  Initially defined by the IAU in 1958, the galactic coordinate system was
thereafter in 1984 transformed from the B1950.0 FK4-based system to the J2000.0
FK5-based system. In 1994, the IAU recommended that the dynamical reference
system FK5 be replaced by the ICRS, which is a kinematical non-rotating system
defined by a set of remote radio sources. However the definition of the
galactic coordinate system was not updated. We consider that the present
galactic coordinates may be problematic due to the unrigorous transformation
method from the FK4 to the FK5, and due to the non-inertiality of the FK5
system with respect to the ICRS. This has led to some confusions in
applications of the galactic coordinates. We tried to find the transformation
matrix in the framework of the ICRS after carefully investigating the
definition of the galactic coordinate system and transformation procedures,
however we could not find a satisfactory galactic coordinate system that is
connected steadily to the ICRS. To avoid unnecessary misunderstandings, we
suggest to re-consider the definition of the galactic coordinate system which
should be directly connected with the ICRS for high precise observation at
micro-arcsecond level.

Astrophysics of Galaxies