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

SI

The base unit in the International System of Units 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 centimeter–gram–second system of units, the basic unit of length is the [centimeter">centimetre–gram–second system of units">centimeter–gram–second system of units, the basic unit of length is the [centimeter, or of a meter.
Other non-SI units are derived from decimal multiples of the meter.
NameSymbolSI value
fermifm1 femtometer
ångströmÅ100 picometers
micronμm1 micrometer
Norwegian/Swedish mil or myriameter10,000 meters
x unitxu0.1 picometer

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:
In addition, the following are used by sailors:
use feet for altitude worldwide and nautical miles for distance.

Surveying

in the United States continue to use:
The Australian building trades adopted the metric system in 1966 and the units used for measurement of length are meters and millimeters. Centimeters are avoided as they cause confusion when reading plans. 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.

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

measure uses:
  • Earth radius ≈ 6,371 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 propertySymbolLength, in metersReference
The classical electron radiusre
The Compton wavelength of the electronλC
The reduced Compton wavelength of the electronC
The Compton wavelength of any fundamental particlex
The Bohr radius of the hydrogen atom a0
The reduced wavelength of hydrogen radiation1 / R∞
The Planck length?P
Stoney unit of lengthlS
Quantum chromodynamics unit of lengthlQCD
Natural units based on the electronvolt1 eV−1

Archaic

units of distance include:
  • cana
  • cubit
  • rope
  • league
  • li
  • pace
  • verst

    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:
and other equestrian activities keep alive: