Division 1 / Working Group. Numerical Standards of Fundamental Astronomy. Triennial Report 2006-2009

The IAU Working Group (WG) on Numerical Standards for Fundamental Astronomy has been tasked with updating the IAU Current Best Estimates (CBEs). In addition to the need to update the CBEs because of improved estimates, there have also been significant changes that impact the IAU CBEs. Since the IAU CBEs were adopted, the IERS Conventions 2003, a document widely used by the astronomical and geodetic communities, has been produced. This reference contains estimates of many of the constants included in the IAU CBEs. One significant change was the adoption of a new precession model with IAU 2006 Resolution B1. This resolution accepted the conclusion of the IAU Division I Working Group on Precession and the Ecliptic (Hilton et al. 2006) and adopted the P03 precession theory of Capitaine et al. (2003). This resolution also replaced the terms lunisolar precession and planetary precession with precession of the equator and precession of the ecliptic . Another change is the redefinition of Barycentric Dynamical Time (TDB) that occurred with the adoption of IAU 2006 Resolution B3. In addition to updating the list of CBEs, the WG is addressing the larger issues surrounding the adoption of IAU CBEs. These include the mechanism to keep the CBEs current and the way in which these constants will be provided, the procedure to document the theoretical context of the constants, and whether the IAU should revise its current list of adopted constants to correspond with the new list of CBEs. There are now significant differences between the CBEs and the current IAU system of constants. This is due to both increasing accuracy of estimates and to changes in astronomical theory. As a result of this, there is a consensus to recommend to the 2009 IAU General Assembly that the IAU System of Constants be updated. There is also a consensus to recommend that the IAU should seek a mechanism to periodically update the value of the astronomical constants and CBEs in the future. Published in the Transactions IAU (International Astronomical Union), v27a: Reports on Astronomy 2006-2009, p60-62, 2009.