Temperature is measured by the property of liquid expanding when it gets warmer (and contracting when it gets cooler). The original thermometers used mercury (a liquid metal), but since mercury has been discovered to be toxic, alcohol with a red or black dye is more often used. If the liquid expands or contracts up or down a narrow tube, it goes to certain lengths of the thermometer at different temperatures. Using different known melting points and boiling points of known substances, particularly water, a scale can be made on the thermometer.
There are three major temperature scales: Fahrenheit, Celsius, and Kelvin. The first thermometer was devised by Fahrenheit in Germany in the early 1800s or late 1700s. He mixed together salt and ice to get a temperature that he thought was as cold as one could get with that mixture. He called that temperature zero and marked his thermometer. He then used the thermometer to measure body temperature and called the resulting temperature 100 degrees. (The person must have had a slight fever, since we now know that body temperature is 98.6 degrees Fahrenheit.) For this system, ice melts at 32 degrees and water boils at 212 degrees.
The Celsius scale was developed in Sweden and was based on the properties of water. Thus, ice melts at zero degrees and water boils at 100 degrees. Body temperature is 37 degrees.
The third scale, named after Lord Kelvin, is based on absolute zero. Absolute zero is a theoretical temperature where all molecular motion stops. Absolute zero is equivalent to negative 273 degrees Celsius. Scientists have come within a degree or two of absolute zero, but it may not be possible to reach.
Converting from Fahrenheit degrees to Celsius degrees is based on the different number of degrees between the boiling point of water and the melting point of ice. For Fahrenheit, the difference is 212 32 = 180 degrees. For Celsius, the difference is 100 0 = 100 degrees. Thus, there are 180 Fahrenheit degrees for every 100 Celsius degrees. 180/100 = 9/5, which is part of the equation for conversion. Alternatively 100/180 = 5/9, which is also used to convert the other direction. You also need to account for the melting point of ice being 32 degrees higher in the Fahrenheit scale than the Celsius scale.
The following equations were derived using the above information:
degrees Fahrenheit = (9/5 x degrees Celsius) + 32
degrees Celsius = 5/9 x (degrees Fahrenheit 32)
So, for example, to convert 212 degrees Fahrenheit to Celsius, we subtract 32, giving 180. Multiplying 180 times 5/9 gives exactly 100 degrees Celsius.