Why is temperature in Celsius not double in Fahrenheit?

Answer 1

The size of the units comes from how the scales were developed:

Daniel Fahrenheit (in Germany) developed his scale in the early 1700s. He used three points to define his scale: the temperature of a freezing brine solution at 0°, the freezing point of water at 32° and body heat (average body temperature) at 96°. The different between 32° and 96° is 64° which being 2^6 makes it easy to mark each degree on the scale by bisecting the two reference points 6 times. He noticed that water on this scale boiled at about 212°. Later the scale was slightly adjusted using two fixed points: water freezing at 32° and boiling at 212°; this difference is of 180°.

Anders Celsius (in Sweden) developed his scale in the mid 1700s. He used two points of reference: the boiling point of water at 0° and the freezing point of water at 100°. (Note that this is upside down to the scale we named after him: it was reversed in his death year of 1744 by another scientist.) This scale has also be slightly adjusted by using two different reference points: absolute zero (0K = -273.15°) and the triple point of water (273.16K = 0.01°). Other scales were developed at the same time, all using a 100° (hence the old name for the scale of centigrade) difference between the freezing and boiling points of water.

As a result of the origins of the scales, the equivalences (using the difference between boiling and freezing points of water) are 100°C = 180°F → 1°C = 180/100°F = 9/5°F = 1.8°F.

Answer 2

Because the two scales start from different places (they have zero at different temperatures),

and the sizes of their degrees are not 2 to 1.