Unit of length
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Many different units of length have been used around the world. The main units in modern use are U.S. customary units in the United States and the Metric system 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.
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 1/299,792,458 of a second." It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:
For example, a kilometre is 1000 metres.
Non-SI units of length include:
- fermi (fm) (= 1 femtometre in SI units)
- ångström (Å) (= 100 picometres in SI units)
- micron (= 1 micrometre in SI units)
- Norwegian/Swedish mil/myriametre (= 10,000 metres)
- x unit (= 0.1 picometre)
Common Imperial units and U.S. customary units of length include:
- thou or mil (1/1000 of an inch)
- line (1/12 of an inch)
- inch (2.54 cm)
- foot (12 inches, 0.3048 m)
- yard (3 ft, 0.9144 m)
- (terrestrial) mile (5280 ft, 1609.344 m)
- (land) league (3 miles)
In addition, the following are used by mariners:
- fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
- nautical mile (one minute of arc of latitude = 1852 m)
Aviators use feet (same as US) for altitude worldwide (except in Russia and China) and nautical miles for distance.
Astronomical measure uses:
- Earth radius () (≈6,371 km)
- astronomical unit (au or ua) (defined in 2012 as 149,597,870,700 m) Approximately the distance between the Earth and Sun.
- light year (ly) (≈9,460,730,472,580.8 km) The distance that light travels in a vacuum in one Julian year.
- parsec (pc) (≈30,856,775,814,671.9 km or about 3.26156 ly)
- Hubble length (13.8 billon light year/306593922 parsec)
Archaic units of distance include:
- li (China)
- pace (the "double pace" of about 5 feet used in Ancient Rome)
- verst (Russia)
See also English units of length.
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 (9.5–10.9 metres in length)
- Football field (generally around 110 metres, depending on the country)
- Thickness of a human hair (around 80 micrometres)
- A beard-second is a unit created as a teaching concept. It is the distance that a beard grows in a second (about 5 nanometres)
- Smoot, a jocular unit of length created as part of an MIT fraternity prank
Horse racing and other equestrian activities keep alive:
- Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins. Springer. ISBN 9781852336820.
- Hinkelman, Edward G.; Sibylla Putzi (2005). Dictionary Of International Trade: Handbook Of The Global Trade Community. World Trade Press. p. 245. ISBN 9781885073723.
- 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.
- "17th General Conference on Weights and Measures (1983), Resolution 1.". Retrieved 2012-09-19.
- 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.
- Cardarelli 2003, pp. 29–30
- Moritz, H. (March 2000). "Geodetic Reference System 1980". Journal of Geodesy 74 (1): 128–133. Bibcode:2000JGeod..74..128.. doi:10.1007/s001900050278.
- Geoff Brumfiel (14 Sep 2012). "The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number.". Retrieved 14 Sep 2012.
- The IAU and astronomical units, International Astronomical Union, retrieved 2008-07-05
- "atomic unit of length". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012.
- "natural unit of length". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012.
- "Planck length". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012.
- Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN 9780312370268.