Fig. 1.

Geometry and parameters relevant to occultation and transit. In occultation, rays enter from the right, with impact parameter b, are bent by atmospheric refraction through an angle ω, and have a distance of closest approach $r_{min}$. In transit, rays follow the opposite trajectory. Note also the planetary radius, $R_p$, the radial coordinate, r, the corresponding vertical height coordinate, $z=r-R_p$, and the polar angle, ϕ.