Samiksha Jaiswal (Editor)

Unit of length

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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.

Contents

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 1/299,792,458 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.

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.

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.

Common Imperial units and U.S. customary units of length include:

  • 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)

    • 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.

    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, or 5.0292 m)

    • Astronomy

    Astronomical measure uses:

  • Earth radius R⊕ ≈ 6,371 km
  • Lunar distance LD ≈ 7008384402000000000♠384402 km average distance between the center of Earth and the center of the Moon.
  • astronomical unit AU, au or ua. Defined as 7011149597870700000♠149597870700 m. Approximately the distance between the Earth and Sun.
  • light-year ly ≈ 7015946073047258080♠9460730472580.8 km The distance that light travels in a vacuum in one Julian year.
  • parsec pc ≈ 7016308567758146719♠30856775814671.9 km or about 7016308567400801506♠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:

    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)
  • Football field (generally around 110 metres in length)
  • Thickness of a human hair (around 80 micrometres)

    • Other

    Horse racing and other equestrian activities keep alive:

  • furlong ≈ 0.125 miles (201 m)
  • horse length ≈ 8 feet (2.4 m)

    • References

    Unit of length Wikipedia
    (Text) CC BY-SA