Degree Celsius
In 1742 the Swedish astronomer Anders Celsius described a thermometer with two fixed points. On his scale water boiled at zero and snow melted at one hundred. The warmer it was, the smaller the number.
Three years later Carl Linnaeus ordered a thermometer with the reversed scale for the greenhouse in Uppsala. That one stuck. Celsius noticed something else: snow melts at the same temperature everywhere, but water boils differently depending on pressure. Today the degree Celsius is defined through the kelvin, and 0 °C is exactly 273.15 K.
| Notation | °C |
| Definition | t = T − 273.15 K, the degree is the same size as the kelvin |
| Water points | ice melts at 0.00 °C · triple point 0.01 °C · boiling 99.97 °C at 101.325 kPa |
| Introduced | 1742, Uppsala · the name «degree Celsius» — CGPM 1948 |
| Standards | ISO 80000-5 · SI Brochure 9th ed. · ITS-90 |
| Typical values | −89.2 °C — coldest on record, Antarctica · 37 °C — the body · 1538 °C — iron melts |
01 · Definition
The same kelvin, with zero in a different place
Celsius temperature t is the thermodynamic temperature T shifted by 273.15 K. A degree Celsius is the same size as a kelvin. That is why a temperature difference is written with the same number in both units.
Water no longer defines the scale. At normal pressure ice melts at 0.00 °C, the triple point of water lies at 0.01 °C, and boiling at 99.97 °C, not exactly at 100. These fractions of a degree are the trace of the fixed points being refined first and then abandoned.
Why boiling water was zero for Celsius
Two identical thermometers stand side by side, and their columns are always at the same height. Only the labels differ. On the left is the 1742 scale, where one hundred is at melting snow; on the right, the modern one. The blue mark is snow, the red one boiling water.
Zero on the Celsius scale is set by melting ice, not by the absence of heat. Such numbers cannot be divided. In kelvins 20 °C is 293.15 K and 10 °C is 283.15 K. The ratio is 1.035: only three and a half per cent warmer. The difference, though, is honest: 10 °C and 10 K are the same thing.
Where a thermometer has its zero and its hundred
The glass tube has no divisions. You have to mark them the way Celsius did: dip the thermometer into melting snow and mark zero, then into boiling water and mark one hundred. The ochre marks are yours; the instrument divides the gap between them into ten parts. On the left is a glass of snow, on the right a pot on a burner and a barometer. The mountain in the middle shows the altitude of the place: the higher, the lower the pressure and the sooner water boils. Mark one hundred on Everest and see what your thermometer shows in the room.
Dip the thermometer into melting snow and wait until the column stops. That spot will be zero.
02 · Conversion
| Unit | Value | Fixed points |
|---|---|---|
| {n} | {v} | {note} |
At −40 the Celsius and Fahrenheit scales show the same number.
Delisle and Celsius in 1742 counted downwards: the hotter, the smaller the number.
A difference of 1 °C is always 1 K and 1.8 °F.
03 · Orders of magnitude
04 · Measuring instruments
05 · Writing rules
06 · Neighbouring units
07 · Historical section
Celsius checked the main thing: snow melts at the same temperature in Uppsala and in the north of Sweden, while the boiling point depends on pressure. He proposed a correction from the barometer reading. So one hundred degrees was marked only at normal pressure.
In 1954 the reference moved to the triple point of water: 273.16 K, that is 0.01 °C. Since 2019 the kelvin has been defined through the Boltzmann constant, k = 1.380 649·10⁻²³ J/K. Water no longer defines anything, and the relation 0 °C = 273.15 K stayed exact by definition.
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