refframe vocabulary

[Steve Allen]

On Mon 2019-04-08T16:41:10+0200 Markus Demleitner hath writ:
> * Speaking of references, once you start digging, it turns out
>   ECLIPTIC becomes a tricky term.  If you can supply a reference for
>   what this probably means where it's being used, I'd be really
>   happy.  Oh, and if you have something ready for SUPER_GALACTIC, I'd
>   like that as well.

I also would like to see a reference for these, but my impression is
that they have never been used for precision purposes.  To decode my
abbreviations below I point to the current reference system synopsis
http://www.iaufs.org/res.html

In the case of ECLIPTIC I think that the FK4 and FK5 versions can be
retained for historical purposes, but starting from the IAU 2000
conventions that came into use in 2003 that concept should be used
with caution.  I think that using the notion of ECLIPTIC is akin to
the notion of "heliocentric", which is to say that some approximations
for the sake of tabulation and manual calculation are being used.

I have not done a search, but my impression is that most of the usage
of ECLIPTIC is as a form of (probably non-relativistic, possibly not
even aberrated) GCRS which has been rotated using what was
ideally/hopefully the precession model corresponding to the rest of
the astrometry in that usage.  As such EQUINOX always required the
specification of the date of the mean equinox.

That kind of usage could be continued into the future using the ICRS
and P03 precession (likely one of the lower-precision "concise"
versions of P03 as given by Capitaine and Wallace A&A 478, 277 2008).
Ideally this new usage would specify both the date of the equinox and
the precession model (which may change again, but at the precision
that is likely expected for usage of ECLIPTIC the name of the
precession model is probably irrelevant).

If there are reasons for wanting a precise version of ECLIPTIC in the
future then I will guess that it wants to be reconsidered and renamed.
In order to correspond to ICRS the new notions should be expressed in
the BCRS and it should be named something like "INVARIABLE PLANE".
My impression is this would be a rotation of the ICRS to re-orient
things according to the angular momentum vector of the solar system
and the intersection of that plane with the ICRS equator.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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