Celsius

The degree Celsius is a fundamental unit used to measure temperature. It is part of the Celsius temperature scale, which is one of two scales used in the International System of Units (SI). The other closely related scale is the Kelvin scale. The Celsius scale is used by most countries across the globe. However, some places like the United States, Liberia, and certain island territories still use the Fahrenheit scale.

This scale is vital for science, weather reporting, and daily life because it provides a standardized way to describe heat.

Historically, the scale was defined by the physical properties of water. For much of the 19th and early 20th centuries, scientists used the freezing and boiling points of water. These points were measured at one standard atmosphere of pressure. Under these specific conditions, water freezes at 0 °C and boils at 100 °C. This created a system of 100 equal steps between the two states.

This method allowed for consistent measurements in laboratories and homes.

Between 1954 and 2019, the definition of the scale became more complex and precise. Scientists moved away from simple boiling and freezing points. Instead, they used the triple point of water to define the scale. The triple point is a specific condition where water can exist as a solid, liquid, and gas at once. In 2005, they even specified the exact isotopic composition of the water used. Since 2007, the Celsius scale has been defined in terms of the Kelvin scale. This ensures that the magnitude of one degree Celsius is exactly equal to one Kelvin.

The history of the scale involves several different scientists and surprising changes. In 1742, a Swedish astronomer named Anders Celsius proposed the first version. Interestingly, his original scale was reversed from the one we use today. He set 0 °C as the boiling point and 100 °C as the freezing point.

In 1743, a French physicist named Jean-Pierre Christin inverted the scale. Some believe Christin invented this version independently, while others say he simply flipped Celsius's work. Later, in 1744, the botanist Carl Linnaeus also used a reversed scale for his greenhouses.

For many years, the scale was commonly known as the centigrade scale. The name comes from Latin words meaning "one hundred steps." In 1948, the International Committee for Weights and Measures officially renamed it to honor Anders Celsius. They did this to prevent confusion with the term "gradian," which refers to angular measurements in some languages. While "Celsius" is the standard in scientific work, the term "centigrade" is still used informally in some English and French-speaking regions. In Australia, weather reports switched exclusively to Celsius in 1972. The BBC in the United Kingdom also switched from "centigrade" to "Celsius" in 1985.

We can see the scale in action by looking at common temperatures. For instance, the average normal human body temperature is about 37.0 °C. A common room temperature is often around 20 °C. At the extreme low end, absolute zero is defined as exactly -273.15 °C.

This is the lowest possible temperature. On the high end, the boiling point of liquid nitrogen is -195.8 °C. These numbers help scientists and engineers communicate exact thermal states across the world.

It is important to understand how the Celsius scale relates to other systems. While it is used for specific temperatures, it can also express temperature intervals. An interval is the difference between two temperatures, such as a change of 40 °C. The scale follows an interval system rather than a ratio system. This means that an object at 20 °C does not have twice the thermal energy of an object at 10 °C. Because the scale is relative and not absolute, 0 °C is not the lowest possible value. In professional science, the Celsius scale and the Kelvin scale are often used together to provide highly accurate data.