Unit of length

History

Metric system

Main article: Metric system

SI

Main article: International System of 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 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 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.

Imperial/U.S.

Main article: Imperial units, United States customary units, English units of measurement#Length

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 3 mile

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

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 )
• 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 meters (m) and millimeters (mm). Centimeters (cm) 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 66 ft 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

Main article: Astronomical system of units

Astronomical 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 .{{cite journal | access-date = 14 Sep 2012 | url-access = subscription
• 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:

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

Other

Horse racing and other equestrian activities keep alive:

• furlong = 1/8 mi
• horse length ≈ 8 ft

References

References

1. Cardarelli, François. (2003). "Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins". Springer.
2. Hinkelman, Edward G.. (2005). "Dictionary Of International Trade: Handbook Of The Global Trade Community". World Trade Press.
3. Judson, Lewis Van Hagen. (1960). "Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue,233". U.S. Department of Commerce, National Bureau of Standards.
4. "17th General Conference on Weights and Measures (1983), Resolution 1.".
5. Donald Fenna. (2002). "A dictionary of weights, measures, and units". Oxford University Press.
6. {{harvnb. Cardarelli. 2003
7. Wilks, Kevin Joseph.. (1992). "Metrication in Australia : a review of the effectiveness of policies and procedures in Australia's conversion to the metric system". Australian Govt. Pub. Service.
8. "Metrication in Australia".
9. Moritz, H.. (March 2000). "Geodetic Reference System 1980". Journal of Geodesy.
10. (January 2009). "The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range". Astronomical Society of the Pacific.
11. "The IAU and astronomical units". International Astronomical Union.
12. (August 2000). "Recent advances in metrology and fundamental constants". IOS Press, 2001. Proceedings of the International School of Physics "Enrico Fermi".
13. "Compton wavelength over 2 pi". NIST.
14. "Planck length". [[NIST]].