VOUnits RFC

[Arnold Rots]

This reminded me that I should quote the units allowed in the draft
of FITS WCS Paper IV:

recommended:

s: second
d: day (= 86400 s)
a: Julian year (= 365.25 d)
cy: Julian century (= 100 a)

also acceptable:

min: minute (= 60 s)
h: hour (= 3600 s)
yr: Julian year (= a)
ta: tropical year
Ba: Besselian year

The last two were added reluctantly, necessitated by usage in existing
records.

  - Arnold

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Fri, Jul 26, 2013 at 1:02 PM, Rob Seaman <seaman at noao.edu> wrote:

> Hi Norman and the Semantic Faculties,
>
> > The VOUnits recommendation process trundles on...
>
> I wonder if you have seen the proposed new SI system:
>
>         http://nvlpubs.nist.gov/nistpubs/jres/116/6/V116.N06.A01.pdf
>
> There may be some implications for VOUnits from their seven-fold path.
>
> Also, Andrew Main (zefram of perl) gave a presentation (draft attached) at
> the recent UTC meeting with his vision of how to support multiple
> timescales.  He addresses the issue of (the equivalent of) unit conversions:
>
>         "It is worth examining the extent to which multiple dispatch is
> required. In many non-time conversion situations, all conversions can pivot
> around a single canonical format, thus breaking the general double-dispatch
> conversion case into two single-dispatch legs. That is not possible in the
> general case for time scale conversions. For example, consider the UT1 ↔
> UT2 conversion, which is exactly defined for any time for which UT1 is
> defined. If forced to go via an atomic time scale such as TAI, the
> conversion would be incorrectly rendered imprecise for any future time (for
> which the requisite EOP measurements don’t yet exist). Conversely, UTC ↔
> TAI would be incorrectly rendered imprecise if forced to go via an Earth
> rotation time scale. There is no pivot time scale that satisfies both
> cases, so single dispatch does not suffice." (p. 7)
>
> The assertion is not that all non-time conversions benefit from canonical
> formats, but only that many do.  Some may not.
>
> > The VOUnits Recommendation still intends _not_ to be innovative, in the
> sense that it is intended to indicate the _intersection_ of the existing
> syntaxes for unit strings, as much as possible.
>
>
> I guess innovative might imply the conveyance of semantics within the
> format / thesaurus?  Whereas the intent of the recommendation is to convey
> only syntax and leave semantics for applications to sort out?  Whichever it
> is, the structure of the conversions should be general enough not to assume
> that all time-like or mass-like or distance-like conversions are
> commutative / idempotent / reversible / transitive.
>
> Rob
> --
>
>
>
>
>
>
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