Summary of the discussion on "Nomenclature in Fundamental Astronomy"
on Tuesday 21 September 2004, during the Journées 2004, Paris, 20-22 September 2004
(Nicole Capitaine, 2 November 2004)
This discussion took place (1700-1830) after Session III "Nomenclature
in fundamental astronomy" chaired by D.D. McCarthy and including the
following contributions:
- Capitaine N. : Report of the NFA Working Group
- de Viron O., Dehant V. : 3D representation of the Non-Rotating Origin
- Hohenkerk C. : Implementation of the new nomenclature in the
Astronomical Almanac
- Wallace P. : Post-IAU-2000 nomenclature for the telescope pointing
application
- Soffel M., Klioner S. : The ICRS, BCRS and GCRS: astronomical
reference-systems and frames in the framework of Relativity, problems of
nomenclature.
The following points were introduced by the panel, composed of
N.Capitaine, C.Hohenkerk, C.Ma, D.McCarthy, M.Soffel and P.Wallace, and
were submitted to the audience for comments and discussion:
(1) The draft NFA WG recommendations
(2) The draft Resolution proposal to the IAU 2006 GA
(3) The WG explanatory documents
(4) Future actions of the NFA
The discussion on these points is summarized below.
(1) The draft NFA WG recommendations
The discussion concluded by supporting Recommendations 1 to 12, except
for, in Recommendation 9, the comment on the use of Greenwich meridian
which was considered unnecessary by a number of participants.
The most contentious points in the other recommendations were:
- in Recommendation 3, whether "intermediate" was the best term for the
CIP/CIO triad,
- in Recommendations 5 and 6, whether to use "system" or "frame"; this
was debated not only for the "intermediate frame/system" itself, which
is quoted in this recommendation, but also for the ICRS/ICRF,
- in Recommendation 8, whether "right ascension" could legitimately be
used for CIO-based positions, but the final conclusion was to support these Recommendations.
The conclusion on the system/frame question was using "system" in the
broadest sense as possible (especially in the case of the intermediate
system), but keeping "frame" for the set of coordinates as with the
ICRF; a suggestion by M. Soffel to suppress the use of "frame" was
debated but not supported, especially because of the necessity of having
ITRF in the terrestrial case. Using special designations for particular
realizations of the "intermediate celestial system" was supported, but
not for the ICRS or ITRS which are considered as being theoretical ideal
versions of the ICRF and ITRF, respectively, although a certain
asymmetry was recognized between the celestial and terrestrial cases
(e.g. special designations exist for the ITRF, such as ITRF2000, but not
for the ICRF).
In Recommendation 12, there was the requirement that "ITRF prime
meridian" be replaced by "ITRF zero-meridian" which was to be verified,
and agreed afterwards, by the ITRS IERS Product Center (C. Boucher).
(2) The draft Resolution proposal to the IAU 2006 GA
This proposal was supported, except by one participant who was strongly
against.
(3) The WG explanatory documents
- B1 NFA document (Chart)
There have been some criticisms of showing all the successive steps and
giving them names: a suggestion to leave the chart as it is, simply
adding an introductory sentence explaining that these steps are set out
"as supplementary information for the general user community", was
agreed. Some warning should also be given regarding the step from the
intermediate terrestrial system to the local system that in fact it
would require a more complicated process in the General Relativity
framework in order to achieve a microarcecond accuracy.
- B2 NFA document (summary of terms and definitions)
There was a consensus that a more complete list of acronyms and
abbreviations was needed to be added to the Chart.
- B3 NFA document (terminology list)
This document will be revised when the ICRS/BCRS/CRS issue is clarified.
- The ICRS/BCRS/GCRS issue
The recent WG question on how the GCRS is defined from ICRS was
discussed in detail; the conclusion was that regarding the WG documents
and recommendations, it may be simply necessary to specify at the start
that the BCRS is considered together with the cosmological hypothesis of
non-rotation of the directions of quasars(*); introducing a new name for
the GCRS having its axes aligned to those of the ICRS was debated but
without no definitive conclusion.
(4) Future actions of the NFA
Further educational efforts were strongly recommended. Part A of the NFA
explanatory document is considered as being a key document to be
realized by the WG. The 3D representation of the Non-Rotating Origin
presented by O. de Viron will be made available (through a link to the
appropriate URL) after some improvements on the NFA web page.
(*) More recent discussions with Sergei Klioner during the GAIA meeting
in Meudon, leads me to think, that in this case, we can assume in addition,
that the BCRS considered here, has its spatial axes fixed to those of the
ICRS; this would avoid having to introduce a new name for the GCRS that
has its axes aligned to those of the ICRS.