[fitsbits] Celsius unit in FITS keyword comments

[Lucio Chiappetti]

On Thu, 3 Mar 2016, Forveille thierry wrote:

> The FITS standards explicitly states, from day 1 if my memory is 
> correct, that the only supported units are SI, plus degrees for angles.

Sorry to contradict you even I agreed with all your suggestions (and 
therefore wanted to be silent) :-)

I also *believed* FITS accepted only SI units plus something, but then 
Celsius apparently are listed as "derived units" for SI (at least on 
wikipedia).

The reference for units in the FITS standard is section 4.3, which DOES 
NOT list Celsius explicitly or implicitly in the text and in tables 3/4, 
nor does forbid it.

However it refers to the IAU style manual, which at the link mentioned by 
Rob https://www.iau.org/publications/proceedings_rules/units/ also does 
not list Celsius explicitly, except that in footnote 5 says "The degree 
Celsius (oC) is used in specifying temperature for meteorological 
purposes, but otherwise the kelvin (K) should be used."

I cannot talk for IVOA, but somebody more familiar with that environment 
should (after all there is no sense in FITS and IVOA diverging, nor in 
FITS ruling over this matter).

Being ill at ease with wikipedia as reference, I am now checking the 
official SI reference http://www.bipm.org/en/publications/si-brochure/
and indeed http://www.bipm.org/en/publications/si-brochure/section2-2.html
gives Celsius as a "SI coherent derived unit"

It is perhaps a pity that FITS standard 4.3.1 and Table 6 defining 
compound units do NOT allow to define a compound unit as an unit 
plus/minus a constant, so one cannot use K–273.15.

Presumably nobody is going to express a stellar temperature or galaxy 
cluster temperature or cosmic background temperature in Celsius, so the 
only possible references to Celsius in a FITS file should be 
non-astronomical, related to environmental or meteorological temperature.

And this usage is presumably bound to local/site conventions.

I see only two possible ways out: either convert temperature to K, or use 
explicitly a wording like "Celsius".

After all FITS 4.3 rules are a "should". It also says "If a requirement 
exists within this standard for the units of a keyword, then those units 
must be used." Since there is no requirement for Celsius, one is free to 
use the name under a local convention.

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