The History of Temperature Measurement
Before the Thermometer
For most of human history, temperature was entirely subjective — hot, warm, cold. Physicians judged fever by touch. Blacksmiths judged metal temperature by color. There was no way to communicate a specific temperature or compare readings between different people or places.
Galileo's Thermoscope (~1592)
Galileo Galilei invented one of the earliest temperature-measuring devices — the thermoscope — around 1592. It used a glass tube where liquid rose and fell with temperature changes, but it had no numbered scale. It could show that something was hotter or colder, but not by exactly how much.
Fahrenheit (1724)
German physicist Daniel Gabriel Fahrenheit invented the mercury thermometer in 1714 and proposed his temperature scale in 1724. He set 0°F as the coldest temperature he could achieve in his lab (a mixture of ice, water, and ammonium chloride) and 96°F as human body temperature. Later calibrated, the scale placed water's freezing point at 32°F and boiling at 212°F.
Celsius (1742)
Swedish astronomer Anders Celsius proposed a scale in 1742 based on water — 0 for boiling and 100 for freezing (the reverse of what we use today). His colleague Carl Linnaeus flipped it to the intuitive 0°C (freezing) and 100°C (boiling) system we know now. The simplicity of a 100-point scale tied to water made it ideal for science.
Kelvin — Absolute Zero (1848)
William Thomson (Lord Kelvin) proposed the absolute temperature scale in 1848, setting 0 K as absolute zero — the theoretical point where all molecular motion stops (-273.15°C). Kelvin is used in physics and astronomy because it starts at the true physical bottom. There are no negative Kelvin temperatures.
Why the US Still Uses Fahrenheit
The US adopted Fahrenheit in the colonial era when it was the standard in British science. When most of the world shifted to Celsius in the 20th century, the US never made the switch in everyday life — largely due to the practical cost of changing every textbook, signage, and cultural habit. Scientists in the US use Celsius and Kelvin.