Celsius

A thermometer calibrated in degrees Celsius

The Celsius scale, also known as the centigrade scale, [1] [2] is an SI scale and unit of measurement for temperature. As an SI derived unit, it is used by most countries in the world. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval, a difference between two temperatures or an uncertainty. Before being renamed to honour Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps.

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 following a change introduced by Jean-Pierre Christin to reverse the Celsius thermometer scale (from water boiling at 0 degrees and ice melting at 100 degrees). This scale is widely taught in schools today. By international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: 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; 32.02 °F). [3]

Thus, a temperature difference of one degree Celsius and that of one kelvin are exactly the same, with the null point of the Kelvin scale (0 K) at exactly −273.15 °C, and the null point of the Celsius scale (0 °C) at exactly 273.15 K. [4]

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 which was the reverse of the scale now known by the name "Celsius": 0 represented the boiling point of water, while 100 represented the freezing point of water. [5] 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 1013250 dynes per square centimetre (101.325  kPa). [6]

In 1743, the Lyonnais physicist Jean-Pierre Christin, permanent secretary of the Académie des sciences, belles-lettres et arts de Lyon FR, working independently of Celsius, developed a scale where zero represented the freezing point of water and 100 represented the boiling point of water. [7] [8] 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. [9] [10] [11]

In 1744, coincident with the death of Anders Celsius, the Swedish botanist Carolus Linnaeus (1707–1778) reversed Celsius's scale. [12] 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 and 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; [13] 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 [14] 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...

Centigrade, hectograde and Celsius

Since the 19th century, the scientific and thermometry communities worldwide referred to this scale as the 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 (1/100 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". Furthermore, in the context here, centigrade/hectograde is referring to the whole 0–100 range, not the given part thereof, hence "20° centigrade" means "20ᵍ per 100 gradians" (or 20% hectograde), not its literal description, "0.2 gradians".

(To be descriptively correct, "20° centigrade" should be "20° hectocentigrade", or just "20 gradians" {20ᵍ}.)

To eliminate such confusion, the 9th CGPM and the CIPM ( Comité international des poids et mesures) formally adopted "degree Celsius" in 1948, [15] [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" otherwise continuing to be in common but decreasing use, especially in informal contexts in English-speaking countries. [16] It was not until February 1985 that the forecasts issued by the BBC switched from "centigrade" to "Celsius". [17]

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
(precisely, by definition)
0 K −273.15 °C −459.67 °F
Boiling point of liquid nitrogen 77.4 K −195.8 °C [18] −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) [19] 273.1499 K −0.0001 °C 31.9998 °F
Water's triple point
(precisely, by definition)
273.16 K 0.01 °C 32.018 °F
Normal human body temperature (approximate average) [20] 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
dansk: Celsius
Deutsch: Grad Celsius
español: Grado Celsius
Esperanto: Grado celsia
euskara: Celsius gradu
فارسی: سلسیوس
føroyskt: Celsiusstigin
français: Degré Celsius
Frysk: Celsius
galego: Celsius
贛語: 攝氏
한국어: 섭씨
हिन्दी: सेल्सियस
hrvatski: Celzij
Bahasa Indonesia: Celsius
interlingua: Celsius
íslenska: Selsíus
italiano: Grado Celsius
ქართული: ცელსიუსი
Kiswahili: Selsiasi
latviešu: Celsija grāds
Lëtzebuergesch: Grad Celsius
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
Nordfriisk: Graad Celsius
norsk nynorsk: Celsius
occitan: Gra Celsius
ਪੰਜਾਬੀ: ਸੈਲਸੀਅਸ
پنجابی: سیلسیس
Patois: Selsios
Plattdüütsch: Grad Celsius
português: Grau Celsius
română: Celsius
Scots: Celsius
shqip: Celcius
sicilianu: Gradu cintigradu
Simple English: Celsius
slovenčina: Stupeň Celzia
српски / srpski: Степен целзијуса
srpskohrvatski / српскохрватски: Celzijus
Basa Sunda: Celsius
svenska: Grad Celsius
తెలుగు: సెల్సియస్
Türkçe: Celsius
українська: Градус Цельсія
اردو: سیلسیس
Tiếng Việt: Độ Celsius
West-Vlams: Celsius
Winaray: Sentigrado
吴语: 摄氏温标
粵語: 攝氏
中文: 摄氏温标