 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Unit of length" – news ·newspapers ·books ·scholar ·JSTOR(October 2012) (Learn how and when to remove this message) |
Aunit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are themetric units, used in every country globally. In the United States theU.S. customary units are also in use. BritishImperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided intoSI and non-SI units.[1][2][3]
Before the establishment of the decimalmetric system inFrance during theFrench Revolution in the late18th century,[4] many units of length were based on parts of thehuman body.[5][6]
The Nippurcubit was one of the oldest known units of length. The oldest known metalstandard forlength corresponds to this Sumerian unit and dates from 2650 BCE.[7][8] Thiscopper bar was discovered inNippur, on the banks of theEuphrates, and is kept in theIstanbul Archaeological Museum. Archaeologists consider that this 51.85 centimetres long unit was the origin of theRoman foot. Indeed, theEgyptians divided theSumerian cubit into 28fingers and 16 of these fingers gave a Roman foot of 29.633 cm.[8][6]Thebase unit in the International System of Units (SI) is themeter, defined as "the length of the path travelled bylight in vacuum during a time interval of1⁄299792458 seconds."[9] It is approximately equal to1.0936 yd. Other SI units are derived from the meter by addingprefixes, as in millimeter or kilometer, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, akilometer is1000 m.
In thecentimeter–gram–second system of units, the basic unit of length is thecentimeter, or1⁄100 of a meter.Other non-SI units are derived from decimal multiples of the meter.
| Name | Symbol | SI value |
|---|---|---|
| fermi | fm | 1femtometer |
| ångström | Å | 100picometers |
| micron | μm | 1micrometer |
| Norwegian/Swedish mil ormyriameter | 10,000 meters | |
| x unit | xu | 0.1 picometer |
The basic unit of length in the imperial and U.S. customary systems is theyard, defined as exactly0.9144 m by international treaty in 1959.[2][10]
Common imperial units and U.S. customary units of length include:[11]
In addition, the following are used bysailors:
Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.[citation needed]
Surveyors in the United States continue to use:
The Australian building trades adopted themetric system in 1966 and the units used for measurement of length aremeters (m) andmillimeters (mm).Centimeters (cm) are avoided as they cause confusion when readingplans. 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.[12][13]
American surveyors use a decimal-based system of measurement devised byEdmund Gunter in 1620. The base unit isGunter's chain of 66 feet (20 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.
Astronomical measure uses:
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 commonnatural units of length are included in this table:
| Atomic property | Symbol | Length, in meters | Reference |
|---|---|---|---|
| The classical electron radius | re | 2.817940285(31)×10−15 | [18] |
| The Compton wavelength of the electron | λC | 2.426310215(18)×10−12 | [18] |
| The reduced Compton wavelength of the electron | 3.8615926764(18)×10−13 | [19] | |
| The Compton wavelength (or reduced Compton wavelength) of any fundamental particle | |||
| TheBohr radius of the hydrogen atom (Atomic unit of length) | a0 | 5.291772083(19)×10−11 | [18] |
| The reduced wavelength of hydrogen radiation | 1 /R∞ | 9.112670505509(83)×10−8 | [18] |
| ThePlanck length | 𝓁P | 1.616199(97)×10−35 | [20] |
| 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 units of distance include:
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:
Horse racing and other equestrian activities keep alive:
| International | |
|---|---|
| National | |
| Other | |
