USNO Circular 179 - U.S. Naval Observatory

6 RELATIVITY
1.4 Concluding Remarks
The 2000 IAU resolutions on relativity define a framework for future dynamical developments
within the context of general relativity. For example, Klioner (2003) has described how to use the
framework to compute the directions of stars as they would be seen by a precise observing system
in Earth orbit. However, there is much unfinished business. The apparently familiar concept of
the ecliptic plane has not yet been defined in the context of relativity resolutions. A consistent
relativistic theory of Earth rotation is still some years away; the algorithms described in Chapter 5
are not such a theory, although they contain all the main relativistic effects and are quite adequate
for the current observational precision.
A local reference system similar to the GCRS can be easily constructed for any body of an
N-body system in exactly the same way as the GCRS, simply by changing the notation so that the
subscript E denotes a body other than the Earth. In particular, a celenocentric reference system
for the Moon plays an important role in lunar laser ranging.
It is also worth noting that the 2000 resolutions do not describe the proper reference system of
the observer — the local, or topocentric, system in which most measurements are actually taken.
(VLBI observations are unique in that they exist only after data from various individual antennas
are combined; therefore they are referred to the GCRS ab initio.) A kinematically non-rotating
version of the proper reference system of the observer is just a simplified version of the GCRS: xi E
should be understood to be the BCRS position of the observer (vi E and aiE are then the observer’s
velocity and acceleration) and one should neglect the internal potentials. See Klioner & Voinov
(1993); Kopeikin (1991); Kopeikin & Vlasov (2004); Klioner (2004).
One final point: the 2000 IAU resolutions as adopted apply specifically to Einstein’s theory of
gravity, i.e., the general theory of relativity. The Parameterized Post-Newtonian (PPN) formalism
(see, e.g., Will & Nordtvedt (1972)) is more general, and the 2000 resolutions have been discussed in
the PPN context by Klioner & Soffel (2000) and Kopeikin & Vlasov (2004). In the 2000 resolutions,
it is assumed that the PPN parameters β and γ are both 1.