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The degree Celsius (°C) is a unit of temperature named for the Swedish astronomer Anders Celsius (1701-1744), who first proposed it in 1742. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees, and the boiling point is 100 degrees at standard atmospheric pressure.

Since there are one hundred graduations between these two reference points, the original term for this system was centigrade (100 parts). In 1948 the system's name was officially changed to Celsius by the 9th General Conference on Weights and Measures, both in recognition of Celsius himself and to eliminate confusion caused by conflict with the SI (metric) use of the centi- prefix.

While the values for freezing and boiling of water remain approximately correct, the original definition is unsuitable as a formal standard: it depends on the definition of standard atmospheric pressure which in turn depends on the definition of temperature. The current official definition of the Celsius sets 0.01°C to be at the triple point of water and a degree to be one 1/273.16 the difference in temperature between the triple point of water and absolute zero. This definition ensures that one degree Celsius represents the same temperature difference as one kelvin.

Anders Celsius originally proposed that the freezing point should be 100 degrees and that the boiling point should be 0 degrees. This was reversed, possibly at the instigation of Carl von Linné or Daniel Ekström, the manufacturer of most of the thermometers used by Celsius.

To convert Celsius to Fahrenheit: multiply the Celsius temperature by 1.8 and add 32 degrees.

F = 1.8 C + 32
To convert Fahrenheit to Celsius: subtract 32 degrees from the Fahrenheit temperature and divide the quantity by 1.8.
C = (F - 32) / 1.8.

A temperature of -40 degrees is the same for Celsius and Fahrenheit. Correspondingly, another method for converting Celsius to Fahrenheit is to add 40, multiply by 1.8, and subtract 40. Similarly, to convert Fahrenheit to Celsius add 40, divide by 1.8, and subtract 40.

The Celsius scale is used throughout most of the world for day-to-day purposes, though in broadcast media was still frequently referred to as centigrade until the late 1980s or early 1990s, particularly by weather forecasters on European networks such as the BBC, ITV, and RTÉ. United States media still exclusively uses the Fahrenheit scale for temperatures, which puzzles European viewers watching US TV or satellite or cable. Having not been taught fahrenheit for decades, they have no comprehension of how 'extreme' the weather being described is.

Other temperature scales include Fahrenheit (1724), Réaumur (1730), Rømer[?] (1730+), Kelvin (1862), and Rankine (ca. 1860). The kelvin is the official SI (metric) temperature unit.

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