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1. Background on IAU Resolutions on reference systems
At its 23rd General Assembly in
1997, the IAU adopted the International Celestial Reference System (ICRS) as
specified by IAU Resolution A4, 1991 and the International Celestial Reference
Frame (ICRF) (Ma et al. 1998) that realizes the ICRS. At the 24th IAU GA in 2000, a number of
resolutions were passed that concern the definition of the astronomical
reference systems and transformations between them, which are required when
dealing with the Earth's rotation or when computing directions of celestial
objects in various systems:
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Resolution B1.3 specifies that the systems of space-time coordinates as
defined by IAU Resolution A4 (1991) for the solar system and the Earth within
the framework of General Relativity are now named the Barycentric Celestial
Reference System (BCRS) and the Geocentric Celestial Reference System (GCRS)
respectively. It also provides a general framework for expressing the metric
tensor and defining coordinate transformations at the first post-Newtonian
level (see Soffel et al. 2003).
-
Resolution
B1.6 recommends the adoption of the new
precession-nutation model that came into force on 1 January 2003 and
is designated IAU 2000 (version A corresponding to the complete model of
Mathews et al. (2002), of 0.2~mas accuracy and version B corresponding to its
shorter version (McCarthy and Luzum 2002) with an accuracy at 1 mas
level).
-
Resolution
B1.8 recommends the use of the ‘non-rotating
origin’ (Guinot,1979), designated CEO (Celestial Ephemeris Origin) and TEO
(Terrestrial Ephemeris Origin), as origins on the moving equator in the
celestial and terrestrial reference systems, respectively, and defines UT1 as
linearly proportional to the Earth Rotation Angle (ERA) between the CEO and the
TEO on the moving equator (Capitaine et al. 2000). This resolution recommends
that the transformation between the International Terrestrial Reference System
(ITRS) and the GCRS be specified by the position of the Celestial Intermediate
Pole, CIP, (defined by Resolution B1.7) in the GCRS, the position of the
CIP in the ITRS, and the ERA. This resolution also recommends that
the IERS continue to provide users with data and algorithms for the
conventional transformations.
-
Resolution
B1.9 provides the conventional linear relation
between TT and TCG.
2. Terminology for Implementing the IAU Resolutions
The
implementation of these Resolutions (especially B1.3, B1.7 and B1.8) for
various astronomical applications requires that a consistent and well defined terminology is recognized and adopted by the astronomical
community for all the quantities based on the new concepts.
The
terminology issue began to be discussed within the ICRS Working Group in 2003 and in recent papers (Seidelmann and
Kovalevsky 2002, Capitaine et al. 2003 a, b). The 2003 ICRS discussions resulted in
documents that were circulated and discussed within the WG ICRS before the
last IAU General Assembly, but no resolutions were presented.
3. The Working Group ``Nomenclature for Fundamental Astronomy'' (NFA)
The IAU
Working Group on NFA was created by Division I at the 24th IAU General Assembly
in July 2003 and officially installed by the IAU in November 2003. The
general task of this WG is to provide proposals for new nomenclature associated
with the implementation of the IAU 2000 Resolutions and to make related
educational efforts for addressing the issue to the large community of scientists. The webpage of the
NFA WG is: http://syrte.obspm.fr/iauWGnfa/
where all
the documents regarding the WG discussion and its Newsletters are posted, and
includes those documents mentioned above. NFA Newsletters 1 to 4 (October 2003 to April
2004) have discussed the different steps to be followed by the WG in order to
select the proposed terminology. One
important step has been the preparation of a NFA questionnaire.
4. The NFA Questionnaire
Based on
(i) the document “Draft of Nomenclature and Terminology for the IAU WG'' by C. Hohenkerk
(Annex 6 of Newsletter 1) and (ii) the preliminary list of questions
about terminology provided in Newsletter 1, a draft questionnaire was
prepared and submitted to the WG (November 2003). The comments and suggestions of the WG were incorporated
(January 2004), including splitting the Questionnaire into two parts;
Questionnaire NFA/A on ‘terminology choices’ which was intended for
the astronomical community, and Questionnaire NFA/B, for the Almanac Offices and related organizations. All
the WG members responded to QA and the main Almanac offices responded to QA and
QB. The documents collecting the responses
and comments to both Questionnaires have been distributed and posted on the NFA
webpage. They have been the basis for the draft WG recommendations which are to
be discussed by the WG.
The summary provided in the NFA Newsletter 4 and the
responses to the questions of this Newsletter have led to draft Working Group
recommendations and guidelines on terminology. These guidelines are to be discussed by the WG
and then by a larger community before being submitted to the IAU together with
a Resolution proposal to the IAU 2006 GA.
6. References
Capitaine, N., Guinot, B., McCarthy, D.D.,
2000, "Definition of the Celestial Ephemeris Origin and of UT1 in the
International Celestial Reference Frame", A&A 355, 398.
Capitaine, N., Chapront, J., Lambert, S.,
and Wallace, P.T. 2003a, Expressions for the Celestial Intermediate Pole and
Celestial Ephemeris origin consistent with the IAU 2000A precession-nutation
model, A&A, 400, 1145
Capitaine, N., Wallace, P.T.,
and McCarthy, D.D. 2003b, Expressions to implement the IAU 2000 definition of
UT1, A&A, 406, 1135
Ma, C., Arias, E. F.,
Eubanks, T. M., Fey, A., Gontier, A.-M., Jacobs, C. S., Sovers, O. J.,
Archinal, B. A., and Charlot, P., 1998, ``The International Celestial Reference
Frame as realized By Very Long Baseline Interferometry, AJ 116, 516
Mathews, P. M., Herring, T.
A., and Buffett B. A., 2002, ``Modeling of nutation-precession: New nutation
series for nonrigid Earth, and insights into the Earth's Interior," JGR, 107,
B4, 10.1029/2001JB000390.
Seidelmann, P.K. and
Kovalevsky, J. 2002, Application of the new concepts and definitions (ICRS,
CIP, and CEO) in fundamental astronomy, A&A, 392, 341-351.
Soffel,M., Klioner, S.,
Petit, G., Bretagnon, P., Capitaine, N., et al., 2003, "Explanatory
supplement for the IAU'2000 resolutions on relativity", AJ 126, 2687