Add to ORKGIt is shown how all global Wigner distribution moments of arbitrary order in the output plane of a (generally anamorphic) two-dimensional fractional Fourier transform system can be expressed in terms of the moments in the input plane. This general input-output relationship is then broken down into a number of rotation-type input-output relationships between certain combinations of moments. As an important by-product we get a number of moment combinations that are invariant under (anamorphic) fractional Fourier transformation