  Polarization of light rotates in a gravitational field. The accrued phase is
operationally meaningful only with respect to a local polarization basis. In
stationary space-times, we construct local reference frames that allow us to
isolate the Machian gravimagnetic effect from the geodetic (mass) contribution
to the rotation. The Machian effect is supplemented by the geometric term that
arises from the choice of standard polarizations. The phase accrued along a
close trajectory is gauge-independent and is zero in the Schwarzschild
space-time. The geometric term may give a dominant contribution to the phase.
We calculate polarization rotation for several trajectories and find it to be
more significant than is usually believed, pointing to its possible role as a
future gravity probe.
