  Extended source size effects have been detected in photometric monitoring of
gravitational microlensing events. We study similar effects in the centroid
motion of an extended source lensed by a point mass. We show that the centroid
motion of a source with uniform surface brightness can be obtained
analytically. For a source with circularly symmetric limb-darkening profile,
the centroid motion can be expressed as a one-dimensional integral, which can
be evaluated numerically. We found that when the impact parameter is comparable
to the source radius, the centroid motion is significantly modified by the
finite source size. In particular, when the impact parameter is smaller than
the source radius, the trajectories become clover-leaf like. Such astrometric
motions can be detected using space interferometers such as the Space
Interferometry Mission. Such measurements offer exciting possibilities of
determining stellar parameters such as stellar radius to excellent accuracy.
