"Nomenclature for Fundamental Astronomy" (NFA)

Working Group objectives and methods



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:


-         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.



5. NFA Working group recommendations and guidelines on terminology


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