Unit of length
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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.[1][2][3]
Metric system[]
SI[]
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."[4] It is approximately equal to 1.0936 yd. Other units are derived from the metre by adding prefixes from the table below:
| Prefix name | N/A | deca- | hecto- | kilo- | mega- | giga- | tera- | peta- | exa- | zetta- | yotta- |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Prefix symbol | da- | h- | k- | M- | G- | T- | P- | E- | Z- | Y- | |
| Factor | 100 | 101 | 102 | 103 | 106 | 109 | 1012 | 1015 | 1018 | 1021 | 1024 |
| Prefix name | N/A | deci- | centi- | milli- | micro- | nano- | pico- | femto- | atto- | zepto- | yocto- |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Prefix symbol | d- | c- | m- | μ- | n- | p- | f- | a- | z- | y- | |
| Factor | 100 | 10–1 | 10–2 | 10–3 | 10–6 | 10–9 | 10–12 | 10–15 | 10–18 | 10–21 | 10–24 |
For example, a kilometre is 1000 m. A slang term for the kilometre in the US and UK militaries is klick.[5][6]
Non-SI[]
In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or 1⁄100 of a metre. Other non-SI units are derived from decimal multiples of the metre.
| Name | Symbol | SI value |
|---|---|---|
| fermi | fm | femtometre |
| ångström | Å | 100 picometres |
| micron | μm | 1 micrometre |
| Norwegian/Swedish mil or myriametre | 10,000 metres | |
| x unit | xu | 0.1 picometre |
Imperial/US[]
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.[2][7]
Common imperial units and U.S. customary units of length include:[8]
- 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)
Marine[]
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)
Aviation[]
Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.[citation needed]
Surveying[]
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)
Science[]
Astronomy[]
Astronomical measure uses:
- Earth radius R⊕ ≈ 6,371 km[9]
- Lunar distance LD ≈ 384402 km.[10] Average distance between the center of Earth and the center of the Moon.
- astronomical unit au. Defined as 149597870700 m.[11] 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.[12]
- parsec pc ≈ 30856775814671.9 km or about 3.26156 ly
- Hubble length 14.4 billion light-years or 4.55 gigaparsecs
Physics[]
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 | [13] |
| The Compton wavelength of the electron | λC | 2.426310215(18)×10−12 | [13] |
| The reduced Compton wavelength of the electron | 3.8615926764(18)×10−13 | [14] | |
| 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 | [13] |
| The reduced wavelength of hydrogen radiation | 1 / R∞ | 9.112670505509(83)×10−8 | [13] |
| The Planck length |
|