Unit of length
A Unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension. The most common units in modern use are U.S. customary units in the United States and metric units 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.
|Norwegian/Swedish mil or myriametre||10,000 metres|
|x unit||xu||0.1 picometre|
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.
- 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 sailors:
- 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 for altitude worldwide (except in Russia and China) and nautical miles for distance.
Astronomical measure uses:
- Earth radius R⊕ ≈ 6,371 km
- Lunar distance LD ≈ 402 km384 average distance between the center of Earth and the center of the Moon.
- astronomical unit AU, au or ua. Defined as 597870700 m. 149 Approximately the distance between the Earth and Sun.
- light-year ly ≈ 460730472580.8 km The distance that light travels in a 9vacuum in one Julian year.
- parsec pc ≈ 856775814671.9 km or about 3056 ly3.261
- 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 meters||Reference|
|The classical electron radius||re||940285(31)×10−152.817|
|The Compton wavelength of the electron||λC||310215(18)×10−122.426|
|The reduced Compton wavelength of the electron||26764(18)×10−15386.159|
|The Compton wavelength (or reduced Compton wavelength) of any fundamental particle.|
|The Bohr radius of the hydrogen atom (Atomic unit of length)||a0||772083(19)×10−115.291|
|The reduced wavelength of hydrogen radiation||1 / R∞||670505509(83)×10−89.112|
|The Planck length||𝓁P||199(97)×10−351.616|
|Stoney unit of length||lS||×10−351.381|
|Quantum chromodynamics (QCD) unit of length||lQCD||×10−162.103|
|Natural units based on the electron-Volt||1 eV−1||×10−71.97|
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 (9.5–10.9 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:
- List of examples of lengths
- Medieval weights and measures
- Orders of magnitude (length)
- System of measurement
- Units of measurement
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- "Compton wavelength over 2 pi". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012.
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- Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN 9780312370268.