Illustration of an occultation event in spacetime. The past light cone of the reception point-event x_B intersects , the world-line of the emitter, at the point-event x_A. The zeroth-order null light path (red line) joining the emission point-event x_A to the reception point-event x_B lies on the surface of . The domain represents the limit of the refractive region while x₋ and x₊ are the intersection point-events between and the zeroth-order null light path. The path of integration (dashed red line) is limited to the portion of the zeroth-order null light path crossing through .