Why Kelvin and Celsius Exist

Walk into any chemistry lab in the world and you'll find Kelvin on the thermometer. Step outside and check the weather app on your phone and you'll find Celsius — or Fahrenheit if you're in the United States. Three scales, measuring the same thing, all in common use. Why?

Anders Celsius and the Rational Scale

In 1742, Swedish astronomer Anders Celsius proposed a temperature scale with exactly 100 degrees between two fixed, reproducible reference points: the freezing and boiling points of water at sea level. His original scale was inverted (0° was boiling, 100° was freezing), but Carl Linnaeus and others quickly flipped it to the form we know today.

The appeal was immediate. A scale that runs from 0 to 100 between two phenomena every person on Earth can observe made intuitive sense. By the time the metric system was standardized in the late 18th century, Celsius (then called centigrade) was its natural companion.

The scale was renamed Celsius in 1948 to honor its inventor and to avoid confusion with the word "centigrade," which has geometric meaning in some languages.

Lord Kelvin and Absolute Zero

William Thomson, later ennobled as Lord Kelvin, proposed a different kind of temperature scale in 1848. Instead of anchoring to water, he asked: what is the lowest temperature physically possible?

Experiments with gases had shown that as temperature drops, gas pressure and volume drop proportionally — and if you extrapolate, they would reach zero at approximately −273°C. This point, called absolute zero, is where molecular motion theoretically ceases entirely.

Kelvin proposed a scale starting at absolute zero. Its degrees are the same size as Celsius degrees, making conversion simple: K = °C + 273.15. The Kelvin scale has no negative numbers, which is essential in physics: you can't have negative kinetic energy.

Why Science Needs Kelvin

Many of the fundamental laws of physics and chemistry only work correctly when temperature is expressed in Kelvin. The ideal gas law (PV = nRT), the Stefan-Boltzmann law for radiation, and Boltzmann's entropy formula all require absolute temperature. Plug in −10°C instead of 263.15 K and you get nonsense.

This is why scientists universally use Kelvin, while engineers and everyday users prefer Celsius for its human-scale anchors.

Two Scales, Two Purposes

Celsius excels at describing the world we live in. Water freezes at 0° and boils at 100°. Body temperature is 37°. A comfortable room is around 20-22°. These numbers are memorable and meaningful.

Kelvin excels at describing physics as it is. The surface of the sun is 5778 K. Liquid nitrogen is 77 K. The cosmic microwave background — the faint afterglow of the Big Bang — is 2.725 K.

Both scales are indispensable. One for living, one for understanding.