An angular displacement of an object is measured interferometrically by splitting a laser beam into a reference beam and a measuring beam. The reference beam is directed at a stationary reference retroreflector and then a phase shift detector. The measuring beam is directed at a rotatable reflective surface of the object and then a stationary measuring retroreflector and then back to the rotatable reflective surface and then to the phase shift detector such that the phase shift detector measures an angular displacement of the rotatable reflective surface when the length of the path of the measuring beam changes when the rotatable reflective surface is displaced.