Abstract: An apparatus and method for measuring in real time the eccentric error of a rotating body, particularly an encoded optical disk. An eccentricity measurement pattern comprising a plurality of concentric spaced apart diffraction rings are included on an encoded disk. An error read head with a plurality of laser beams is positioned to reflect laser light off the eccentricity measurement pattern on the disk. The relative radial spacing of the laser beams on the error read head is adjusted to a non-integral multiple of the radial spacing of the diffraction rings. Output signals from the error read head are generated according to reflected laser light from the eccentricity measurement pattern which is detected by the error read head. The output signals are converted to count and direction information.