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The degree Celsius is the unit of
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
(SI), the other being the
Kelvin scale The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and p ...
. The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference or range between two temperatures. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale in 1742. Before being renamed in 1948 to honour Anders Celsius, the unit was called ''centigrade'', from the Latin ''centum'', which means 100, and ''gradus'', which means steps. Most major countries use this scale; the other major scale, Fahrenheit, is still used in the United States, some island territories, and
Liberia Liberia (), officially the Republic of Liberia, is a country on the West African coast. It is bordered by Sierra Leone to Liberia–Sierra Leone border, its northwest, Guinea to Guinea–Liberia border, its north, Ivory Coast to Ivory Coast� ...
. The Kelvin scale is of use in the sciences, with representing absolute zero. Since 1743 the Celsius scale has been based on 0 °C for the freezing point of water and 100 °C for the boiling point of water at 1  atm pressure. Prior to 1743 the values were reversed (i.e. the boiling point was 0 degrees and the freezing point was 100 degrees). The 1743 scale reversal was proposed by
Jean-Pierre Christin Jean-Pierre Christin (31 May 1683 – 19 January 1755) was a French physicist, mathematician, astronomer, and musician. His proposal in 1743 to reverse the Celsius thermometer scale (from water boiling at 0 degrees and ice melting at 100 degree ...
. By international agreement, between 1954 and 2019 the unit and the Celsius scale were defined by absolute zero and the
triple point In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the ...
of water. After 2007, it was clarified that this definition referred to Vienna Standard Mean Ocean Water (VSMOW), a precisely defined water standard. This definition also precisely related the Celsius scale to the scale of the
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
, the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 °C. Until 19 May 2019, the temperature of the triple point of water was defined as exactly . On 20 May 2019, the kelvin was redefined so that its value is now determined by the definition of the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...
rather than being defined by the triple point of VSMOW. This means that the triple point is now a measured value, not a defined value. The newly-defined exact value of the Boltzmann constant was selected so that the measured value of the VSMOW triple point is exactly the same as the older defined value to within the limits of accuracy of contemporary
metrology Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fran ...
. The temperature in degree Celsius is now defined as the temperature in kelvins subtracted by 273.15, meaning that a temperature difference of one degree Celsius and that of one kelvin are exactly the same, and that the degree Celsius remains exactly equal to the kelvin (i.e., 0 °C remains exactly 273.15 K).


History

In 1742, Swedish astronomer Anders Celsius (1701–1744) created a temperature scale that was the reverse of the scale now known as "Celsius": 0 represented the boiling point of water, while 100 represented the freezing point of water. In his paper ''Observations of two persistent degrees on a thermometer'', he recounted his experiments showing that the melting point of ice is essentially unaffected by pressure. He also determined with remarkable precision how the boiling point of water varied as a function of atmospheric pressure. He proposed that the zero point of his temperature scale, being the boiling point, would be calibrated at the mean barometric pressure at mean sea level. This pressure is known as one standard atmosphere. The BIPM's 10th General Conference on Weights and Measures (CGPM) in 1954 defined one standard atmosphere to equal precisely 1,013,250 dynes per square centimeter (101.325  kPa). In 1743, the
Lyon Lyon,, ; Occitan: ''Lion'', hist. ''Lionés'' also spelled in English as Lyons, is the third-largest city and second-largest metropolitan area of France. It is located at the confluence of the rivers Rhône and Saône, to the northwest of ...
nais physicist
Jean-Pierre Christin Jean-Pierre Christin (31 May 1683 – 19 January 1755) was a French physicist, mathematician, astronomer, and musician. His proposal in 1743 to reverse the Celsius thermometer scale (from water boiling at 0 degrees and ice melting at 100 degree ...
, permanent secretary of the
Academy of Lyon __NOTOC__ The Academy of Sciences, Humanities and Arts of Lyon (French: Académie des Sciences, Belles-Lettres et Arts de Lyon) is a French learned society founded in 1700. Its founders included: * Claude Brossette, lawyer, alderman of Lyons, and ...
, inverted the Celsius scale so that 0 represented the freezing point of water and 100 represented the boiling point of water. Some credit Christin for independently inventing the reverse of Celsius' original scale, while others believe Christin merely reversed Celsius' scale.
Don Rittner Don Rittner is an American historian, archeologist, anthropologist, environmental activist, educator, author and film maker living in the Capital District, Schenectady County, New York. He is the former Schenectady County Historian, responsibl ...
; Ronald A. Bailey (2005)
''Encyclopedia of Chemistry.''
Facts On File,
Manhattan Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state ...
, New York City. p. 43.
On 19 May 1743 he published the design of a
mercury thermometer Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Mercur ...
, the "Thermometer of Lyon" built by the craftsman Pierre Casati that used this scale.''
Mercure de France The was originally a French gazette and literary magazine first published in the 17th century, but after several incarnations has evolved as a publisher, and is now part of the Éditions Gallimard publishing group. The gazette was published ...
'' (1743)
''MEMOIRE sur la dilatation du Mercure dans le Thermométre.''
Chaubert; Jean de Nully, Pissot, Duchesne, Paris. pp. 1609–1610.
''Journal helvétique'' (1743)
''LION.''
Imprimerie des Journalistes,
Neuchâtel , neighboring_municipalities= Auvernier, Boudry, Chabrey (VD), Colombier, Cressier, Cudrefin (VD), Delley-Portalban (FR), Enges, Fenin-Vilars-Saules, Hauterive, Saint-Blaise, Savagnier , twintowns = Aarau (Switzerland), Besançon (Fra ...
. pp. 308–310.
''Memoires pour L'Histoire des Sciences et des Beaux Arts'' (1743)
''DE LYON.''
Chaubert, París. pp. 2125–2128.
In 1744, coincident with the death of Anders Celsius, the Swedish botanist
Carl Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, ...
(1707–1778) reversed Celsius's scale.Citation: Uppsala University (Sweden)
''Linnaeus' thermometer''
/ref> His custom-made "linnaeus-thermometer", for use in his greenhouses, was made by Daniel Ekström, Sweden's leading maker of scientific instruments at the time, whose workshop was located in the basement of the Stockholm observatory. As often happened in this age before modern communications, numerous physicists, scientists, and instrument makers are credited with having independently developed this same scale;Citation for Christin of Lyons: Le Moyne College

citation for Linnaeus's connection with Pehr Elvius and Daniel Ekström: Uppsala University (Sweden)

general citation: The Uppsala Astronomical Observatory

/ref> among them were Pehr Elvius, the secretary of the Royal Swedish Academy of Sciences (which had an instrument workshop) and with whom Linnaeus had been corresponding; , the instrument maker; and Mårten Strömer (1707–1770) who had studied astronomy under Anders Celsius. The first known Swedish documentCitations: University of Wisconsin–Madison

and; Uppsala University

/ref> reporting temperatures in this modern "forward" Celsius scale is the paper ''Hortus Upsaliensis'' dated 16 December 1745 that Linnaeus wrote to a student of his, Samuel Nauclér. In it, Linnaeus recounted the temperatures inside the orangery at the
University of Uppsala Botanical Garden The University of Uppsala Botanical Garden (in Swedish ''Botaniska trädgården''), near Uppsala Castle, is the principal botanical garden belonging to Uppsala University. It was created on land donated to the university in 1787 by Sweden's Ki ...
:


Centigrade vis-à-vis Celsius

Since the 19th century, the scientific and thermometry communities worldwide have used the phrase "centigrade scale" and temperatures were often reported simply as "degrees" or, when greater specificity was desired, as "degrees centigrade", with the symbol °C. In the French language, the term ''centigrade'' also means one hundredth of a gradian, when used for
angular measurement In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles a ...
. The term ''centesimal degree'' was later introduced for temperatures but was also problematic, as it means gradian (one hundredth of a right angle) in the French and Spanish languages. The risk of confusion between temperature and angular measurement was eliminated in 1948 when the 9th meeting of the General Conference on Weights and Measures and the Comité International des Poids et Mesures (CIPM) formally adopted "degree Celsius" for temperature. While "Celsius" is the term commonly used in scientific work, "centigrade" remains in common use in English-speaking countries, especially in informal contexts. While in Australia from 1 September 1972, only Celsius measurements were given for temperature in weather reports/forecasts, it was not until February 1985 that the weather forecasts issued by the BBC switched from "centigrade" to "Celsius".


Common temperatures

Some key temperatures relating the Celsius scale to other temperature scales are shown in the table below.


Name and symbol typesetting

The "degree Celsius" has been the only SI unit whose full unit name contains an uppercase letter since 1967, when the SI base unit for temperature became the
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
, replacing the capitalized term ''degrees Kelvin''. The plural form is "degrees Celsius". The general rule of the
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry ...
(BIPM) is that the numerical value always precedes the unit, and a space is always used to separate the unit from the number, (not "" or ""). The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle (°, ′, and ″, respectively), for which no space is left between the numerical value and the unit symbol.For more information on conventions used in technical writing, see the informative
SI Unit rules and style conventions
' by the
NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
as well as the BIPM's SI brochure: Subsection 5.3.3,
Formatting the value of a quantity.
''
Other languages, and various publishing houses, may follow different typographical rules.


Unicode character

Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, ...
provides the Celsius symbol at code point . However, this is a compatibility character provided for roundtrip compatibility with legacy encodings. It easily allows correct rendering for vertically written East Asian scripts, such as Chinese. The Unicode standard explicitly discourages the use of this character: "In normal use, it is better to represent degrees Celsius "°C" with a sequence of + , rather than . For searching, treat these two sequences as identical."


Temperatures and intervals

The degree Celsius is subject to the same rules as the kelvin with regard to the use of its unit name and symbol. Thus, besides expressing specific temperatures along its scale (e.g. " Gallium melts at 29.7646 °C" and "The temperature outside is 23 degrees Celsius"), the degree Celsius is also suitable for expressing temperature ''intervals'': differences between temperatures or their uncertainties (e.g. "The output of the heat exchanger is hotter by 40 degrees Celsius", and "Our standard uncertainty is ±3 °C").Decision No. 3 o
Resolution 3 of the 13th CGPM
Because of this dual usage, one must not rely upon the unit name or its symbol to denote that a quantity is a temperature interval; it must be unambiguous through context or explicit statement that the quantity is an interval. This is sometimes solved by using the symbol °C (pronounced "degrees Celsius") for a temperature, and C° (pronounced "Celsius degrees") for a temperature interval, although this usage is non-standard.H.D. Young, R. A. Freedman (2008). University Physics with Modern Physics (12th ed.). Addison Wesley. p. 573. Another way to express the same is , which can be commonly found in literature. Celsius measurement follows an interval system but not a ratio system; and it follows a relative scale not an absolute scale. For example, an object at 20 °C does not have twice the energy of when it is 10 °C; and 0 °C is not the lowest Celsius value. Thus, degrees Celsius is a useful interval measurement but does not possess the characteristics of ratio measures like weight or distance.This fact is demonstrated in the book ''Biostatistics: A Guide to Design, Analysis, and Discovery'' By Ronald N. Forthofer, Eun Sul Lee and Mike Hernandez


Coexistence of Kelvin and Celsius scales

In science and in engineering, the Celsius scale and the Kelvin scale are often used in combination in close contexts, e.g. "a measured value was 0.01023 °C with an uncertainty of 70 μK". This practice is permissible because the magnitude of the degree Celsius is equal to that of the kelvin. Notwithstanding the official endorsement provided by decision no. 3 of Resolution 3 of the 13th CGPM, which stated "a temperature interval may also be expressed in degrees Celsius", the practice of simultaneously using both °C and K remains widespread throughout the scientific world as the use of SI-prefixed forms of the degree Celsius (such as "μ°C" or "microdegrees Celsius") to express a temperature interval has not been well adopted.


Melting and boiling points of water

The melting and boiling points of water are no longer part of the definition of the Celsius scale. In 1948, the definition was changed to use the triple point of water. In 2005 the definition was further refined to use water with precisely defined isotopic composition (VSMOW) for the triple point. In 2019, the definition was changed to use the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...
, completely decoupling the definition of the kelvin from the properties of water. Each of these formal definitions left the numerical values of the Celsius scale identical to the prior definition to within the limits of accuracy of the
metrology Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fran ...
of the time. When the melting and boiling points of water ceased being part of the definition, they became measured quantities instead. This is also true of the triple point. In 1948 when the 9th General Conference on Weights and Measures ( CGPM) in Resolution 3 first considered using the triple point of water as a defining point, the triple point was so close to being 0.01 °C greater than water's known melting point, it was simply defined as precisely 0.01 °C. However, later measurements showed that the difference between the triple and melting points of VSMOW is actually very slightly (< 0.001 °C) greater than 0.01 °C. Thus, the actual melting point of ice is very slightly (less than a thousandth of a degree) below 0 °C. Also, defining water's triple point at 273.16 K precisely defined the magnitude of each 1 °C increment in terms of the absolute thermodynamic temperature scale (referencing absolute zero). Now decoupled from the actual boiling point of water, the value "100 °C" is hotter than 0 °C – in absolute terms – by a factor of ''exactly'' (approximately 36.61% thermodynamically hotter). When adhering ''strictly'' to the two-point definition for calibration, the boiling point of VSMOW under one standard atmosphere of pressure was actually 373.1339 K (99.9839 °C). When calibrated to ITS-90 (a calibration standard comprising many definition points and commonly used for high-precision instrumentation), the boiling point of VSMOW was slightly less, about 99.974 °C.Citation: London South Bank University
''Water Structure and Behavior, notes c1 and c2''
/ref> This boiling-point difference of 16.1 millikelvins between the Celsius scale's original definition and the previous one (based on absolute zero and the triple point) has little practical meaning in common daily applications because water's boiling point is very sensitive to variations in
barometric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
. For example, an altitude change of only 28 cm (11 in) causes the boiling point to change by one millikelvin.


See also

* Comparison of temperature scales * Degree of frost * Thermodynamic temperature


Notes


References


External links

*NIST
''Basic unit definitions: Kelvin''
*The Uppsala Astronomical Observatory

*London South Bank University

*BIPM

*TAMPILE
''Comparison of temperature scales''
{{authority control SI derived units Scales of temperature Swedish inventions 1742 introductions 18th-century inventions Scales in meteorology