How did we end up with both Fahrenheit and Celsius scales?

As scientific knowledge grew, scientists had to develop new ways in which to record their discoveries and compare them with one another. Because different scientists had different tools, different backgrounds, and different goals, they came up with different systems.

The Celsius scale is named after Swedish astronomer Anders Celsius, who developed it in 1742. Celsius measured the freezing and boiling points of water at sea level and divided the distance between the two into hundredths. He originally had the scale in the opposite order of the scale used today — 0°C was the boiling point of water, and 100°C was the freezing point — but other scientists later reversed the scale.

The Fahrenheit scale was first proposed in 1724 by the German physicist Daniel Gabriel Fahrenheit. Using a tube filled with mercury or alcohol, Fahrenheit determined his scale based on three points:

  • He set the point at which water, ice, and ammonium chloride (a salt) stabilized as 0°F.
  • He set the point at which still water just began to form ice on its surface as 32°F.
  • He set the temperature of the human body taken in the mouth or under the arm as 96°F.

These may seem like odd numbers to choose, but consider that 64 degrees separate 32° and 96°. Because 64 is 26, Fahrenheit could simply bisect (divide in half) the distance between the two numbers six times in order to calibrate 1°.

Later, scientists noted that water boiled at nearly 180° higher than it froze, and readjusted the scale to make it exactly 180°. That's how human body temperature moved from 96°F to 98.6°F.

To convert degrees Celsius into degrees Fahrenheit, multiply the number by 1.8 and add 32. To convert from Fahrenheit to Celsius, subtract 32 and divide that number by 1.8.