Definition of Kelvin: The kelvin is the primary unit of temperature measurement in the physical sciences, but is often used in conjunction with the Celsius degree, which has the same magnitude. Subtracting 273.16 K from the temperature of the triple point of water (0.01 °C) makes absolute zero (0 K) equivalent to −273.15 °C (−459.67 °F). (source https://en.wikipedia.org/wiki/Kelvin)
In this definition, Celsius is capitalized but kelvin is not?
Which is correct?
The confusion here stems from the fact that "Celsius" is not the name of the unit, rather it is a classifier for the unit "degree," of which there is also "Fahrenheit." Because "Celsius" and "Fahrenheit" are names (Anders Celsius and Daniel Fahrenheit), they remain capitalized.
However, a "kelvin" is a unit just like "meter" or "gram," which, despite being named after someone (Lord Kelvin), is spelled lowercase, just like all SI units named after someone (c.f. "watt," "hertz").
Wikipedia's entry on SI units explains this clearly:
Names of units follow the grammatical rules associated with common nouns: in English and in French they start with a lowercase letter (e.g., newton, hertz, pascal), even when the symbol for the unit begins with a capital letter. This also applies to "degrees Celsius", since "degree" is the unit.
(It goes on to explain that they are capitalized in German, because that's how German works. But your question isn't about German.)
Note that "degrees Kelvin" is not correct. Celsius and Fahrenheit are "degrees" because 100° C is not twice 50° C, nor is 100° F twice 50° F (in fact, that hardly makes sense).
But because 0 K is absolute zero, 100 K is twice 50 K, just like how 100 m is twice 50 m, which means the Kelvin scale is not a degree scale. (Oddly, note that "Kelvin" is capitalized here because it is functioning as an adjective modifying "scale," so is capitalized just like "American" would be. It's just the unit that isn't capitalized. Whew.)
(I'll confess, I don't entirely understand this myself, but that's how it was explained to me.)
Even though "kelvin" is not capitalized, its abbreviation is, to distinguish it from the SI prefix "kilo-," which is k: "kg," "km."
See also Wikipedia's entry on "Kelvin."
Both are correct.
I believe the difference is due to that fact that Celsius isn't the actual unit - the unit is called a "degree". Since we have two different ways of measuring temperature in units called "degrees", we distinguish between them with the name of the creator of the system - "Celsius" or "Fahrenheit". These are proper nouns, and so are capitalised. The units themselves (which are usually named after people), aren't capitalised.
So, if we were talking about Lord Kelvin, who gave his name to the kelvin unit of temperature, we capitalise his name, but with the unit itself we don't (as seen in this very sentence).
This gets a bit confusing, appearing to contradict the "a name not a unit" explanation, when people say "60 Fahrenheit" or "40 Celsius", etc - really this is shorthand for "60 degrees Fahrenheit" or "40 degrees Celsius".
EDIT - on a side note, you will sometime see people use "290 degrees Kelvin" - I believe that this is acceptable, even though it's not the official unit. If you do use it like this, capitalize "Kelvin". So, you can say
290 kelvin
or
290 degrees Kelvin
Stick to the former, though, since it's the standard.
Newspaper style, at least in Canada, used to be that the unit was capitalized if named after a person. So Kelvin is capitalized, not centigrade. But then, I have never seen volt (named after Mr. Volta) capitalized, or ampere, (named after Ampere). So no definitive response!
