Report of the Working Group ‘Relativity for Celestial Mechanics and Astrometry’

Abstract

Abstract : This is a brief report on the work done by the International Astronomical Union (IAU) working group Relativity for Celestial Mechanics and Astrometry. The first central task of the working group was to recommend some specific form of the metric tensor that is related to the distance of two neighboring points in space-time with coordinates. The metric tensor allows one to derive translational and rotational equations of motion of bodies, to describe the propagation of light, set the rates of atomic clocks, and to model the processes of observation. Meanwhile, it is widely accepted that to adequately describe modern astronomical observations one has to use several relativistic reference systems. The barycentric celestial reference system (BCRS) can be used to model the light propagation from distant celestial objects as well as the motion of bodies within the solar system. The geocentric celestial reference system (GCRS) is physically adequate to describe processes occurring in the vicinity of the Earth (i.e., Earth's rotation, motion of Earth's satellites). The central task of the working group is to specify the metric tensors both in the BCRS and in the GCRS and the corresponding space-time coordinate transformations between these two systems. The IAU resolution A4(1991) contains nine recommendations, the first five of which are relevant to this discussion: (1) space-time cannot be described by a single coordinate system; (2) definition of the origin and orientation of the spatial coordinate grids for the solar system (BCRS) and for the Earth (GCRS); (3) definitions of the TCB (Barycentric Coordinate Time) and TCG (Geocentric Coordinate Time), the time coordinates of the BCRS and GCRS, respectively; (4) definition of Terrestrial Time (TT); and (5) old barycentric dynamical time (TDB) may still be used where discontinuity with previous work is deemed to be undesirable.