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 systemSIThe 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 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 1000 m. Non-SIIn 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/USThe 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][5] Common imperial units and U.S. customary units of length include:[6]
MarineIn addition, the following are used by sailors:
AviationAviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.[citation needed] SurveyingDetermination of the rod, using the length of the left foot of 16 randomly chosen people coming from church serviceSurveyors in the United States continue to use:
ScienceAstronomyAstronomical measure uses:
PhysicsIn 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:
ArchaicArchaic units of distance include:
InformalIn 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:
OtherHorse racing and other equestrian activities keep alive:
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