The Difference Between Celsius and Centigrade

Celsius uses a more precisely defined zero

Thermometer in garden
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The Celsius and centigrade temperature scales are the same scales, where 0 degrees is the freezing point of water and 100 degrees is the boiling point. However, the Celsius scale uses a zero that can be precisely defined. Here's a closer look at the difference between Celsius and centigrade:

The Origin of the Celsius Scale

Anders Celsius, a professor of astronomy at the University of Uppsala, Sweden, devised a temperature scale in 1741. His original scale had 0 degrees at the point where water boiled and 100 degrees where water froze. Because there were 100 degrees between the defining points of the scale, it was a type of centigrade scale. Upon Celsius' death, the endpoints of the scale were switched (0° C became the freezing point of water and 100° C became the boiling point of water), and the scale became known as the centigrade scale.

Why Centigrade Became Celsius

The confusing part here is that the centigrade scale was invented by Celsius, more or less, so it had been called Celsius' scale or the centigrade scale. However, there were a couple of problems with the scale. First, the grade was a unit of plane angle, so a centigrade could be one-hundredth of that unit. More important, the temperature scale was based on an experimentally determined value that could not be measured with the precision deemed sufficient for such an important unit.

In the 1950s, the General Conference of Weights and Measures set out to standardize several units and decided to define Celsius temperature as Kelvin minus 273.15. The triple point of water was defined to be 273.16 K and 0.01° C. The triple point of water is the temperature and pressure at which water exists simultaneously as a solid, liquid, and gas. The triple point can be measured accurately and precisely, so it was a superior reference to the freezing point of water. Since the scale had been redefined, it was given a new official name: the Celsius temperature scale.