromodynamics (QCD) unit of length lQCD 2.103×10−16
Natural units based on the electronvolt 1 eV−1 1.97×10−7

Archaic

Archaic units of distance include:

Informal

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:

Other

Horse racing and other equestrian activities keep alive:

See also

References

  1. Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins. Springer. ISBN 9781852336820.
  2. 2.0 2.1 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 (2002). A dictionary of weights, measures, and units. Oxford University Press. pp. 130–131. ISBN 978-0198605225. Retrieved 8 January 2012.
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Further reading

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