Metre
Encyclopedia
The metre symbol m, is the base unit
Fundamental unit
A set of fundamental units is a set of units for physical quantities from which every other unit can be generated.In the language of measurement, quantities are quantifiable aspects of the world, such as time, distance, velocity, mass, momentum, energy, and weight, and units are used to describe...

of length in the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

(SI). Originally intended to be one ten-millionth of the distance from the Earth's equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

to the North Pole (at sea level), its definition has been periodically refined to reflect growing knowledge of metrology
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...

. Since 1983, it is defined as the length of the path travelled by light in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

in of a second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

.

Name

The first recorded proposal for a decimal-based unit of length was the universal measure unit proposed by the English philosopher John Wilkins
John Wilkins
John Wilkins FRS was an English clergyman, natural philosopher and author, as well as a founder of the Invisible College and one of the founders of the Royal Society, and Bishop of Chester from 1668 until his death....

in 1668. In 1675, the Italian scientist Tito Livio Burattini
Tito Livio Burattini
Tito Livio Burattini was an inventor, architect, Egyptologist, scientist, instrument-maker, traveller, engineer, and nobleman. He was born in Agordo, Italy, and studied in Padua and Venice...

, in his work Misura Universale, used the words (lit. "catholic [i.e. universal] metre"), which was derived from the Greek (métron katholikón), "a universal measure". This word gave rise to the French mètre which in 1797 was introduced into the English language.

Meridional definition

In 1668, Wilkins proposed using Christopher Wren
Christopher Wren
Sir Christopher Wren FRS is one of the most highly acclaimed English architects in history.He used to be accorded responsibility for rebuilding 51 churches in the City of London after the Great Fire in 1666, including his masterpiece, St. Paul's Cathedral, on Ludgate Hill, completed in 1710...

's suggestion of a pendulum with a half-period
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

of one second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

to measure a standard length that Christiaan Huygens had observed to be 38 Rhineland or 39¼ English inches (997 mm) in length.
In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One approach followed Wilkins in defining the metre as the length of a pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

with a half-period of one second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, a 'seconds pendulum
Seconds pendulum
A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz....

'. The other approach suggested defining the metre as one ten-millionth of the length of the Earth's meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

along a quadrant
Circular sector
A circular sector or circle sector, is the portion of a disk enclosed by two radii and an arc, where the smaller area is known as the minor sector and the larger being the major sector. In the diagram, θ is the central angle in radians, r the radius of the circle, and L is the arc length of the...

, that is the distance from the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

to the North Pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...

. In 1791, the French Academy of Sciences
French Academy of Sciences
The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research...

selected the meridional definition over the pendular definition because the force of gravity varies slightly
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...

over the surface of the Earth, which affects the period of a pendulum.

In order to establish a universally accepted foundation for the definition of the metre, measurements of this meridian more accurate than those available at that time were imperative. The French Academy of Sciences
French Academy of Sciences
The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research...

commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain
Pierre Méchain
Pierre François André Méchain was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep sky objects and comets.-Life:...

, lasting from 1792 to 1799, which measured the distance between the Dunkerque belfry and Montjuïc castle
Montjuïc
Montjuïc is a hill located in Barcelona, Catalonia.-Etymology:Montjuïc is translated as 'Jew Hill' in medieval Catalan, or is perhaps related to the Latin phrase Mons Jovicus . The name is found in several locations in the Catalan Countries: the Catalan cities of Girona and Barcelona both have a...

, Barcelona
Barcelona
Barcelona is the second largest city in Spain after Madrid, and the capital of Catalonia, with a population of 1,621,537 within its administrative limits on a land area of...

to estimate the length of the meridian arc
Meridian arc
In geodesy, a meridian arc measurement is a highly accurate determination of the distance between two points with the same longitude. Two or more such determinations at different locations then specify the shape of the reference ellipsoid which best approximates the shape of the geoid. This...

through Dunkerque (assumed to be the same length as the Paris meridian
Paris Meridian
The Paris Meridian is a meridian line running through the Paris Observatory in Paris, France—now longitude 2°20′14.025″ east. It was a long-standing rival to Greenwich as the prime meridian of the world, as was the Meridian of Antwerp in Antwerp, Belgium....

). This portion of the meridian was to serve as the basis for the length of the half meridian, connecting the North Pole with the equator. The exact shape of the Earth is not a simple mathematical shape (sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

or oblate spheroid) at the level of precision required for defining a standard of length. The irregular and particular shape of the Earth (smoothed to sea level) is called a Geoid
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...

, which means "Earth-shaped".

However, in 1793, France adopted as its official unit of length a metre based on provisional results from the expedition. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre because of miscalculation of the flattening of the Earth, this length became the standard. The circumference of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

through the poles is therefore slightly more than forty million metres (40 007 863).

Prototype metre bar

In the 1870s and in light of modern precision, a series of international conferences was held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

(BIPM: Bureau International des Poids et Mesures) to be located in Sèvres
Sèvres
Sèvres is a commune in the southwestern suburbs of Paris, France. It is located from the centre of Paris.The town is known for its porcelain manufacture, the Manufacture nationale de Sèvres, making the famous Sèvres porcelain, as well as being the location of the International Bureau of Weights...

, France. This new organisation would preserve the new prototype metre and kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

standards when constructed, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

(CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre
International Prototype Meter
The metre was originally defined as one ten-millionth of the distance between the North Pole and the equator at the longitude of Paris. Because of the difficulty of reproducing this measurement, a platinum bar nominally of that length was constructed in 1799 and housed in the Archives de la...

as the distance between two lines on a standard bar composed of an alloy of ninety percent platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

and ten percent iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

, measured at the melting point of ice.

The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. A discussion of measurements of a standard metre bar and the errors encountered in making the measurements is found in a NIST
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

document.

Standard wavelength of krypton-86 emission

In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson
Albert Abraham Michelson
Albert Abraham Michelson was an American physicist known for his work on the measurement of the speed of light and especially for the Michelson-Morley experiment. In 1907 he received the Nobel Prize in Physics...

, the inventor of the device and an advocate of using some particular wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

as a standard of length. By 1925, interferometry
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

defined the metre in the new SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

system as equal to 1,650,763.73 wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s of the orange
Orange (colour)
The colour orange occurs between red and yellow in the visible spectrum at a wavelength of about 585–620 nm, and has a hue of 30° in HSV colour space. It is numerically halfway between red and yellow in a gamma-compressed RGB colour space, the expression of which is the RGB colour wheel. The...

-red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

emission line in the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

of the krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

-86 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

.

Speed of light

To further reduce uncertainty, the seventeenth CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

and the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

:
This definition fixed the speed of light in classical vacuum at exactly 299,792,458 metres per second. Although the metre is now defined as the path length travelled by light in a given time, actual laboratory length measurements in metres are determined by counting the number of wavelengths of laser light of one of the standard types that fit into the length, and converting the selected unit of wavelength to metres. Three major factors limit the accuracy attainable with laser interferometer
Interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

s for a length measurement:
• Uncertainty in vacuum wavelength of the source,
• Uncertainty in the refractive index of the medium,
• Least count resolution of the interferometer.

Of these, the last is peculiar to the interferometer itself. The conversion of a length in wavelengths to a length in metres is based upon the relation:

which converts the unit of wavelength λ to metres using c, the speed of light in classical vacuum in m/s. Here n is the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

of the medium in which the measurement is made; and f is taken for the conversion here as the measured frequency of the source. Although conversion from wavelengths to metres introduces additional error in the overall length due to measurement error in determining the frequency, measurement of frequency is one of the most accurate measurements available.

An intended byproduct of the 17th CGPM’s definition was that it enabled scientists to compare their lasers accurately using frequency, resulting in wavelengths with one-fifth the uncertainty involved in the direct comparison of wavelengths because interferometer errors were eliminated. To further facilitate reproducibility from lab to lab, the 17th CGPM also made the iodine-stabilised helium-neon laser “a recommended radiation” for realising the metre. For purposes of delineating the metre, the BIPM currently considers the HeNe laser wavelength to be as follows: with an estimated relative standard uncertainty (U) of . This uncertainty is currently the limiting factor in laboratory realisations of the metre as it is several orders of magnitude poorer than that of the second . Consequently, a practical realisation of the metre is usually delineated (not defined) today in labs as wavelengths of helium-neon laser light in a vacuum.

Timeline of definition

• 1790 May 8 – The French National Assembly
National Assembly (French Revolution)
During the French Revolution, the National Assembly , which existed from June 17 to July 9, 1789, was a transitional body between the Estates-General and the National Constituent Assembly.-Background:...

decides that the length of the new metre would be equal to the length of a pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

with a half-period of one second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

.
• 1791 March 30 – The French National Assembly accepts the proposal by the French Academy of Sciences
French Academy of Sciences
The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research...

that the new definition for the metre be equal to one ten-millionth of the length of the Earth's meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

along a quadrant through Paris, that is the distance from the equator to the north pole.
• 1795 – Provisional metre bar constructed of brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

.
• 1799 December 10 – The French National Assembly specifies the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
• 1889 September 28 – The first General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

(CGPM) defines the metre as the distance between two lines on a standard bar of an alloy of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

with ten percent iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

, measured at the melting point of ice.
• 1927 October 6 – The seventh CGPM adjusts the definition of the metre to be the distance, at 0 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
• 1960 October 14 – The 11th CGPM defines the metre to be equal to 1,650,763.73 wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

of the radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

-86 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

.
• 1983 October 21 – The 17th CGPM defines the metre as equal to the length of the path travelled by light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

during a time interval of of a second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

.
• 2002  – The International Committee for Weights and Measures
International Committee for Weights and Measures
The Interglobal Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention...

(CIPM) considers the metre to be a unit of proper length
Proper length
In relativistic physics, proper length is an invariant measure of the distance between two spacelike-separated events, or of the length of a spacelike path within a spacetime....

and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

to be negligible with respect to the uncertainties of realisation".

Definitions of the metre since 1795
Basis of definition Date Absolute
uncertainty
Relative

uncertainty
1/10,000,000 part of the quarter of a meridian,
measurement by Delambre and Mechain
1795 0.5–0.1 mm 10−4
First prototype Metre des Archives
platinum bar standard
1799 0.05–0.01 mm 10−5
Platinum-iridium bar at
melting point of ice (1st CGPM)
1889 0.2–0.1 µm 10−7
Platinum-iridium bar at melting point of ice,
atmospheric pressure,
supported by two rollers (7th CGPM)
1927 n.a. n.a.
Hyperfine atomic transition; 1650763.73 wavelengths
of light from a specified transition in Krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

86 (11th CGPM)
1960 0.01–0.005 µm 10−8
Length of the path travelled by light in a vacuum
in 1/299792458 of a second (17th CGPM)
1983 0.1 nm 10−10

SI prefixed forms of metre

SI prefix
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

es are often employed to denote decimal multiples and submultiples of the metre, as shown in the table below. As indicated in the table, some are commonly used, while others are not. Long distances are usually expressed in km, astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s, light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s, or parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s, rather than in Mm, Gm, Tm, Pm, Em, Zm or Ym; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.
The term micron is often used instead of micrometre, but this practice is officially discouraged.

Spelling

Metre is used as the standard spelling of the metric unit for length in all English-speaking nations except the USA, which uses meter.

The most recent official brochure, written in 2006, about the International System of Units (SI), Bureau international des poids et mesures, was written in French by the International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

. An English translation (using the spelling: metre) is included to make the SI standard "more widely accessible".

In 2008, the U.S. English translation published by the U.S. National Institute of Standards and Technology
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

chose to use meter in accordance with the United States Government Printing Office Style Manual.

Measuring devices (such as parking meter, speedometer) are traditionally spelt "...meter" in all countries. The word "meter", signifying any such device, has the same derivation as the word "metre", denoting the unit of length this article is about.

Equivalents in other units

Metric unit
expressed in non-SI units
Non-SI unit
expressed in metric units
1 metre 1.09361 yards 1 yard
Yard
A yard is a unit of length in several different systems including English units, Imperial units and United States customary units. It is equal to 3 feet or 36 inches...

0.9144 metres
1 metre 39.370 inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

es
1 inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

0.0254 metres
1 centimetre 0.39370 inch   1 inch 2.54 centimetres
1 millimetre
Millimetre
The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....

0.039370 inch   1 inch 25.4 millimetres
1 metre 1×1010 ångström
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

1 ångström 1×10−10 metre
1 nanometre 10 ångström   1 ångström 100 picometres

Within this table, "inch" (and "yard") means "international inch" (and yard). though approximate conversions in the left-hand column hold for both international units and survey units.
"≈" means "is approximately equal to".
"≡" means "equals by definition" or equivalently, "is exactly equal to".

One metre is exactly equivalent to inches and to yards.

A simple mnemonic
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

aid exists to assist with conversion, as three "3";
1 metre is nearly equivalent to 3 feet, 3 and 3/8 inches. This gives an over-estimate of 0.125 mm.

See also

• Conversion of units
Conversion of units
Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors.- Process :...

for comparisons with other units
• International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

• ISO 1
ISO 1
ISO 1 is an international standard that specifies the standard reference temperature for geometrical product specification and verification. The temperature is fixed at 20 degrees centigrade, which is equal to 293.15 Kelvins and 68 degrees Fahrenheit....

– standard reference temperature for length measurements
• Length measurement
Length measurement
Length measurement is implemented in practice in an amazing variety of ways. This article is restricted to only a few methods, in particular, those used with SI units. The most commonly used approaches are the transit-time methods and the interferometer methods based upon the speed of light...

• Metre Convention
• Metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

• Metrication
Metrication
Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in...

• Orders of magnitude (length)
• SI prefix
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

• Speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

Further reading

• Alder, Ken. (2002). The Measure of All Things : The Seven-Year Odyssey and Hidden Error That Transformed the World. Free Press, New York ISBN 0-7432-1675-X
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