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Celsius

Definition: The Celsius scale (symbol: °C) is a temperature scale used by the International System of Units (SI). As defined today, zero degrees Celsius is equal to 273.15 K, with a temperature difference of 1 deg C equivalent to a difference of 1 K, meaning the scale is now defined by two fixed points: absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW), a specially purified water.

History/origin: The Celsius scale was proposed by the Swedish astronomer Anders Celsius in 1742. In Celsius's original scale, zero represented the boiling point of water and 100 represented the freezing point. This was reversed after his death in 1744 to create the current scale, where zero represents the freezing point of water and 100 represents the boiling point, at standard atmospheric pressure.

The scale was known as the centigrade scale until 1948 when the 9th General Conference on Weights and Measures renamed it to honor Anders Celsius. The term "centigrade" refers to the 100-degree interval between the defined points.

In 1954, the 10th General Conference on Weights and Measures defined the Celsius scale in terms of the triple point of water (0.01 °C) and absolute zero (-273.15 °C), making it directly derivable from the Kelvin scale. In 2019, the definition was updated to be based on the Boltzmann constant, maintaining the same temperature values but with a more fundamental definition.

Current use: The Celsius scale is used worldwide for most temperature measurements, including weather forecasts, body temperature, and in scientific contexts. It's part of the metric system and is used in nearly all countries, with the notable exception of the United States, where Fahrenheit remains common for non-scientific applications.

Fahrenheit

Definition: The Fahrenheit scale (symbol: °F) is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit. In this scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point is 212 °F at standard atmospheric pressure.

History/origin: The Fahrenheit scale was developed by Daniel Gabriel Fahrenheit, a German-Dutch-Polish physicist, engineer, and glass blower, in the early 18th century. Fahrenheit's scale was based on three points: the temperature of a mixture of ice, water, and ammonium chloride (set at 0 °F); the freezing point of water (32 °F); and human body temperature (originally set at 96 °F, later adjusted to 98.6 °F).

There are several accounts of how Fahrenheit originally established the 0°F point. According to one account, Fahrenheit established 0 °F as the temperature of an equal mixture of ice and salt, with 32 °F being the temperature at which water freezes and 96 °F being the temperature of the human body. He then observed that the boiling point of water with this scale was 212 °F, and defined that as the third reference point.

Current use: The Fahrenheit scale is primarily used in the United States and its territories for non-scientific measurements. It is also used in the Bahamas, Belize, the Cayman Islands, Palau, and a few other countries. Most countries worldwide use the Celsius scale for everyday temperature measurement.