Metre

The metre (Commonwealth spelling) or meter (American spelling) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 of a second. The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. In 1799, the metre was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and slightly updated in 2019.

This article is about the unit of length. For other uses of "metre" or "meter", see Meter (disambiguation).

metre
Seal of the International Bureau of Weights and Measures (BIPM) – Use measure (Greek: ΜΕΤΡΩ ΧΡΩ)
General information
Unit systemSI base unit
Unit oflength
Symbolm[1] 
Conversions
1 m[2] in ...... is equal to ...
   SI units   1000 mm
 0.001 km
   Imperial/US units   ≈ 1.0936 yd
 ≈ 3.2808 ft
 ≈ 39.370 in 
   Nautical units   ≈ 0.00053996 nmi 

Spelling

Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States[3][4][5][6] and the Philippines,[7] which use meter. Other Germanic languages, such as German, Dutch, and the Scandinavian languages[8] likewise spell the word meter.

Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English.[9] The suffix "-meter" has the same Greek origin as the unit of length.[10][11]

Etymology

The etymological roots of metre can be traced to the Greek verb μετρέω (metreo) (to measure, count or compare) and noun μέτρον (metron) (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (metior, mensura), French (mètre, mesure), English and other languages. The motto ΜΕΤΡΩ ΧΡΩ (metro chro) in the seal of the International Bureau of Weights and Measures (BIPM), which was a saying of the Greek statesman and philosopher Pittacus of Mytilene and may be translated as "Use measure!", thus calls for both measurement and moderation. The use of the word metre (for the French unit mètre) in English began at least as early as 1797.[12]

History of definition
Main article: History of the metre
Meridian room of the Paris Observatory (or Cassini room): the Paris meridian is drawn on the ground.

In 1671 Jean Picard measured the length of a "seconds pendulum" (a pendulum with a period of two seconds) at the Paris observatory. He found the value of 440.5 lines of the Toise of Châtelet which had been recently renewed. He proposed a universal toise (French: Toise universelle) which was twice the length of the seconds pendulum.[13][14] However, it was soon discovered that the length of a seconds pendulum varies from place to place: French astronomer Jean Richer had measured the 0.3% difference in length between Cayenne (in French Guiana) and Paris.[15][16][17]

Jean Richer and Giovanni Domenico Cassini measured the parallax of Mars between Paris and Cayenne in French Guiana when Mars was at its closest to Earth in 1672. They arrived at a figure for the solar parallax of 9.5 arcseconds, equivalent to an Earth–Sun distance of about 22000 Earth radii. They were also the first astronomers to have access to an accurate and reliable value for the radius of Earth, which had been measured by their colleague Jean Picard in 1669 as 3269 thousand toises. Picard's geodetic observations had been confined to the determination of the magnitude of the Earth considered as a sphere, but the discovery made by Jean Richer turned the attention of mathematicians to its deviation from a spherical form. In addition to its significance for cartography, the determination of the Figure of the Earth became a problem of the highest importance in astronomy, inasmuch as the diameter of the Earth was the unit to which all celestial distances had to be referred.[18] [19][20][21]

Meridional definition
Paris Panthéon

As a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, and on 19 March 1791 advised the adoption of the term mètre ("measure"), a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator along the meridian through Paris.[22][23][24][25][26] In 1793, the French National Convention adopted the proposal.[12]

The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to accurately measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona at the longitude of Paris Panthéon (see meridian arc of Delambre and Méchain).[27] The expedition was fictionalised in Denis Guedj, Le Mètre du Monde.[28] Ken Alder wrote factually about the expedition in The Measure of All Things: the seven year odyssey and hidden error that transformed the world.[29] This portion of the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. From 1801 to 1812 France adopted this definition of the metre as its official unit of length based on results from this expedition combined with those of the Geodesic Mission to Peru.[30][31] The latter was related by Larrie D. Ferreiro in Measure of the Earth: The Enlightenment Expedition that Reshaped Our World.[32]

Triangulation near New York City, 1817.

A more accurate determination of the Figure of the Earth would soon result from the measurement of the Struve Geodetic Arc (1816–1855) and would have given another value for the definition of this standard of length. This did not invalidate the metre but highlighted that progresses in science would allow better measurement of Earth's size and shape.[21] After the July Revolution of 1830 the metre became the definitive French standard from 1840. At that time it had already been adopted by Ferdinand Rudolph Hassler for the U.S Survey of the Coast.[30][33][34]

"The unit of length to which all distances measured in the Coast Survey are referred is the French metre, an authentic copy of which is preserved in the archives of the Coast Survey Office. It is the property of the American Philosophical Society, to whom it was presented by Mr. Hassler, who had received it from Tralles, a member of the French Committee charged with the construction of the standard metre by comparison with the toise, which had served as unit of length in the measurement of the meridional arcs in France and Peru. It possesses all the authenticity of any original metre extant, bearing not only the stamp of the Committee but also the original mark by which it was distiguished from the other bars during the operation of standarding. It is always designated as the Committee metre" (French : Mètre des Archives).[35]

In 1830 President Andrew Jackson mandated Ferdinand Rudolf Hassler to work out new standards for all U.S. states. According to the decision of the Congress of the United States, the British Parlementary Standard from 1758 was introduced as the unit of length.[36] Another geodesist with metrology skills was to play a pivotal role in the process of internationalization of weights and measures, Carlos Ibáñez e Ibáñez de Ibero who would become the first president of both the International Geodetic Association and the International Committee for Weights and Measures.[37]

International prototype metre bar
Creating the metre-alloy in 1874 at the Conservatoire des Arts et Métiers. Present Henri Tresca, George Matthey, Saint-Claire Deville and Debray

In 1867 at the second general conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth.[38][39][40] The conference recommended the adoption of the metre in replacement of the toise and the creation of an international metre commission, according to the proposal of Johann Jacob Baeyer, Adolphe Hirsch and Carlos Ibáñez e Ibáñez de Ibero who had devised two geodetic standards calibrated on the metre for the map of Spain.[34][38][40][41] Measurement traceability between the toise and the metre was ensured by comparison of the Spanish standard with the standard devised by Borda and Lavoisier for the survey of the meridian arc connecting Dunkirk with Barcelona.[37][41][42]

A member of the Preparatory Committee since 1870 and Spanish representative at the Paris Conference in 1875, Carlos Ibáñez e Ibáñez de Ibero intervened with the French Academy of Sciences to rally France to the project to create an International Bureau of Weights and Measures equipped with the scientific means necessary to redefine the units of the metric system according to the progress of sciences.[43]

Gravimeter with variant of Repsold pendulum

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 (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation distributed such bars in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice.[44]

The comparison of the new prototypes of the metre with each other and with the Committee metre (French: Mètre des Archives) involved the development of special measuring equipment and the definition of a reproducible temperature scale. The BIPM's thermometry work led to the discovery of special alloys of iron-nickel, in particular invar, for which its director, the Swiss physicist Charles-Edouard Guillaume, was granted the Nobel Prize for physics in 1920.[45]

Artist's impression of a GPS-IIR satellite in orbit.

As Carlos Ibáñez e Ibáñez de Ibero stated, the progress of metrology combined with those of gravimetry through improvement of Kater's pendulum led to a new era of geodesy. If precision metrology had needed the help of geodesy, the latter could not continue to prosper without the help of metrology. Indeed, how to express all the measurements of terrestrial arcs as a function of a single unit, and all the determinations of the force of gravity with the pendulum, if metrology had not created a common unit, adopted and respected by all civilized nations, and if in addition one had not compared, with great precision, to the same unit all the standards for measuring geodesic bases, and all the pendulum rods that had hitherto been used or would be used in the future? Only when this series of metrological comparisons would be finished with a probable error of a thousandth of a millimetre would geodesy be able to link the works of the different nations with one another, and then proclaim the result of the last measurement of the Globe. As the figure of the Earth could be inferred from variations of the seconds pendulum length with latitude, the United States Coast Survey instructed Charles Sanders Peirce in the spring of 1875 to proceed to Europe for the purpose of making pendulum experiments to chief initial stations for operations of this sort, in order to bring the determinations of the forces of gravity in America into communication with those of other parts of the world; and also for the purpose of making a careful study of the methods of pursuing these researches in the different countries of Europe. In 1886 the association of geodesy changed name for the International Geodetic Association, which Carlos Ibáñez e Ibáñez de Ibero presided up to his death in 1891. During this period the International Geodetic Association (German: Internationale Erdmessung) gained worldwide importance with the joining of United States, Mexico, Chile, Argentina and Japan.[37][46][47][48][49][50]

Efforts to supplement the various national surveying systems, which begun in the 19th century with the foundation of the Mitteleuropäische Gradmessung, resulted in a series of global ellipsoids of the Earth (e.g., Helmert 1906, Hayford 1910/1924) which would later lead to develop the World Geodetic System. Nowadays the practical realisation of the metre is possible everywhere thanks to the atomic clocks embedded in GPS satellites.[51][52]

Wavelength definition

In 1873, James Clerk Maxwell suggested that light emitted by an element be used as the standard both for the meter and for the second. These two quantities could then be used to define the unit of mass.[53]

In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of length. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1650763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.[54]

Speed of light definition

To further reduce uncertainty, the 17th 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 and the speed of light:[55]

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

This definition fixed the speed of light in vacuum at exactly 299792458 metres per second (≈300000 km/s).[55] An intended by-product of the 17th CGPM's definition was that it enabled scientists to compare 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.[56] For the purpose of delineating the metre, the BIPM currently considers the HeNe laser wavelength, λHeNe, to be 632.99121258 nm with an estimated relative standard uncertainty (U) of 2.1×10−11.[56][57][58] This uncertainty is currently one limiting factor in laboratory realisations of the metre, and it is several orders of magnitude poorer than that of the second, based upon the caesium fountain atomic clock (U = 5×10−16).[59] Consequently, a realisation of the metre is usually delineated (not defined) today in labs as 1579800.762042(33) wavelengths of helium-neon laser light in a vacuum, the error stated being only that of frequency determination.[56] This bracket notation expressing the error is explained in the article on measurement uncertainty.

Practical realisation of the metre is subject to uncertainties in characterising the medium, to various uncertainties of interferometry, and to uncertainties in measuring the frequency of the source.[60] A commonly used medium is air, and the National Institute of Standards and Technology (NIST) has set up an online calculator to convert wavelengths in vacuum to wavelengths in air.[61] As described by NIST, in air, the uncertainties in characterising the medium are dominated by errors in measuring temperature and pressure. Errors in the theoretical formulas used are secondary.[62] By implementing a refractive index correction such as this, an approximate realisation of the metre can be implemented in air, for example, using the formulation of the metre as 1579800.762042(33) wavelengths of helium–neon laser light in vacuum, and converting the wavelengths in a vacuum to wavelengths in air. Air is only one possible medium to use in a realisation of the metre, and any partial vacuum can be used, or some inert atmosphere like helium gas, provided the appropriate corrections for refractive index are implemented.[63]

The metre is defined as the path length travelled by light in a given time and practical 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,[66] and converting the selected unit of wavelength to metres. Three major factors limit the accuracy attainable with laser interferometers for a length measurement:[60][67]

  • 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 vacuum in m/s. Here n is the refractive index of the medium in which the measurement is made, and f is the measured frequency of the source. Although conversion from wavelengths to metres introduces an additional error in the overall length due to measurement error in determining the refractive index and the frequency, the measurement of frequency is one of the most accurate measurements available.[67]

Timeline
Closeup of National Prototype Metre Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) and given to the United States, which served as the standard for defining all units of length in the US from 1893 to 1960
DateDeciding bodyDecision
8 May 1790French National AssemblyThe length of the new metre to be equal to the length of a pendulum with a half-period of one second.[30]
30 Mar 1791French National AssemblyAccepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to one ten-millionth of the length of a great circle quadrant along the Earth's meridian through Paris, that is the distance from the equator to the north pole along that quadrant.[68]
1795Provisional metre bar made of brass and based on Paris meridan arc (French: Méridienne de France) measured by Nicolas-Louis de Lacaillle and Cesar-François Cassini de Thury, legally equal to 443.44 lines of the toise du Pérou (a standard French unit of length from 1766).[30][31][42][52] [The line was 1/864 of a toise.]
10 Dec 1799French National AssemblySpecifies the platinum metre bar, presented on 22 June 1799 and deposited in the National Archives, as the final standard. Legally equal to 443.296 lines on the toise du Pérou.[52]
24–28 Sept 18891st General Conference on Weights and Measures (CGPM)Defines the metre as the distance between two lines on a standard bar of an alloy of platinum with 10% iridium, measured at the melting point of ice.[52][69]
27 Sept – 6 Oct 19277th CGPMRedefines the metre as the distance, at 0 °C (273 K), 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 and supported on two cylinders of at least 10 mm (1 cm) diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm (57.1 cm) from each other.[70]
14 Oct 196011th CGPMDefines the metre as 1650763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom.[71]
21 Oct 198317th CGPMDefines the metre as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second.[72][73]
2002International Committee for Weights and Measures (CIPM)Considers the metre to be a unit of proper length and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by general relativity to be negligible with respect to the uncertainties of realisation".[74]
Definitions of the metre since 1795[75]
Basis of definition Date Absolute
uncertainty		 Relative
uncertainty
1/10 000 000 part of the quadrant along the meridian, measurement by Delambre and Méchain (443.296 lines) 1795 500–100 μm 10−4
First prototype Mètre des Archives platinum bar standard 1799 50–10 μm 10−5
Platinum-iridium bar at melting point of ice (1st CGPM) 1889 0.2–0.1 μm (200–100 nm) 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-86 (11th CGPM) 1960 4 nm 4×10−9[76]
Length of the path travelled by light in a vacuum in 1/299 792 458 second (17th CGPM) 1983 0.1 nm 10−10

SI prefixed forms of metre

SI prefixes 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 units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), 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 terms micron and (occasionally) millimicron are often used instead of micrometre (μm) and nanometre (nm), but this practice is officially discouraged.[77]

SI multiples of metre (m)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1 m dm decimetre 101 m dam decametre
10−2 m cm centimetre 102 m hm hectometre
10−3 m mm millimetre 103 m km kilometre
10−6 m µm micrometre 106 m Mm megametre
10−9 m nm nanometre 109 m Gm gigametre
10−12 m pm picometre 1012 m Tm terametre
10−15 m fm femtometre 1015 m Pm petametre
10−18 m am attometre 1018 m Em exametre
10−21 m zm zeptometre 1021 m Zm zettametre
10−24 m ym yoctometre 1024 m Ym yottametre
Common prefixed units are in bold face.

Equivalents in other units
Metric unit
expressed in non-SI units		 Non-SI unit
expressed in metric units
1 metre≈1.0936yard 1 yard≡0.9144metre
1 metre≈39.370inches 1 inch≡0.0254metre
1 centimetre≈0.39370inch 1 inch≡2.54centimetres
1 millimetre≈0.039370inch 1 inch≡25.4millimetres
1 metre≡1 × 1010ångström 1 ångström≡1 × 10−10metre
1 nanometre≡10ångström 1 ångström≡100picometres

Within this table, "inch" and "yard" mean "international inch" and "international yard"[78] respectively, though approximate conversions in the left column hold for both international and survey units.

"≈" means "is approximately equal to";
"≡" means "equal by definition" or "is exactly equal to".

One metre is exactly equivalent to 5 000/127 inches and to 1 250/1 143 yards.

A simple mnemonic aid exists to assist with conversion, as three "3"s:

1 metre is nearly equivalent to 3 feet ​3 3⁄8 inches. This gives an overestimate of 0.125 mm; however, the practice of memorising such conversion formulas has been discouraged in favour of practice and visualisation of metric units.

The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm).[79] Scottish and English definitions of the ell (two cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively.[80][81] The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly ​1⁄2 toise.[82] The Russian verst was 1.0668 km.[83] The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.[84]

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See also

Notes
  1. "Base unit definitions: Meter". National Institute of Standards and Technology. Retrieved 28 September 2010.
  2. "Base unit definitions: Meter". National Institute of Standards and Technology. Retrieved 28 September 2010.
  3. "The International System of Units (SI) – NIST". US: National Institute of Standards and Technology. 26 March 2008. The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary. Thus the spellings "meter,"…rather than "metre,"...as in the original BIPM English text...
  4. The most recent official brochure about the International System of Units (SI), written in French by the Bureau international des poids et mesures, International Bureau of Weights and Measures (BIPM) uses the spelling metre; an English translation, included to make the SI standard more widely accessible also uses the spelling metre (BIPM, 2006, p. 130ff). However, in 2008 the U.S. English translation published by the U.S. National Institute of Standards and Technology (NIST) chose to use the spelling meter in accordance with the United States Government Printing Office Style Manual. The Metric Conversion Act of 1975 gives the Secretary of Commerce of the US the responsibility of interpreting or modifying the SI for use in the US. The Secretary of Commerce delegated this authority to the Director of the National Institute of Standards and Technology (Turner). In 2008, NIST published the US version (Taylor and Thompson, 2008a) of the English text of the eighth edition of the BIPM publication Le Système international d'unités (SI) (BIPM, 2006). In the NIST publication, the spellings "meter", "liter" and "deka" are used rather than "metre", "litre" and "deca" as in the original BIPM English text (Taylor and Thompson (2008a), p. iii). The Director of the NIST officially recognised this publication, together with Taylor and Thompson (2008b), as the "legal interpretation" of the SI for the United States (Turner). Thus, the spelling metre is referred to as the "international spelling"; the spelling meter, as the "American spelling".
  5. Naughtin, Pat (2008). "Spelling metre or meter" (PDF). Metrication Matters. Retrieved 12 March 2017.
  6. "Meter vs. metre". Grammarist. Retrieved 12 March 2017.
  7. The Philippines uses English as an official language and this largely follows American English since the country became a colony of the United States. While the law that converted the country to use the metric system uses metre (Batas Pambansa Blg. 8) following the SI spelling, in actual practice, meter is used in government and everyday commerce, as evidenced by laws (kilometer, Republic Act No. 7160), Supreme Court decisions (meter, G.R. No. 185240), and national standards (centimeter, PNS/BAFS 181:2016).
  8. "295–296 (Nordisk familjebok / Uggleupplagan. 18. Mekaniker – Mykale)" [295–296 (Nordic Family Book / Owl Edition. 18. Mechanic – Mycular)]. Stockholm. 1913.
  9. Cambridge Advanced Learner's Dictionary. Cambridge University Press. 2008. Retrieved 19 September 2012., s.v. ammeter, meter, parking meter, speedometer.
  10. American Heritage Dictionary of the English Language (3rd ed.). Boston: Houghton Mifflin. 1992., s.v. meter.
  11. "-meter – definition of -meter in English". Oxford Dictionaries.
  12. Oxford English Dictionary, Clarendon Press 2nd ed.1989, vol.IX p.697 col.3.
  13. texte, Picard, Jean (1620–1682). Auteur du (1671). Mesure de la terre [par l'abbé Picard]. Gallica. pp. 3–4. Retrieved 13 September 2018.
  14. Bigourdan, Guillaume (1901). Le système métrique des poids et mesures ; son établissement et sa propagation graduelle, avec l'histoire des opérations qui ont servi à déterminer le mètre et le kilogramme. University of Ottawa. Paris : Gauthier-Villars. pp. 6–8.
  15. Poynting, John Henry; Thomson, Joseph John (1907). A Textbook of Physics. C. Griffin. pp. 20.
  16. Picard, Jean (1620–1682) Auteur du texte (1671). Mesure de la terre [par l'abbé Picard]. pp. 3–5.
  17. Bond, Peter, (1948- ...). (2014). L'exploration du système solaire. Dupont-Bloch, Nicolas. ([Édition française revue et corrigée] ed.). Louvain-la-Neuve: De Boeck. pp. 5–6. ISBN 9782804184964. OCLC 894499177.CS1 maint: multiple names: authors list (link)
  18.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Clarke, Alexander Ross; Helmert, Friedrich Robert (1911). "Earth, Figure of the". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 8 (11th ed.). Cambridge University Press. p. 801.
  19. "Première détermination de la distance de la Terre au Soleil | Les 350 ans de l'Observatoire de Paris". 350ans.obspm.fr. Retrieved 14 May 2019.
  20. Buffet, Loriane. "Cassini, l'Astronome du roi et le satellite – Exposition virtuelle". expositions.obspm.fr (in French). Retrieved 14 May 2019.
  21. "Nomination of the Struve geodetic arc for inscription on the World Heritage List" (PDF). Retrieved 13 May 2019.
  22. Tipler, Paul A.; Mosca, Gene (2004). Physics for Scientists and Engineers (5th ed.). W.H. Freeman. p. 3. ISBN 0716783398.
  23. ('decimalization is not of the essence of the metric system; the real significance of this is that it was the first great attempt to define terrestrial units of measure in terms of an unvarying astronomical or geodetic constant.) The metre was in fact defined as one ten-millionth of one-quarter of the earth's circumference at sea-level.' Joseph Needham, Science and Civilisation in China, Cambridge University Press, 1962 vol.4, pt.1, p.42.
  24. Agnoli, Paolo (2004). Il senso della misura: la codifica della realtà tra filosofia, scienza ed esistenza umana (in Italian). Armando Editore. pp. 93–94, 101. ISBN 9788883585326. Retrieved 13 October 2015.
  25. Rapport sur le choix d'une unité de mesure, lu à l'Académie des sciences, le 19 mars 1791 (in French). Gallica.bnf.fr. 15 October 2007. Retrieved 25 March 2013.: "Nous proposerons donc de mesurer immédiatement un arc du méridien, depuis Dunkerque jusqu'a Bracelone: ce qui comprend un peu plus de neuf degrés & demi." [We propose then to measure directly an arc of the meridian between Dunkirk and Barcelona: this spans a little more than nine-and-a-half degrees."] p. 8
  26. Paolo Agnoli and Giulio D’Agostini,'Why does the meter beat the second?,' December, 2004 pp.1–29.
  27. Ramani, Madhvi. "How France created the metric system". www.bbc.com. Retrieved 21 May 2019.
  28. Guedj 2001.
  29. Alder 2002.
  30. Larousse, Pierre (1817–1875) (1866–1877). Grand dictionnaire universel du XIXe siècle : français, historique, géographique, mythologique, bibliographique.... T. 11 MEMO-O / par M. Pierre Larousse.CS1 maint: date format (link)
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References
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