Physical unit
Unit symbol ${\ displaystyle \ mathrm {^ {\ circ} C}}$
Physical quantity (s) temperature
Formula symbol ${\ displaystyle t \, (\ vartheta)}$
dimension ${\ displaystyle {\ mathsf {\ Theta}}}$
system International system of units
In SI units ${\ displaystyle \ left \ {t \ right \} {} _ {\ mathrm {^ {\ circ} C}} = \ left \ {T \ right \} _ {\ mathrm {K}} -273 {,} 15}$
Named after Different Celsius
Derived from Kelvin

The degree Celsius is a unit of measurement of temperature , which was named after Anders Celsius .

## definition

Celsius temperature is over the absolute temperature (thermodynamic temperature) with the unit Kelvin defined (K) as follows: . This means that the numerical values ​​when using the units degrees Celsius and Kelvin differ by the constant value 273.15. For example, 293.15 K and 20 ° C denote the same temperature, and the absolute zero at 0 K is −273.15 ° C. ${\ displaystyle t}$${\ displaystyle T}$${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} = \ left \ {T \ right \} _ {\ mathrm {K}} -273 {,} 15}$

With this definition of the Celsius scale, the melting and boiling points of water are very close to 0 ° C and 100 ° C (at 0.002519 ° C and 99.9839 ° C (99.9743 ° C according to ITS-90 ) ).

As a symbol for the Celsius temperature, the small t complies with the standard according to SI ; alternatively, this ( theta , also to distinguish ) is common. The use of the capital T is wrong because T is reserved for the absolute temperature in Kelvin. ${\ displaystyle \ vartheta}$${\ displaystyle \ theta}$

## history

The Celsius scale goes back to the Swedish astronomer Anders Celsius , who introduced a hundred-part temperature scale in 1742. Like the Réaumur scale presented in 1730 , he used the temperatures of the freezing and boiling points of water at normal pressure , i.e. an air pressure of 1013.25 hectopascals or 760  millimeters of mercury , as fixed points . The area between these fixed points, measured with a mercury thermometer , is divided into 100 equally long sections, which are designated as degrees. This led to the historical name of the "hundred-part thermometer". Unlike the modern Celsius scale, however, Celsius assigned the value of 0 ° C to the boiling point of water and 100 ° C to the freezing point. Thus, the temperature value of a body decreased when heated.

The modern Celsius scale, in which the boiling point of water is assigned the value 100 ° C and the freezing point the value 0 ° C, was introduced by Carl von Linné , a friend of Celsius, shortly after his death in 1744.

In 1948, about 200 years after the introduction of the scale, the scale distance of a centigrade or centesimal degree on a Celsius thermometer was officially renamed to the temperature unit degrees Celsius by the 9th International General Conference on Weights and Measures .

In 1954 the Kelvin scale and, based on it, the definition of degrees Celsius that is valid today were introduced. The freezing and boiling points of water thus lost their role as fixed points on the Celsius scale.

## symbol

The symbol for the unit of measurement is a combination of the degree symbol and the capital letter " C ". These are to be viewed as a unit and must not be separated. The numerical value is in front of it, as usual with units of measurement, separated by a space. For compatibility contains Unicode standard in Unicode block "Letterlike Symbols" in addition the representation by a sign ℃ ( U+2103), but the Unicode Consortium discourages use. According to the rules of the organs of the international meter convention , degrees Celsius may also be used together with SI prefixes . However, this regulation has not been adopted in the national German standard of the German Institute for Standardization ( DIN 1301-1 , DIN 1345 ) and is therefore not permitted under German unit law.

## Temperature difference

The temperature difference is the difference in the temperature of two measuring points that differ in time or spatial position. Since the Kelvin and Celsius scales are shifted by a fixed value, the numerical values ​​of temperature differences match when using the units Kelvin and degrees Celsius: ${\ displaystyle \ Delta t}$${\ displaystyle \ left \ {\ Delta t \ right \} _ {\ mathrm {{} ^ {\ circ} C}} = \ left \ {\ Delta T \ right \} _ {\ mathrm {K}}}$

As a unit for temperature differences, DIN recommends the Kelvin in adaptation to the International System of Units (SI) with the DIN 1345 standard (December 1993 edition). DIN adds: "According to the resolution of the 13th General Conference on Weights and Measures (1967–1968), the difference between two Celsius temperatures may also be given in the unit degree Celsius (° C)." The unit name is degrees Celsius used as a special name for the Kelvin.

Example: The difference between the temperature (corresponds to ) and the temperature ( ) is . This can also be written as, but of course this difference is not to be equated with . ${\ displaystyle t_ {b} = 20 \, \ mathrm {{} ^ {\ circ} C}}$${\ displaystyle T_ {b} = 293 {,} 15 \, \ mathrm {K}}$${\ displaystyle t_ {a} = 10 \, \ mathrm {{} ^ {\ circ} C}}$${\ displaystyle T_ {a} = 283 {,} 15 \, \ mathrm {K}}$${\ displaystyle \ Delta t = t_ {b} -t_ {a} = 10 \, \ mathrm {K}}$${\ displaystyle \ Delta t = 10 \, \ mathrm {{} ^ {\ circ} C}}$${\ displaystyle 283 {,} 15 \, \ mathrm {K}}$

The same applies to multiples: It would be wrong to  call t  = 60 ° C ( T = 333.15 K) compared to t  = 30 ° C ( T  = 303.15 K) as "twice as warm"; However, such a statement is correct for the absolute temperature, e.g. B. "600 K is twice as warm as 300 K".

## Comparison with other scales

### conversion

Temperatures in degrees Celsius can be exactly converted using a numerical equation as follows:

 Kelvin: ${\ displaystyle \ left \ {T \ right \} _ {\ mathrm {K}} = \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} +273 {,} 15}$ ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} = \ left \ {T \ right \} _ {\ mathrm {K}} -273 {,} 15}$ Fahrenheit degree : ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} F}} = \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} \ cdot 1 {,} 8 + 32}$ ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} = (\ left \ {t \ right \} _ {\ mathrm {^ {\ circ} F}} - 32) \ cdot {\ tfrac {5} {9}}}$ Rankine degree : ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} Ra}} = \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} \ cdot 1.8 + 491 {,} 67}$ ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} = \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} Ra}} \ cdot {\ tfrac {5} {9}} - 273 {,} 15}$ Degree Réaumur : ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} R}} = \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} \ cdot 0 {,} 8}$ ${\ displaystyle \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} C}} = \ left \ {t \ right \} _ {\ mathrm {^ {\ circ} R}} \ cdot 1 {,} 25}$

### Fixed points

Fixed points of common temperature scales
Kelvin ° Celsius ° Fahrenheit ° Rankine ° Réaumur
Boiling point of water at normal pressure  373.150K 100,000 ° C 212,000 ° F 671.670 ° Ra 80,000 ° Ré
" Human body temperature " according to Fahrenheit 308.70 5  K 35, 555  ° C 96,000 ° F 555.670 ° Ra 28, 444  ° Ré
Triple point of water 273.160K 0.010 ° C 32.018 ° F 491.688 ° Ra 0.008 ° Ré
Freezing point of water at normal pressure 273.150K 0.000 ° C 32,000 ° F 491.670 ° Ra 0.000 ° Ré
Cold mixture of water, ice and NH 4 Cl 255.37 2  K −17, 777  ° C 0.000 ° F 459.670 ° Ra −14, 222  ° Ré
absolute zero 0 K −273.150 ° C −459.670 ° F 0 ° Ra −218.520 ° Ré

The fixed points with which the scales were originally defined are highlighted in color and converted exactly into the other scales. Today they have lost their role as fixed points and are only approximate. Only the absolute zero point still has exactly the specified values.

## Individual evidence

1. according to DIN 1301 part 1 - October 2010: units - part 1: unit names, unit symbols. : “The unit names 'degrees Celsius' and 'degrees' used to be male according to DIN 1301-1. Since degree is not only used as a unit, but also as a masculine noun in the sense of extent (see also DIN 5485), the neuter forms were specified to differentiate between the units, which the Duden, Volume 1, also allows. "
2. Information on water on the London South Bank University website.
3. a b BIPM (Ed.): The International System of Units (SI) . 8th edition. 2014, Section 2.1.1.5 (English, bipm.org ( memento of March 25, 2015 in the Internet Archive ) [accessed March 8, 2018]).
4. A. Celsius: Observationer om two constant graders på en thermometer . In: Kungliga Vetenskapsakademiens handlingar . 1742, p. 171–180 (Swedish, facsimile in the Google book search).
5. a b Jakow Abramowitsch Smorodinskij, Paul Ziesche: What is temperature? Concept, history, laboratory and cosmos. Harri Deutsch, Thun 2000. p. 11 ( limited preview in Google book search).
6. a b c Joachim Blüthgen, Wolfgang Weischet: General climate geography. Textbook of General Geography. de Gruyter, Berlin / New York 1980, p. 118 ( limited preview in the Google book search).
7. Nikos Psarros: The chemistry and its methods . John Wiley & Sons, 2008, ISBN 978-3-527-62463-8 , pp. 109 ( limited preview in Google Book search).
8. ^ Minutes of the 9th General Conference on Weights and Measures , 1948, p. 64 (accessed June 4, 2020), French
9. Resolution 7 of the 9th meeting of the CGPM (1948) ( online , accessed June 4, 2020), English
10. BIPM (ed.): The International System of Units (SI) . 9th edition. 2019, chap. 5.4.3: Formatting the value of a quantity , p. 149 (English, bipm.org [PDF; accessed November 3, 2019]).
11. Unicode Consortium: The Unicode Standard, Version 10.0. (PDF) 2017, p. 785 , accessed on February 26, 2018 (English).
12. Section 1 (3) of the Unit Ordinance