Top panel: sketch of the sky-projected apparent stellar disk and illustration of the impact parameter p and integration variables x (along the abscissa) and r (along the radius). The transiting planet is shown as a black circle. Bottom panel: sketch of the stellar intensity or apparent stellar brightness across the disk. The variables r and φ are the integration variables used to calculate the disk-averaged intensities for the various limb darkening laws. The curved line represents I(γ) in the quadratic limb darkening law with a = 0.4 and b = 0.4 as used in Fig. 1.