Temperature is measured using a thermometer (thermo meaning “heat” and meter meaning “to measure”). The inventor of the thermometer was probably Galileo Galilei (1564–1642), who used a device called the thermoscope to measure hot and cold.

Temperatures are determined using various scales, the most popular being Celsius, Fahrenheit, and Kelvin. Invented by Swedish astronomer, mathematician, and physicist Anders Celsius (1701–1744) in 1742, Celsius used to be called the Centigrade scale (it can be capitalized or not; centigrade means “divided into 100 degrees”). He used 0 degrees Celsius as the freezing point of water; the point where water boils was marked 100 degrees Celsius. Because of its ease of use (mainly because it is based on an even 100 degrees), it is the scale most used by scientists. It is also the scale most associated with the metric system.

A thermometer showing degrees Fahrenheit on the left and Celsius on the right.

Fahrenheit is the scale invented by Polish-born German physicist Daniel Gabriel Fahrenheit (1686–1736) in 1724. His thermometer contained mercury in a long, thin tube, which responded to changes in temperatures. He arbitrarily decided that the difference between water freezing and boiling—32 degrees Fahrenheit and 212 degrees Fahrenheit, respectively—would be 180 degrees.

The Kelvin scale was invented in 1848 by Lord Kelvin (1824–1907), who was also known as Sir William Thomson, Baron Kelvin of Largs. His scale starts at 0 degrees Kelvin, a point that is called absolute zero, the temperature at which all molecular activity ceases and the coldest temperature possible. His idea was that there was no limit to how hot things can get, but there was a limit to how cold. Kelvin’s absolute zero is equal to -273.15 degrees Celsius or -459.67 degrees Fahrenheit. So far, scientists believe nothing in the universe can get that cold.