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Celsius (also known as centigrade) is a temperature scale that is named after the Swedish astronomer Anders Celsius (1701-1744), who developed a similar temperature scale two years before his death. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as serve as a unit increment to indicate a temperature interval (a difference between two temperatures or an uncertainty).
From 1744 until 1954, 0 degrees C was defined as the freezing point of water and 100 °C was defined as the boiling point of water, both at a pressure of one standard atmosphere. Although these defining correlations are commonly taught in schools today, by international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different points: absolute zero, and the triple point of VSMOW (specially prepared water). This definition also precisely relates the Celsius scale to the Kelvin scale, which is the SI base unit of temperature (symbol: K). Absolute zero, the hypothetical but unattainable temperature at which matter exhibits zero entropy, is defined as being precisely 0 K and -273.15 °C. The temperature value of the triple point of water is defined as being precisely 273.16 K and 0.01 °C.
This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water. Thus, it sets the magnitude of one degree Celsius and that of one kelvin as exactly the same. Additionally, it establishes the difference between the two scales' null points as being precisely 273.15 degrees Celsius (-273.15 °C = 0 K and 0 °C = 273.15 K).
In 1742 Swedish astronomer Anders Celsius (1701-1744) originally created a "reversed" version of the modern Celsius temperature scale whereby zero represented the boiling point of water and one hundred represented the freezing point of water. In his paper Observations of two persistent degrees on a thermometer, he recounted his experiments showing that ice's melting point was effectively unaffected by pressure. He also determined with remarkable precision how water's boiling point 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 mean sea level. This pressure is known as one standard atmosphere. (The BIPM's 10th CGPM later defined one standard atmosphere to equal precisely 1,013,250 dynes per cm2 (101.325 kPa)).
In 1744, coincident with the death of Anders Celsius, the famous Swedish botanist Carolus Linnaeus (1707-1778) effectively reversed Celsius's scale upon receipt of his first thermometer featuring 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 Ekstrom, 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; 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 Ekstrom, the instrument maker; and Marten Stromer (1707-1770) who had studied astronomy under Anders Celsius.
The first known document 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 Naucler. In it, Linnaeus recounted 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 (1/10000 of a right angle) and had a similar connotation in other languages, the term "centesimal degree" was used when very precise, unambiguous language was required by international standards bodies such as the Bureau international des poids et mesures (BIPM). The 9th CGPM (Conference generale des poids et mesures) and the CIPM (Comite international des poids et mesures) formally adopted "degree Celsius" (symbol: °C) in 1948. Some people still use the old term.