  The vorticity of a congruence is often considered to be the rate of rotation
for the precession of a gyroscope moving along a world-line belonging to that
congruence. Our aim here was to determine the evolution equation for the
angular momentum of a gyrosocope with respect to an arbitrary time-like
congruence: i.e, a reference congruence not containing the curve described by
the gyroscope. In particular, we show the specific conditions needed to support
the introductory statement about the vorticity. We thus establish a
well-founded theoretical description for the analysis of the precession of
gyroscopes, providing suitable conclusions for possible experiments.
