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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.
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." It is approximately equal to 1.0936 yd. Other units are derived from the metre by adding prefixes from the table below:
|Norwegian/Swedish mil or myriametre||10,000 metres|
|x unit||xu||0.1 picometre|
Common imperial units and U.S. customary units of length include:
- thou or mil (1⁄1000 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)
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)
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, 51⁄2 yards, or 5.0292 m)
Astronomical measure uses:
- Earth radius R⊕ ≈ 6,371 km
- Lunar distance LD ≈ 384402 km. Average distance between the center of Earth and the center of the Moon.
- astronomical unit au. Defined as 149597870700 m. Approximately the distance between the Earth and Sun.
- light-year ly ≈ 9460730472580.8 km. The distance that light travels in a vacuum in one Julian year.
- parsec pc ≈ 30856775814671.9 km or about 3.26156 ly
- Hubble length 14.4 billion light-years or 4.55 gigaparsecs
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|||
|The Compton wavelength of the electron||λC||2.426310215(18)×10−12|||
|The reduced Compton wavelength of the electron||3.8615926764(18)×10−13|||
|The Compton wavelength (or reduced Compton wavelength) of any fundamental particle|
|The Bohr radius of the hydrogen atom (Atomic unit of length)||a0||5.291772083(19)×10−11|||
|The reduced wavelength of hydrogen radiation||1 / R∞||9.112670505509(83)×10−8|||
|The Planck length||𝓁P||1.616199(97)×10−35|||
|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:
- li (China)
- pace (the "double pace" of about 5 feet used in Ancient Rome)
- verst (Russia)
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 (100 yards in length)
- Thickness of a human hair (around 80 micrometres)
Horse racing and other equestrian activities keep alive:
- 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
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