Unit of length

A unit of length is 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.

History

Metric system

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 seconds." It is approximately equal to . Other SI units are derived from the metre by adding prefixes, as in millimetre or kilometre, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometre is .

Non-SI In the centimetre–gram–second system of units, the basic unit of length is the centimetre, or of a metre. Other non-SI units are derived from decimal multiples of the metre. Name Symbol SI value fermi fm1 femtometre ångström Å 100 picometres micron μm 1 micrometre Norwegian/Swedish mil or myriametre 10,000 metres x unit xu 0.1 picometre

Imperial/U.S.

The basic unit of length in the imperial and U.S. customary systems is the yard, defined as exactly by international treaty in 1959.

Common imperial units and U.S. customary units of length include: thou or mil ( of an inch) inch () foot (12 inches, 0.3048 m) yard (3 feet, 0.9144 m) (terrestrial) mile (5280 feet, or 1760 yards 1609.344 m) (land) league

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 = )

Aviation Air traffic control uses feet for altitude worldwide,International Civil Aviation Organization - International Standards and Recommended Practices - Annex 5 to the Convention on International Civil Aviation - Units of Measurement to be Used in Air and Ground Operations Fifth Edition - July 2010 except in a minority of countries including Russia, China, North Korea, and many CIS countries. Distance is measured in nautical miles.

Surveying

Surveyors in the United States continue to use: chain (22 yards, or ) rod (also called pole or perch) (quarter of a chain, 5 yards, or )

Australian building trades The Australian building trades adopted the metric system in 1966 and the units used for measurement of length are metres (m) and millimetres (mm). Centimetres (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half metres is usually recorded as 2500 mm or 2.5 m; it would be considered non-standard to record this length as 250 cm.

Surveyor's trade American surveyors use a decimal-based system of measurement devised by Edmund Gunter in 1620. The base unit is Gunter's chain of 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.

Science

Astronomy

Astronomical measure uses: Earth radius ≈ 6,371.008 km Lunar distance LD ≈ . Average distance between the center of Earth and the center of the Moon. astronomical unit au. Defined as . Approximately the distance between the Earth and Sun. light-year ly ≈ . The distance that light travels in a vacuum in one Julian year. parsec pc ≈ or about 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 The Compton wavelength of the electronλC The reduced Compton wavelength of the electronC The Compton wavelength (or reduced Compton wavelength) of any fundamental particlex The Bohr radius of the hydrogen atom (Atomic unit of length)a0 The reduced wavelength of hydrogen radiation1 / R∞ The Planck length𝓁P Stoney unit of length lS Quantum chromodynamics (QCD) unit of length lQCD Natural units based on the electronvolt 1 eV−1

Archaic

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

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) American football field (100 yards in length) Thickness of a human hair (around 80 micrometres)

Other Horse racing and other equestrian activities keep alive: furlong = horse length ≈

Unit of length

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