  The centrifugial acceleration of the Solar system, resulting from the
gravitational attraction of the Galaxy centre, causes a phenomenon known as
'secular aberrration drift'. This acceleration of the Solar system barycentre
has been ignored so far in the standard procedures for high-precision
astrometry. It turns out that the current definition of the celestial reference
frame as epochless and based on the assumption that quasars have no detectable
proper motions, needs to be revised. In the future, a realization of the
celestial reference system (realized either with VLBI, or GAIA) should correct
source coordinates from this effect, possibly by providing source positions
together with their proper motions. Alternatively, the galactocentric
acceleration may be incorporated into the conventional group delay model
applied for VLBI data analysis.
