Talk:Celsius

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An editor has asked for a discussion to address the redirect °С. Please participate in the redirect discussion if you wish to do so. Pkbwcgs (talk) 10:54, 19 February 2020 (UTC)

History in the top section?

Shouldn't most of the evolution of the definition of celsius be in the history section. People coming to this page expect the lede paragraphs to give an overview of what the thing is, not multiple definitions over several paragraphs which give conflicting descriptions. —Memotype::T 00:55, 23 February 2023 (UTC)

Appropriate mention of Kelvin's derivation in opening paragraph

I've made edits with various wordings, three-times reversed, to indicate that Kelvin has a relationship with Celsius; it's not simply another scale. Historically Kelvin is derived from Celsius; in modern times obviously Celsius is defined in terms of Kelvin. I don't agree that the previous wording is entirely adequate.

I think we can put something in the opening para, perhaps "with same size intervals" or perhaps even just "related scale Kelvin".

First below is how it was; 2 and 3 are two of my wordings.

1. Most countries use this scale; the other major scale, Fahrenheit, is still used in the United States, some island territories, and Liberia. The Kelvin scale is of use in the sciences, with 0 K (−273.15 °C) representing absolute zero.
2. ... The Kelvin scale is of use in the sciences, and was originally derived from Celsius, but with 0 K (−273.15 °C) representing absolute zero.
3. ... The Kelvin scale is of use in the sciences, and is based on Celsius but shifted by -273.15 so that 0 K is absolute zero.

My last edit was to add "closely-related" to "[...] one of two temperature scales used in the International System of Units (SI), the other being the closely-related Kelvin scale." This is in my opinion the best compromise between brevity and informative.

What do others think?

88.19.0.37 (talk) 13:12, 9 January 2024 (UTC)

I think it's worth mentioning in the lead. Kelvin is already brought up in the first paragraph, so it's a logical place to put this information. Justin Kunimune (talk) 12:30, 10 January 2024 (UTC)

At present we mention the Kelvin scale in the first sentence of the lead, describe it in the last sentence of the first paragraph, repeat that description in the second half of the third paragraph with more details, and then focus the fourth paragraph on the Kelvin scale too.

The mention at the end of the first paragraph is only forced on us by the preceding description of Fahrenheit as "the other major scale", as that seems contrary to Kelvin's place in science. Let's change

Most countries use this scale; the other major scale, Fahrenheit, is still used in the United States, some island territories, and Liberia. The Kelvin scale is of use in the sciences, with 0 K (−273.15 °C) representing absolute zero

to

Most countries use this scale (the Fahrenheit scale is still used in the United States, some island territories, and Liberia).

Together with that, we could implement the IP's third, briefest suggestion and change the end of the first sentence from

the other being the Kelvin scale.

to

the other being the closely related Kelvin scale.

without further overwhelming the lead of this article, Celsius. NebY (talk) 13:53, 10 January 2024 (UTC)

Liberia

In the opening paragraph, Liberia is stated as one of the countries still using Fahrenheit. However, in the map further down, Liberia is shown in green, denoting counties using Celcius. From what I could find Liberia is in the process of metrication and is switching to using Celcius officially. Should the opening paragraph be changed to reflect this? SamanthaPantha (talk) 14:57, 4 January 2026 (UTC)

Physical quantity "Celsius temperature"

It should be noted that BIPM defines a separate quantity "Celsius temperature" t (SI Brochure chapter 2.3.1). The reason is that conversion to thermodynamical temperature T is not a factor. For example, a length can be written als $l=20\;\mathrm {inches} =50.8\;\mathrm {cm}$. By simple division you get $1\;\mathrm {inch} =2.54\;\mathrm {cm}$. A similar equation $T=100\;{}^{\circ }\mathrm {C} =373.15\,\mathrm {K}$ would lead to a nonsense result $1\;{}^{\circ }\mathrm {C} =3.7315\;\mathrm {K}$. Likewise, physical equations such as $v=s/t$ can be used with any unit of length, but the formula for black body radiation $P/A=\sigma T^{4}$ only works for a scale that is 0 at the absolute zero (such as Kervin oder Rankine) but not for Celsius. Therefore there is need of a separate quantity.

This should be added explicitely in the article (by a native speaker, not by me). -- Wassermaus (talk) 09:28, 12 May 2026 (UTC)

The Celsius temperature isn't a different physical quantity. As the Brochure says, the Celsius temperature is an expression of "a thermodynamic temperature ... in terms of its difference from the reference temperature". We shouldn't distinguish the Celsius temperature from a thermodynamic temperature, only from the thermodynamic temperature expressed in kelvins, and the article does do this. NebY (talk) 15:06, 12 May 2026 (UTC)

I disagree: i) Yes, t is the difference T-T₀, no one denies that. But this means that T and t cannot be interchanged in physical formulas - therefore thermodynamic temperature and Celsius temperature are two different "things". (Unlike e.g. length - there us no such thing like "metric kength" and "US customary length".) I hope, everyone agrees on this. ii) They are different "things", but are they different "physical quantities"? The brochure says yes: in table #4, Celsius temperature is listed as a separate "derived quantity". -- Wassermaus (talk) 20:12, 23 May 2026 (UTC)

The fact that the Celsius scale isn't an absolute scale doesn't mean that it's measuring anything other than thermodynamic temperature, or that thermometers can be made to measure different physical quantities merely by adding markings to them. The Celsius scale's described in the SI brochure as a derived quantity only inasmuch as it's offset (and to facilitate its listing). That you can't use it in formulae suitable for absolute temperatures without supplying the offset constant doesn't render it a different physical quantity; it's still a measure of thermodynamic temperature, but not with absolute zero as the scale's null point. Such situations aren't unusual in measurement, in which for perfectly reasonable and historic reasons quantities are expressed in a variety of ways without implying the discovery of new physical quantities in the universe, e.g. absolute, gauge and differential pressures. NebY (talk) 11:43, 28 May 2026 (UTC)