new SI prefixes

[Norman Gray]

Alberto, hello.

On 28 Nov 2022, at 14:01, alberto micol wrote:

> Our VOUnit REC tells us we should use “barn” and not “b”
>  (see page 13 of https://www.ivoa.net/documents/VOUnits/20140523/VOUnits-REC-1.0-20140523.pdf <https://www.ivoa.net/documents/VOUnits/20140523/VOUnits-REC-1.0-20140523.pdf> ).

You are perfectly correct.

>  In that sense, while 1 Qb = 100 m**2    is correct in SI,
>  It seems to me that it is wrong to a VOUnits-compliant parser, where instead it should be written:
>
>  1 barn = 100 m**2

Hmm: come to think of it, I'm now not sure if 'b' _is_ legitimate in SI.  The VOUnits document says, in Sect.2.5, that 'b' "is the SI symbol for 'barn'", in a parenthetical remark saying that 'b' shouldn't be used for 'bit' or 'barn'; but doesn't give a reference for this.  Aha: the 'barn' (symbol 'b') is mentioned in Table 8 of the SI brochure, 8th edition [1], but isn't mentioned at all in the 9th edition.  The IAU units rules [2] mention the barn in table 6, with symbol 'b', but asserts that it is a deprecated unit.

So I think we are being reasonable in including 'barn' in VOUnits Table 2, and in discouraging using symbol 'b' for it, but I should edit out the reference to it in Sect.2.5.

Does that sound reasonable?

Parenthetically (and (a) for curiosity's sake, and (b) because I have some marking to do), in sorting this out in my head I realise that -- pace François -- the unit 'are' appears currently to have a rather ghostly definition.  The 'are' was defined by the French Revolutionary Government in year III as 100m^2 (alongside the metre, litre, gramme and stère, for volume), but it was only the hectare that made it in to the first SI definition, and which is still there in the 9th edition.  And thus the 'hectare' is indeed etymologically 100 ares, rather than being named via a square hectometre.  It's not _solely_ le monde anglophone that has its metrological atavisms.

Best wishes,

Norman


[1] https://www.bipm.org/en/publications/si-brochure
[2] https://www.iau.org/publications/proceedings_rules/units/


-- 
Norman Gray  :  https://nxg.me.uk
SUPA School of Physics and Astronomy, University of Glasgow, UK