Question

Why 1.8 is divided when we convert fahrenheit into celsius?

Answer 1

Because Fahrenheit was defined before Celsius, and Celsius chose a different scale.

Fahrenheit was defined in 1724, with 32°F being the freezing point of water and 212°F being the boiling point of water, with 180 degrees of separation [1].

Celsius was defined in 1742, with 0°C being the freezing point of water and 100°C being the boiling point of water, with 100 degrees separation.

Obviously, 180 ≠ 100, and hence you cannot just add 32 and must account for the 1.8:1 scaling difference.

You can't really blame Fahrenheit for this, since his scale came first, even though arguably Celsius's scale "makes more sense."

Except that Fahrenheit's scale also makes sense.