# Celsius

Celsius
A thermometer calibrated in degrees Celsius
General information
Unit systemSI derived unit
Unit oftemperature
Symbol°C
Named afterAnders Celsius
Conversions
1 °C in ...... is equal to ...
K   c + 273.15

The Celsius scale, also known as the centigrade scale,[1][2] is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used by all countries except the United States[3], the Bahamas, Belize, the Cayman Islands and Liberia. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. Before being renamed to honor Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps.

From 1743 to 1954, the Celsius scale was 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 scale was also based on the boiling and melting points of water, but the values were reversed (i.e. the boiling point was at 0 degrees and the melting point was at 100 degrees). The 1743 scale reversal was proposed by Jean-Pierre Christin.

By international agreement, since 1954 the unit degree Celsius and the Celsius scale are defined by absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW), a specially purified water. This definition also precisely relates the Celsius scale to the Kelvin scale, which defines 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. The temperature of the triple point of water is defined as exactly 273.16 K (0.01 °C).[4] This means that a temperature difference of one degree Celsius and that of one kelvin are exactly the same.[5]

## History

An illustration of Anders Celsius's original thermometer. Note the reversed scale, where 100 is the freezing point of water and 0 is its boiling point.

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.[6] 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) later defined one standard atmosphere to equal precisely 1,013,250 dynes per square centimetre (101.325 kPa).[7]

In 1743, the Lyonnais physicist Jean-Pierre Christin, permanent secretary of the Académie des sciences, belles-lettres et arts de LyonFR, working independently of Celsius, developed a scale where zero represented the freezing point of water and 100 represented the boiling point of water.[8][9] On 19 May 1743 he published the design of a mercury thermometer, the "Thermometer of Lyon" built by the craftsman Pierre Casati that used this scale.[10][11][12]

In 1744, coincident with the death of Anders Celsius, the Swedish botanist Carl Linnaeus (1707–1778) reversed Celsius's scale.[13] 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;[14] 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; Daniel Ekström[SV], the instrument maker; and Mårten Strömer (1707–1770) who had studied astronomy under Anders Celsius.

The first known Swedish document[15] 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:

...since the caldarium (the hot part of the greenhouse) by the angle of the windows, merely from the rays of the sun, obtains such heat that the thermometer often reaches 30 degrees, although the keen gardener usually takes care not to let it rise to more than 20 to 25 degrees, and in winter not under 15 degrees...

Since the 19th century, the scientific and thermometry communities worldwide have used the phrase "centigrade scale". Temperatures on the centigrade scale were often reported simply as degrees or, when greater specificity was desired, as degrees centigrade (symbol: °C). Because the term centigrade was also the Spanish and French language name for a unit of angular measurement (1/10000 of a right angle) and had a similar connotation in other languages, the term centesimal degree (known as the gradian, "grad" or "gon": 1ᵍ = 0.9°, 100ᵍ = 90°) was used when very precise, unambiguous language was required by international standards bodies such as the BIPM. More properly, what was defined as "centigrade" then would now be "hectograde".

To eliminate any confusion, the 9th CGPM and the CIPM (Comité international des poids et mesures) formally adopted "degree Celsius" in 1948,[16][a] formally keeping the recognized degree symbol, rather than adopting the gradian/centesimal degree symbol.

For scientific use, "Celsius" is the term usually used, with "centigrade" remaining in common but decreasing use, especially in informal contexts in English-speaking countries.[17] It was not until February 1985 that the weather forecasts issued by the BBC switched from "centigrade" to "Celsius".[18]

### Common temperatures

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

Key scale relations
Kelvin Celsius Fahrenheit
Absolute zero (exactly) 0 K −273.15 °C −459.67 °F
Boiling point of liquid nitrogen 77.4 K −195.8 °C[19] −320.4 °F
Sublimation point of dry ice 195.1 K −78 °C −108.4 °F
Intersection of Celsius and Fahrenheit scales 233.15 K −40 °C −40 °F
Melting point of H2O (purified ice)[20] 273.1499 K −0.0001 °C 31.9998 °F
Water's triple point (exactly) 273.16 K 0.01 °C 32.018 °F
Normal human body temperature (average)[21] 310.15 K 37.0 °C 98.6 °F
Water's boiling point at 1 atm (101.325 kPa)
(approximate: see Boiling point)[b]
373.1339 K 99.9839 °C 211.971 °F
Other Languages
Afrikaans: Celsius
aragonés: Grau Celsius
asturianu: Grau Celsius
azərbaycanca: Selsi şkalası
Bân-lâm-gú: Liap-sī
беларуская: Градус Цэльсія
беларуская (тарашкевіца)‎: Градус Цэльсія
български: Градус Целзий
བོད་ཡིག: སེ་དྲོད།
bosanski: Stepen Celzija
brezhoneg: Derez Celsius
català: Grau Celsius
čeština: Stupeň Celsia
Cymraeg: Celsius
dansk: Celsius
فارسی: سلسیوس
føroyskt: Celsiusstigin
français: Degré Celsius
Frysk: Celsius
galego: Celsius

한국어: 섭씨
हिन्दी: सेल्सियस
hrvatski: Celzij
Bahasa Indonesia: Celsius
interlingua: Celsius
íslenska: Selsíus
ქართული: ცელსიუსი
Kiswahili: Selsiasi
latviešu: Celsija grāds
lietuvių: Celsijus
Limburgs: Celsiussjaol
la .lojban.: jacke'o
македонски: Целзиусов степен
മലയാളം: സെൽഷ്യസ്
मराठी: सेल्सियस
Bahasa Melayu: Celsius
Mirandés: Grau Celsius
Nederlands: Celsius
Nedersaksies: Celsius
norsk nynorsk: Celsius
occitan: Gra Celsius
ਪੰਜਾਬੀ: ਸੈਲਸੀਅਸ
پنجابی: سیلسیس
Patois: Selsios
Piemontèis: Scala Celsius
português: Grau Celsius
română: Celsius
Scots: Celsius
shqip: Celcius
Simple English: Celsius
slovenčina: Stupeň Celzia
српски / srpski: Степен целзијуса
srpskohrvatski / српскохрватски: Celzijus
Basa Sunda: Celsius