Although initially defined by the freezing point of water (and later the melting point of ice), the Celsius scale is now officially a derived scale, defined in relation to the Kelvin temperature scale.
Zero on the Celsius scale (0℃) is now defined as the equivalent to 273.15K, with a temperature difference of 1 deg C equivalent to a difference of 1K, meaning the unit size in each scale is the same. This means that 100℃, previously defined as the boiling point of water, is now defined as the equivalent to 373.15K.
The Celsius scale is an interval system but not a ratio system, meaning it follows a relative scale but not an absolute scale. This can be seen because the temperature interval between 20℃ and 30℃ is the same as between 30℃ and 40℃, but 40℃ does not have twice the air heat energy of 20℃.
A temperature difference of 1 deg C is the equivalent of a temperature difference 1.8°F.
Rømer is a temperature scale named after the Danish astronomer Ole Christensen Rømer, who proposed it in 1701. In this scale, the zero was initially set using freezing brine. The boiling point of water was defined as 60 degrees. Rømer then saw that the freezing point of pure water was roughly one eighth of the way (about 7.5 degrees) between these two points, so he redefined the lower fixed point to be the freezing point of water at precisely 7.5 degrees. The inventor of the Fahrenheit scale Daniel Gabriel Fahrenheit learned of Rømer's work increasing the number of divisions by a factor of four and establishing what is now known as the Fahrenheit scale.