  Equations of light, propagating from quasar to observer on earth, are
integrated in the time-dependent gravitational field of the solar system by
making use of either retarded or advanced solutions of the Einstein field
equations. This technique allows to separate explicitly the effects associated
with the propagation of gravity from those associated with light in the
integral expression for the relativistic time delay of light. We prove that the
relativistic correction to the Shapiro time delay discovered by Kopeikin (ApJ,
556, L1, 2001) changes sign if one retains direction of the light propagation
but replaces retarded with advanced solution of the gravitational field
equations. Hence, this correction is due to the propagation of gravity and
allows to measure its speed. Relativistic VLBI experiment conducted by Fomalont
and Kopeikin in September 2002 proves that the propagation of gravitational
field is characterized by the retarded potentials.
