Newsletter 1 (16 June 1998)


1. Composition of T5
2. Task of T5
3. Currrent situation at the beginning of the work
4. Questions relevant to T5
5. Connection of T5 with the IAU/IUGG WG on the "Thoery of nutation for a non-rigid Earth"
6. Work of T5
Appendix 1
Appendix 2

Newsletter 1
(16 June 1998) 

1. Composition of T5

The membership of T5 (Circ. 2 of the IAU WG ICRS, 17 May 1998 by F. Mignard) is :

of which e-mail adresses are given in Appendix 1.
 

2. Task of T5

The general task of this sub-group, untitled "Computational Consequences", is to discuss the consequences of the adoption of the International Celestial Reference System (ICRS) on the astrometric computations. This task is based upon two Resolutions which have been adopted at the 1997 XXIII rd General Assembly of the IAU (see Appendix 2) :

The subgroup T5 of the Working Group "ICRS" recommended by the Resolution B5 has to :

3. Current situation at the beginning of the work of T5

1) The ICRS adopted by the XXIII rd IAU GA is such that : its axes are fixed to the distant background as implied by observations of extragalactic sources, its realization is based on observations made from the Earth, the axes of which precess and nutate relative to the ICRS.

2) The conventions to be used for the conventional reference systems and the transformation between the celestial and terrestrial systems are described in the IERS Conventions 1996 (McCarthy D.D. 1996, IERS TN 21). 3) The definition and realization of the Celestial Ephemeris Pole (CEP) is not clear, as since the adoption of the CEP in the IAU 1980 theory of nutation, both the theory and the observing procedures have changed which give obsolete the 1980 definition of the CEP.
 

4. Questions relevant to T5

The general task of T5, described in 2., is a consequence of the papers and discussions of the Joint Discussion 3 " Precession, nutation and astronomical constants for the dawn of the 20th century of the XXIII rd IAU General Assembly in Kyoto (published in the Highlights of Astronomy). Other points have also been discussed during the "Journees Systemes de Reference spatio-temporels 1997" in Prague (22-24 September 1997) (quoted here as JSR97). A copy of the Proceedings can be sent to the members of T5 who did not attend this meeting.

Much of the questions relevant to the "Computational Consequences" of the adoption of the ICRS have been considered in the papers listed below:

1) "Introduction of JD3 on 'Precession, nutation and astronomical constants for the dawn of the 20th century' by V. Dehant and T. Fukushima,

2) Paper 1 of JD3 untitled "Overview, formulation and current situation for precession-nutation" by N. Capitaine,

3) Paper 8 of JD3 untitled "Astronomical effects of current changes in fundamental astrometric references" by J. Kovalevsky and D.D. Mc Carthy,

4) "Summary of the Joint Discussion 'Precession, nutation and astronomical constants for the dawn of the 20th century' " presented at the JSR97 by V. Dehant (pp 55-60 of the Proceedings of the JSR97),

5) "Proposals for a new solution of the precession-nutation" presented at the JSR97 by P. Bretagnon (pp 61-64 of the Proceedings of the JSR97),

6) "New precession/nutation and astronomical constants" presented at the JSR97 by D.D. McCarthy (pp 55-60 of the Proceedings of the JSR97),

7) "Formulation of precession and nutation for precise astrometric data" presented at the JSR97 by N. Capitaine (pp 83-86 of the Proceedings of the JSR97).

These questions and proposals will be summarized in the next Newsletter and addressed to T5.
 

5. Connection of T5 with the IAU/IUGG Working Group on the "Theory of Nutation for a non-rigid Earth"

Considering that some of the problems which are relevant to the "Computational Consequences" of the adoption of the ICRS are largely connected to nutation and that an IAU/IUGG Working Group on the "Theory of Nutation for a non-rigid Earth" (chaired by V. Dehant) will be working on the improvement of the theory of nutation up to its discontunuation of the IUGG General Assembly, a close connection between the sub-group T5 of the IAU WG ICRS and this IUGG/IAU WG seems necessary.

In order that the proposals of these two groups be consistent for the common issues, I propose, in agreement with V. Dehant (chair of the IAU/IUGG WG on nutation), that the discussions on the problems connected to nutation are common between T5 and a sub-set of the IAU/IUGG WG (to be specified).
 

6. Work of T5

The present Newsletter gives a look of the questions to be addressed and discussed within this subgroup. The second Newsletter will identify the main issues. Members of T5, in coordination with the IAU/IUGG WG on "The Nutation theory for a non-rigid Earth, are requested to give the additional points that they think relevant to the work of T5.
 
Appendix 1: e-mail addresses of the members of T5

CAPITAINE Nicole France capitain@danof.obspm.fr
DEHANT Veronique Belgium dehant@astr.ucl.ac.be
GONTIER Anne-Marie France gontier@obspm.fr
KOLACZEK Barabara Poland kolaczek@cbk.waw.pl
KOVALEVSKY Jean France kovalevsly@mfg.cnes.fr
LIESKE Jay USA jhl@chives.jpl.nasa.gov
MA Chopo USA cma@gemini.gsfc.nasa.gov
MC CARTHY Denis USA dmc@maia.usno.navy.mil
SOVERS Ojars USA ojs@vlbi07.jpl.nasa.gov
VONDRAK Jan Czech. Rep. vondrak@ig.cas.cz
 


Appendix 2: Resolutions B2 and B5 of the XXXIIIrd IAU General Assembly

(available on the website of IAU Commission 19: http://maia.usno.navy.mil/iauc19/)  

RESOLUTION B2

Proposed by the IAU working group on Reference Frames, On the International Celestial Reference Frame The XXIIIrd International Astronomical Union General Assembly Considering (a) That Recommendation VII of Resolution A4 of the 21st General Assembly specifies the coordinate system for the new celestial reference frame and, in particular, its continuity with the FK5 system at J2000.0; (b) That Resolution B5 of the 22nd General Assembly specifies a list of extragalactic sources for consideration as candidates for the realization of the new celestial reference frame; (c) That the IAU Working Group on Reference Frames has in 1995 finalized the positions of these candidate extragalactic sources in a coordinate frame aligned to that of the FK5 to within the tolerance of the errors in the latter (see note 1); (d) That the Hipparcos Catalogue was finalized in 1996 and that its coordinate frame is aligned to that of the frame of the extragalactic sources in (c) with one sigma uncertainties of ± 0.6 milliarcseconds (mas) at epoch J1991.25 and ± 0.25 mas per year in rotation rate; Noting That all the conditions in the IAU Resolutions have now been met; Resolves (a) That, as from 1 January 1998, the IAU celestial reference system shall be the International Celestial Reference System (ICRS) as specified in the 1991 IAU Resolution on reference frames and as defined by the International Earth Rotation Service (IERS) (see note 2); (b) That the corresponding fundamental reference frame shall be the International Celestial Reference Frame (ICRF) constructed by the IAU Working Group on Reference Frames; (c) That the Hipparcos Catalogue shall be the primary realization of the ICRS at optical wavelengths; (d) That IERS should take appropriate measures, in conjunction with the IAU Working Group on reference frames, to maintain the ICRF and its ties to the reference frames at other wavelengths. Note 1: IERS 1995 Report, Observatoire de Paris, p.II-19 (1996). Note 2: "The extragalactic reference system of the International Earth Rotation Service (ICRS)", Arias, E.F. et al. A & A 303, 604 (1995).

RESOLUTION B5

Proposed by the Joint Discussion N. 3 and on the consequences of the adoption of the International Celestial Reference Frame and of the Hipparcos Catalogue, On the Consequences of the Adoption of the International Celestial Reference Frame and the Hipparcos catalogue The XXIIIrd International Astronomical Union General Assembly considering (1) that the International Astronomical Union (IAU) has adopted an International Celestial Reference System (ICRS) in which the axes are fixed relative to the distant background as implied by observations of extragalactic sources, (2) that the realization of the ICRS is based on observations made from the Earth, the axes of which precess and nutate relative to the ICRS, (3) that there are significant differences between the nutation adopted by the IAU in 1980 and astronomical observations, (4) that a rate of variation of the obliquity is observed, which is not predicted by the 1980 IAU precession-nutation theory, (5) that there is a difference in the precession rate of about -3.0 milliarcseconds per year (mas/year) between the observed and adopted values, recommends (1) that Division I form a new Working Group to report to the IAU General Assembly in 2000 which will a. examine and clarify the effects on astrometric computations, of changes such as the adoption of the International Celestial Reference System, the availability of the Hipparcos catalogue, and the change expected in the conventional precession-nutation model, and b. make recommendations regarding the algorithms to be used, (2) that this Working Group study these questions jointly with the International Earth Rotation Service (IERS) and maintain a close connection with the IAU Working Group on Reference Frames, the IAU Working Group on Astronomical Constants, and the IAU-IUGG Working Group on Non-rigid Earth Nutation Theory (up to its discontinuation at the 1999 IUGG General Assembly), through exchange of representatives.