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Celsius (more precisely, a degree Celsius), sometimes called centigrade, is a unit of measurement used in many countries to measure temperature. This unit was created by Anders Celsius (1701-1744), a Swedish astronomer.

0 degrees (°) Celsius is the melting point of water at what is called normal pressure. 100° Celsius is the boiling point of water at normal pressure.

1 °C is therefore one hundredth (the 100th part) of that difference.

Since 1948 this unit is called Celsius, Centigrade was the name of the unit before the change.

The other commonly used measurement of temperature is the Fahrenheit scale.

History

In 1742, Anders Celsius (1701 – 1744) created a “reversed” version of the modern Celsius temperature scale. In it zero represented the boiling point of water and 100 represented the melting point of ice. In his paper Observations of two persistent degrees on a thermometer, he wrote about his experiments. His could show in his experiments that the melting point of ice was effectively unaffected by pressure. This was not the case for the boiling point of water. It varied as a function of atmospheric pressure. He proposed that zero on his temperature scale (water’s boiling point) would be calibrated at the mean barometric pressure at sea level. This pressure is known as one standard atmosphere. In 1954, Resolution 4 of the 10th CGPM (the General Conference on Weights and Measures) established internationally that one standard atmosphere was a pressure equivalent to 1,013,250 dynes per cm² (101.325 kPa). 

In 1744, the year Anders Celsius died, the famous Swedish botanist Carolus Linnaeus (1707 – 1778) effectively reversed ^[1] Celsius’s scale when he received his first thermometer with a scale where zero represented the melting point of ice and 100 represented water’s boiling point. 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. Eckstöm's 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;^[2] 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; Christian of Lyons; Daniel Ekström, the instrument maker; and Mårten Strömer (1707 – 1770) who had studied astronomy under Anders Celsius.

The first known document^[3] 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 told the temperatures inside the orangery at the Botanical Garden of Uppsala University:

“…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…”

For the next 204 years, 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, “degrees centigrade.” The symbol for temperature values on this scale was °C (in several formats over the years). Because the term “centigrade” was also the Spanish and French language name for a unit of angular measurement (one-hundredth of a right angle) and had a similar connotation in other languages, the term “centesimal degree” was used when very precise, clear language was required by international standards bodies such as the Bureau international des poids et mesures (BIPM). The 9th CGPM (Conférence générale des poids et mesures) and the CIPM (Comité international des poids et mesures) formally adopted “degree Celsius” (symbol: °C) in 1948.^[4] For common people worldwide — including school textbooks — the full change from centigrade to Celsius required nearly two decades after this formal adoption.

Examples

• On the Celsius scale, water freezes at 0° and boils at 100°.
• Room temperature is about 20 °C.
• Absolute zero is -273.15 °C
• Winters in Antarctica can be between -80 and -90 °C.
• A human's body temperature is usually 37 °C.

Temperature conversions

• To make a temperature in degrees Celsius into kelvins a person must add 273.15. For example 0 degrees Celsius, which is the temperature at which water freezes, is 273.15 kelvins.
• To make a temperature in kelvins into degrees Celsius a person must subtract 273.15. For example 310 K is the same temperature as 36.85 °C, which is about the temperature of a human body.
• To make a temperature in degrees Celsius into degrees Fahrenheit a person must multiply it by 9/5 and add 32.
• To make a temperature in degrees Fahrenheit into degrees Celsius a person must subtract 32 and multiply the result by 5/9.

References

1. ↑ Thermodynamics-information.net, A Brief History of Temperature Measurement and; Uppsala University (Sweden), Linnaeus’ thermometer
2. ↑ Citation for Daniel Ekström, Mårten Strömer, Christian of Lyons: The Physics Hypertextbook, Temperature; citation for Christian of Lyons: Le Moyne College, Glossary, (Celsius scale); citation for Linnaeus’ connection with Pehr Elvius and Daniel Ekström: Uppsala University (Sweden), Linnaeus’ thermometer; general citation: The Uppsala Astronomical Observatory, History of the Celsius temperature scale
3. ↑ University of Wisconsin–Madison, Linnæus & his Garden and; Uppsala University, Linnaeus’ thermometer
4. ↑ According to The Oxford English Dictionary (OED), the term “Celsius’s thermometer” had been used at least as early as 1797. The term “The Celsius or Centigrade thermometer” was again used to refer to a particular type of thermometer at least as early as 1850. The OED also cites this 1928 reporting of a temperature: “My altitude was about 5,800 metres, the temperature was 28° Celsius.” However, dictionaries seek to find the earliest use of a word or term and are not a useful resource as regards the terminology used throughout the history of science. According to several writings of Dr. Terry Quinn CBE FRS, Director of the BIPM (1988 – 2004), including Temperature Scales from the early days of thermometry to the 21st century (148 kB PDF, here) as well as Temperature (2nd Edition / 1990 / Academic Press / 0125696817), the term Celsius in connection with the centigrade scale was not used whatsoever by the scientific or thermometry communities until after the CIPM and CGPM adopted the term in 1948. The BIPM wasn’t even aware that “degree Celsius” was in sporadic, non-scientific use before that time. It’s also noteworthy that the twelve-volume, 1933 edition of OED didn’t even have a listing for the word Celsius (but did have listings for both centigrade and centesimal in the context of temperature measurement). The 1948 adoption of Celsius accomplished three objectives:
   

   1) All common temperature scales would have their units named after someone closely associated with them; namely, Kelvin, Celsius, Fahrenheit, Réaumur and Rankine.
   2) Notwithstanding the important contribution of Linnaeus who gave the Celsius scale its modern form, Celsius’s name was the obvious choice because it began with the letter C. Thus, the symbol °C that for centuries had been used in association with the name centigrade could continue to be used and would simultaneously inherit an intuitive association with the new name.
   3) The new name eliminated the ambiguity of the term “centigrade,” freeing it to refer exclusively to the French-language name for the unit of angular measurement.