The Celsius (or centigrade) temperature scale is the most widely used around the world, although the Fahrenheit scale is more popular in the United States. The Celsius system was invented by Swedish astronomer Anders Celsius in 1742. It is based on the difference in temperature between the freezing and boiling points of water at standard atmospheric pressure. In 1954, the definition was changed slightly to base the Celsius scale on absolute zero. This change is important for scientific reasons but not in everyday life, so for most purposes, everyone uses the original definition. It's not hard to read a Celsius thermometer once you understand the difference between the Fahrenheit and Celsius temperature scales.

Learn how the Celsius temperature scale is set up. The temperatures at which water freezes and boils under one standard atmosphere of air pressure are used to define the Celsius scale. The freezing point of water is labeled 0 (zero) degrees, and the boiling point is labeled 100 degrees. The temperature interval between is divided into exactly 100 equal parts or degrees.

Know the difference between the Fahrenheit and Celsius temperature scales. In the Fahrenheit system, 32 degrees is the freezing point of water, and 212 degrees is the boiling point. The Fahrenheit scale therefore divides the interval between the two into 180 degrees (212 minus 32 = 180). Each Celsius degree equals 1.80 Fahrenheit degrees. Another way to say this is that each Fahrenheit degree is 5/9 of a degree Celsius.

Learn how to convert from Celsius to the Fahrenheit scale to read a Celsius thermometer. To do this, multiply the Celsius degrees by 1.80 and add 32. For example, 10 degrees Celsius is converted to Fahrenheit by multiplying 10 degrees by 1.80 (equals 18) plus 32, giving you a Fahrenheit temperature of 50 degrees.

Know how to convert Fahrenheit degrees to Celsius degrees. First subtract 32 degrees, then multiply the result by 5/9 (0.556 on a calculator). For example, suppose a thermometer reads 68 degrees Fahrenheit. Subtract 32 from 68, leaving 36. Multiply 36 by 5/9, which equals 20 degrees Celsius.

Pay attention to the negative sign when you are dealing with temperature conversions. Sometimes a negative Celsius temperature can change to positive when you convert to the Fahrenheit scale. For instance, suppose you read a Celsius thermometer and it gives the temperature as -5 C. You convert to Fahrenheit the same way---multiply -5 degrees by 1.80 (equals -9), then add 32. Adding 32 to -9 equals 23 degrees Fahrenheit. But if you don't pay attention to that minus sign and add 32 and 9, you'll get 41, which is incorrect.