Every measurement system has had its formal definition changed several times. The kilogram, for example, was once formally defined as the mass of a specific block of metal in France, which was later determined to be losing mass, and thus made a pretty terrible standard. Now, the kilogram is formally defined in terms of the meter and the Planck Constant.

Celsius was once defined by the freezing and boiling points of water, but those aren’t actually constant: Fahrenheit’s brine mixture is actually significantly more consistent. Kelvin’s degree spacing comes from that definition of Celsius, but it it was eventually redefined to be more precise by using the triple point of water: pure water at a specific pressure and temperature where it can simultaneously exist as solid, liquid, and gas. Significantly more accurate, but not enough: Kelvin was redefined in 2019 in terms of joules, which are in turn defined by kg, m, s, which are ultimately defined in terms of the Planck constant.

Celsius is now formally defined in terms of Kelvin. Fahrenheit is also formally defined in terms of Kelvin. Fahrenheit’s brine story is just a piece of trivia.