The photometric center of the Gegenschein

Model calculations are used to evaluate two factors which determine the position of the photometric center of the Gegenschein: the increased scattering efficiency of the interplanetary dust near backscattering (scattering angles θ ～ 165–180°), and the spatial density distribution of the dust. Computer-generated brightness contours are used to investigate which of these two factors dominates. The code employs empirical scattering functions with and without a brightness enhancement in the backscattering region. It is found that the effect of the enhanced scattering of light by dust in the backscattering region overrides the effect of the spatial-density distribution of the dust. As a result, the photometric center should be observed at the antisolar point at all times, except possibly when the antisolar point is at its maximum displacement from the symmetry plane.