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For other uses of this word, see Length (disambiguation).


Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end. This may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length. In the physical sciences and engineering, the word "length" is typically used synonymously with "distance", with symbol or .

Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed). In most systems of measurement, length is a fundamental unit, from which other units are derived.

Units of length

In the physical sciences and engineering, when one speaks of "units of length", the word "length" is synonymous with "distance". There are several units that are used to measure length. Units of length may be based on lengths of human body parts, the distance travelled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some fixed object.

In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light. The centimetre and the kilometre, derived from the metre, are also commonly used units. In the English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.

Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the astronomical unit, the light-year, and the parsec.

Units used to denote microscopically small distances, as in chemistry, include the micron and the ångström.

Length of moving rods

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While the length of a resting rod can be measured by direct comparison with a measuring rod, this comparison cannot be performed while the rod is moving. In this case we define its moving length as the distance between its two endpoints at a given instance.

If the world lines of the two endpoints of the rod expressed in the coordinates of an inertial reference frame are

:


and

: ,


then the length of the rod in this reference frame at the instance is

:


Since in special relativity the relation of simultaneity depends on the chosen frame of reference, the length of moving rods also depends.

See also

Length in its basic meaning is the long dimension of an object. The term has the following specific meanings in particular areas.
  • Length (phonetics), in phonetics
  • Vowel length

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dimension (Latin, "measured out") is a parameter or measurement required to define the characteristics of an object—i.e., length, width, and height or size and shape.
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Height is the measurement of vertical distance, but has two meanings in common use. It can either indicate how "tall" something is, or how "high up" it is. For example one could say "That is a tall building", or "That airplane is high up in the sky".
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Distance is a numerical description of how far apart objects are at any given moment in time. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria (e.g. "two counties over").
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Area is a physical quantity expressing the size of a part of a surface. The term Surface area is the summation of the areas of the exposed sides of an object.

Units

Units for measuring surface area include:
square metre = SI derived unit

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The volume of a solid object is the three-dimensional concept of how much space it occupies, often quantified numerically. One-dimensional figures (such as lines) and two-dimensional shapes (such as squares) are assigned zero volume in the three-dimensional space.
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For the use in number theory, see .
A set of fundamental units is a set of units for physical quantities from which every other unit can be generated.
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Distance is a numerical description of how far apart objects are at any given moment in time. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria (e.g. "two counties over").
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units of measurement have played a crucial role in human endeavour from early ages up to this day. Disparate systems of measurement used to be very common. Now there is a global standard, the International System (SI) of units, the modern form of the metric system.
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Measurement is the estimation of the magnitude of some attribute of an object, such as its length or weight, relative to a unit of measuremnt. Measurement usually involves using a measuring instrument, such as a ruler or scale, which is calibrated to compare the object to some
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International System of Units (abbreviated SI from the French Le Système international d'unités) is the modern form of the metric system.
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The international system (SI) of units defines seven SI base units: physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis.
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1 metre =
SI units
1000 mm 0 cm
US customary / Imperial units
0 ft 0 in
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI).
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speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning "swiftness".[1] It is the speed of all electromagnetic radiation, including visible light, in a vacuum.
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Imperial units or the Imperial system is a collection of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced.
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1 inch =
SI units
010−3 m 0 mm
US customary / Imperial units
010−3 ft 010−3 yd


An inch (plural: inches; symbol or abbreviation: in or, sometimes,  
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1 foot =
SI units
0 m 0 mm
US customary / Imperial units
0 yd 0 in
A foot (plural: feet or foot;[1] symbol or abbreviation: ft or, sometimes, ′
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1 yard =
SI units
0 m 0 mm
US customary / Imperial units
0 ft 0 in
A yard (abbreviation: yd) is the name of a unit of length in a number of different systems, including English units, Imperial units, and United States customary
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1 mile =
SI units
0 m 0 km
US customary / Imperial units
0 ft 0 yd
“Miles” redirects here. For other uses, see Miles (disambiguation).

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Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation).
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1 astronomical unit =
SI units
0109 m 0106 km
Astronomical units
010-6 pc 010−6 ly
US customary / Imperial units
0109 ft 0106 mi
The
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1 light-year =
SI units
01015 m 01012 km
Astronomical units
0103 AU 0 pc
US customary / Imperial units
01015 ft 01012 mi
A light-year or lightyear (symbol:
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parsec (symbol pc) is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars.
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1 ångström =
SI units
010−12 m 010−3 nm
Natural units
01024 lP 0 a0
US customary / Imperial units
010−12 ft 010−9
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In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" (such as an orbit in space or a trajectory
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special theory of relativity was proposed in 1905 by Albert Einstein in his article "On the Electrodynamics of Moving Bodies". Some three centuries earlier, Galileo's principle of relativity had stated that all uniform motion was relative, and that there was no absolute and
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The relativity of simultaneity is the concept that simultaneity is not absolute, but dependent on the observer. That is, according to the special theory of relativity formulated by Albert Einstein in 1905, it is impossible to say in an absolute
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Distance is a numerical description of how far apart objects are at any given moment in time. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria (e.g. "two counties over").
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dimension (Latin, "measured out") is a parameter or measurement required to define the characteristics of an object—i.e., length, width, and height or size and shape.
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