Orbits in an anisotropic radiation field

The elliptic‐type motion around a source whose luminosity is anisotropic is being perturbatively treated. Using a Fourier expansion for the perturbing force, exact expressions describing the evolution of each orbital element are determined from Newton‐Euler equations. For near‐unit frequency the resonant solutions are pointed out. An approximate expression for the variation of the nodal period is determined, too. The behaviour of the radius vector of the unstable orbit (resonance case) is pointed out, showing periodic variations of constantly increasing amplitude. The (both angular and physical) time scale for escape is estimated. Concrete astronomical situations modellable in this way are mentioned.