Frequently asked questions on Relativity
Here we would like to collect questions concerning relativity which are or
could be asked by people working in the field of fundamental
astronomy. The questions can be asked here.
The current list of answers is below. This list is permanently updated.
If you have any suggestions or comments concerning the questions and answers given below, please
contact Comission 52.
Why do we need two reference systems - GCRS and BCRS?
can argue in different ways. Basically, one can use any relativistic
reference system to describe physical phenomena since there is no
preferred one in general relativity. However, some of
the reference systems are more convenient than
others. In this sense the BCRS is convenient to describe solar system ephemerides.
Although phenomena located in a certain vicinity of the Earth can
be described in the BCRS as well, that description is not physically
adequate. For example, because of the time-dependent barycentric
velocity of the Earth, the Earth's figure in the BCRS is
time-dependent because of the Lorentz contraction. This effect has
nothing to do with the physics of the Earth.
To avoid this and similar effects a special reference
systems - called GCRS - was introduced. The influences of external matter
(Sun, Moon, etc.) and the translational motion of the Earth are
effaced in the GCRS coordinates as much as it is possible in general
relativity: as it is usual in Newtonian theory the external
gravitational potentials in the GCRS are reduced to tidal
ones. Further explanations can be found in
Soffel et al. 2003, Astron.J, 126, 2687.
What would be a good qualitative description of the time-dependent directions of
the GCRS spatial axes as viewed in the BCRS? (George Kaplan, USNO)
There is no rotation matrix between the spatial axes of the BCRS and those of the GCRS; the orientation of the BCRS spatial axes (determined by the ICRS) completely determines the orientation of GCRS axes.