Celsius

**Celsius** (more precisely, a **degree Celsius**), sometimes called **centigrade**, is a unit of measurement that is used in most countries to measure temperature. The unit was created by Anders Celsius (1701–1744), a Swedish astronomer.

0 °C is the melting point of pure water at sea level (normal atmospheric pressure), and 100 °C is the boiling point of water at sea level. (Water boils at a lower temperature at higher altitudes.)

1 °C is therefore a hundredth of that difference.

Since 1948, the unit has been called "Celsius." "Centigrade" was the name of the unit before the change, with "centi" meaning a hundred and "grade" being a scale.

The other main measurement of temperature is the Fahrenheit scale, but it is less used. The Celsius scale, based on multiples of ten, is used with SI, or metric, measurements.

## History

In 1742, Anders Celsius made a "reversed" version of the modern Celsius temperature scale in which 0 was the freezing point of water and 100 the melting point of ice. In his paper _Observations of Two persistent Degrees on a thermometer,_ he wrote about his experiments. He showed that the melting point of ice was basically unaffected by air pressure. Ice would turn into water at the same temperature whether it was at sea level or on a mountain.

That was not the case for the boiling point of water, which is lower with less pressure, such as on a mountain. He decided that zero on his temperature scale, the boiling point of water, would be set at the standard barometric pressure at  sea level, which is now known as one atmosphere. In 1954, Resolution 4 of the 10th CGPM  (the General Conference on Weights and Measures) set what exactly is one standard atmosphere ().<sub> </sub>

In 1744, the year that Celsius died, the famous Swedish botanist Carolus Linnaeus (1707–1778) used a reversed version of Celsius's scale when he bought his first thermometer. Its scale had zero represent the melting point of ice and 100 represent the boiling point of water, as what is used today.  His custom-made "Linnaeus thermometer" to be used in his greenhouses, was made by Daniel Ekström, who was Sweden's leading maker of scientific instruments. Ekstöm's workshop was in the basement of the Stockholm Observatory. As then often happened before modern communications, many physicists, scientists, and instrument makers are given credit with independently making the same measurement scale; among them were Pehr Elvius, the secretary of the Royal Swedish Academy of Sciences (which had an instrument workshop), who Linnaeus had also talked to; Christin of Lyons; Ekström, the instrument-maker; and Mårten Strömer (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, by Linnaeus, who wrote to one of his students, Samuel Nauclér. In it, Linnaeus reported 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 called this scale the "centigrade scale." Temperatures on the centigrade scale were often reported as "degrees" or "degrees centigrade." The symbol for temperature values on thiescale was °C (in several formats over the years).

Because the name "centigrade" was also the Spanish and the French names for a unit of angular measurement (a hundredth of a right angle) and had a similar meaning in other languages, the term "centesimal degree" was used when very precise and clear language was required for international communication, such as by the International Bureau of Weights and Measures (BIPM). The 9th CGPM (General Conference on Weights and Measures) and the CIPM (International Committee for Weights and Measures) officially decided to use "degree Celsius" (symbol: °C) in 1948.

There were three reasons for the decision to use the word _Celsius_:
1. All common temperature scales would have their units named after someone closely associated with them: Kelvin, Celsius, Fahrenheit, Réaumur and Rankine.
1. The symbol °C that for centuries had been used in association with the name _centigrade_ could continue to be used but now meant _Celsius_. (Linnaeus was an important influence on the modern scale, but Celsius had first developed it.)
1. The new name meant that _centigrade_ could again mean only the French name for the unit of angular measurement.

It would take nearly two decades, however, for school textbooks to change from _centigrade_ to _Celsius_, and many people today still use the old name.

## Examples

- On the Celsius scale, water freezes at 0° and boils at 100°.
- Room temperature is about 20 °C.
- Absolute zero (the coldest possible temperature) is -273.15 °C.
- Liquid nitrogen is about -196 °C.
- Winters in Antarctica can be between -80 and -90 °C.
- A human's body temperature is usually 36.6 °C.
- Cats and dogs have a slightly higher body temperature, about 39 °C.
- ABS plastic starts to soften at about 105 °C, and melts at about 250 °C
- Bread and some other foods are cooked at about 150 °C
- A wood fire burns at up to about 600 °C
- Steel melts at about 1500 °C.
- A halogen light bulb burns at about 2700 °C.
- The surface of the sun is about 5500 °C.
- The core of the sun is about 15 million °C.
- Absolute hot (the highest possible temperature) is about 142 nonillion °C.

## Temperature conversions

- To turn a temperature in degrees Celsius into kelvins, 273.15 is added: K = C + 273.15. For example, 0  °C, the temperature at which water freezes, is 273.15 K.
- To turn a temperature in kelvins into degrees Celsius, 273.15 is subtracted: C = K - 273.15. For example, 310 K is the same temperature as 36.85 °C, about the temperature of a human body.
- To turn a temperature in degrees Celsius into degrees Fahrenheit, the former is multiplied by 9/5 and then 32  °F is added: F = (9/5) * C + 32.
- To turn a temperature in degrees Fahrenheit into degrees Celsius, 32 is subtracted and the result is multiplied by 5/9: C = (F - 32) * (5/9).