  Two-pion correlation functions, measured as a function of azimuthal emission
angle with respect to the reaction plane, provide novel information on the
anisotropic shape and orientation of the pion-emitting zone formed in heavy ion
collisions. We present the first experimental determination of this
information, for semi-central Au+Au collisions at 2-6 AGeV. The source
extension perpendicular to the reaction plane is greater than the extension in
the plane, and tilt of the pion source in coordinate space is found to be
opposite its tilt in momentum space.
