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Many different units of length have been used across 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.
Metric systemEdit
- Main article: Metric system
SI unitsEdit
- Main article: International System of Units
Common units of length in the International System of Units (SI) are:
- meter and its multiples, such as "centimeter" or "kilometer"
Non-SI unitsEdit
Non-SI units of length include:
- fermi (fm) (= 1 femtometer in SI units)
- angstrom (Å) (= 100 picometers in SI units)
- micron (= 1 micrometer in SI units)
- Norwegian/Swedish mil (= 10,000 meters)
Imperial/US unitsEdit
- Main article: Imperial unit
- Main article: U.S. customary units
Common Imperial units and U.S. customary units of length include:
- inch (2.54 cm)
- mil (one thousandth of an inch, one thou)
- foot (12 inches, 0.3048 m)
- yard (3 ft, 0.9144 m)
- (terrestrial) mile (5280 ft, 1609.344 m)
MarineEdit
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)
AviationEdit
Aviators use feet (same as US) for altitude worldwide except in Russia and China.
SurveyingEdit
Surveyors in the United States continue to use:
AstronomicalEdit
- Main article: Astronomical system of units
Astronomical measure uses:
- Earth radius (RE) (~6,370 km)
- astronomical unit (AU) (~150 gigameters)
- light year (ly) (~9.46 petameters)
- parsec (pc) (~30.8 petameters), including kiloparsec (kpc) and megaparsec (Mpc)
Archaic unitsEdit
- Main article: Ancient weights and measures
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 unitsEdit
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 meters in length)
- Football field (generally around 110 meters, depending on the country)
- Widths of a human hair (around 80 micrometers)
- A beard-second is a unit created as a teaching concept. It is the distance that a beard grows in a second (about 5 nanometers)
- Smoot, a jocular unit of length created as part of an MIT fraternity prank
OtherEdit
Horse racing and other equestrian activities keeps alive:
- furlong = 0.125 mi = ~ 201.168 m
- horse length = ~ 8 ft= ~ 2.4 m}}
Physics also uses: