[thaumasiotes]

> it falls apart as soon as you ask about 0 Kelvin

Huh? The whole point of Kelvins is that their zero point is an actual 0 value. That's why Kelvins are a unit and °C aren't.

Q: An object's temperature is 20°C. The object's temperature increases by 10%. What is the new temperature of the object?

A: 49°C.

[peteretep]

> That's why Kelvins are a unit and °C aren't

Could I ask that you update Wikipedia with your discovery? Sadly the page appears to be erroneously using the word "unit" all over the place! https://en.wikipedia.org/wiki/Celsius

Also, NIST might benefit from your guidance: https://physics.nist.gov/cuu/Units/kelvin.html

[thaumasiotes]

It's already there, if you know enough to recognize it:

> The degree Celsius (°C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures

In other words, you can add a quantity of °C to a temperature value and you'll get another temperature value. But you can't measure a temperature in °C.

Compare how, for example, the python datetime library uses a datetime type and a timedelta type. A datetime plus a timedelta is a datetime. datetimes refer to points in time, and timedeltas don't.

°C measures a temperature delta, but not a temperature.

[peteretep]

> It's already there, if you know enough to recognize it

"As an SI derived unit", "or a unit to indicate", "the unit was called"

Three mentions of it being a unit in the first paragraph, alone. I understand the point being made; the conclusion that "celsius is not a unit" is bogus, however, by any common definition, including NIST's.

In a now-deleted comment, you linked to a Wikipedia page on Dimensional Analysis, which includes the sentence "to convert from units of Fahrenheit to units of Celsius".