The Report of the Sub-Group on Numerical Standards of the IAU Working Group on Astronomical Standards (WGAS) is presented. The report is intended to incorporate the majority of the responses received from the e-mail recipients of the series of WGAS Circulars. The report proposes to retain the present (1976) IAU System of Astronomical Constants and also proposes to establish a IAU File of Current Best Estimates. Further, the report proposes the establishment of a Maintenance Committee, governed by a set of proposed by-laws, in order to oversee the maintenance of the new file.

The given mission of the subgroup on Standard Procedures (SGSP) of IAUWGon Astronomical Standards (IAU/WGAS) is to prepare report “on standard procedures needed in fundamental astronomy, which a) should have a maximum degree of compatibility with the IERS Standards, b) should include the implementations of procedures in the form of tested software and/or test cases, c) should be available not only in written form, but also in machine-readable form” as described in the third item of Recommendation VIII of the IAU Resolution A4 (1991). After some general discussions, in 1992 we issued a questionnaire on the image and mechanism on the supposed set of standard procedures (IAU/WGAS Circulrar 51.2.1) and sent it to over 200 scientists in the world. Within a few months, 16 answers were sent back. The detailed answers and the questionnaire was distributed (IAU/WGAS Circular 54.2.1). As a simulation of supposed mechanism, in 1993 we did a test campaign to collect computational procedure(s) to compute the present IAU precession formula (Lieske et al., 1979) in FORTRAN (IAU/WGAS Circular 59.2.1). Within a few months, 5 submissions from 5 countires (Japan, China, USA, Russia, and UK) reached. The analysis of submitted routines was circulated (IAU/WGAS Circular 65.2.1). Based on these responses, we concluded that the establishment of a mechanism to provide such procedures to the community of astronomy is quite useful. To realize this, we will recommend some action items to the IAU as is described in the next Section. Its main purpose is to establish a mechanism by the combination of a comiittee of board and a center, which are to operate within a year after the recommendations are adopted by the IAU. Section 3 will describe some guidelines for the committee and center to perform the tasks to create and maintain standard procedures. Of course, these are to be consulted if necessary and are not to restrict or regulate the activity of the committee and the center.

The task of the Sub-Group on Electronic Distribution (SGED) of the Working Group on Astronomical Standards (WGAS) is to make recommendations for direct worldwide computer access to astronomical standards of various kinds. Specifically, part of the charter of the WGAS was ”to prepare a draft report on possible electronic access to these units, constants, quantities, and procedures...”

Like the other sub-groups, the SGED operated as a ”committee of the whole”, the whole being the entire Working Group on Astronomical Standards. The scope of interest includes numeric data and software that has some sanction by the IAU or has been validated (in some way) as implementing some IAU-adopted model, set of constants, catalog, ephemeris, or algorithm. Thus, the controversial matters of what defines an IAU standard, whether such standards are still useful, what subject areas are appropriate, what specific standards are to be adopted, how software is to be validated, etc., are addressed by the other sub-groups.

Included in the nine adopted recommendations of the IAU Working Group on Reference Systems (Hughes, et. al., in 1991), were recommendations for the introduction of Geocentric Coordinate Time (TCG) and Barycentric CoordinateTime (TCB), the renaming of the Terrestrial Dynamical Time (TDT) as Terrestrial Time (TT), and the approval to continue the use of Barycentric Dynamical Time (TDB) when that is desirable. The relationships between these different time scales and the reason for their introduction was given by Seidelmann and Fukushima (1992). Since it was recognized that there were some unresolved issues as a result of these recommendations, a subcommittee of the Working Group on Astronomical Standards was established for Issues on Time.

Conventional astronomical standards are applied to a wide range of applications, many of which do not require state-of-the-art precision. Stability of the standards is of great importance, although there is a trend by some in the astronomical community to promote frequent revision of the values of astronomical constants. It is possible to employ stable standards and to differentially correct them by use of the IERS formulation when very precise observations require such action.

The IERS formulation for the transformation from a fixed frame to an of-date frame involves precession and nutation along with two parameters δψ and δ which can represent modifications to the standard precession and nutation formulation. The transformation from the mean-of-date frame to the true-of-date frame in the IERS formulation using rotation matrices is rtrue = NrMean where

N = R1(−εm − Δε − δε) R3 (−ΔΨ − δΨ) R1(ε)m

where εm is the mean obliquity, ΔΨ and Δε are the nutations in longitude and obliquity, and where δψ and δε are IERS parameters which allow for changes to the IAU system. They are tabulated in the IERS reports and are generally thought of as observationally-determined values, but the IERS formulation can be used to introduce corrections of an arbitrary nature.

The observations of dψ and dε used in this analysis were taken from the combination solution of the IERS Sub-Bureau for Rapid Service and Precition (McCarthy and Luzum 1991a). Besides corrections to the coefficients determined from VLBI, additional estimates can be derived from lunar laser ranging (LLR) observations (Williams et al. 1991; Whipple 1993). Table ?? shows a comparison of the estimates of the change in longitude and obliquity derived in this analysis with corresponding terms from analyses by other authors using different observations. The theoretical value of obliquity by Williams (1994) is also included for comparison. There are significant unexplained discrepancies among the rate estimates. Difference in the methods of the analyses such as the procesure for the estimation of the nutation coefficients and correlations within the solutions probably account for the greater part of these discrepancies. In considering the adoption of changes in the IAU model for nutation, it is important to recall that changes must also be made in the precession constant. Introduction of changes in nutation without corresponding changes in precession will not improve the agreement between observations and theory.