Schematic of the geometry of a light ray crossing the atmosphere. The inner circle is the arbitrary reference surface of the planet of radius Rp. The lighter-grey region is the atmosphere. The distance from the centre of the planet is r = Rp + z. A light ray is defined by its distance of closest approach to the centre of the planet, ρ, and the corresponding tangential altitude zt = ρ − Rp. The direction of the light ray defines the x direction. As we further discuss the extent of the limb along the ray, we introduce xlimb so that the absorption outside the [−xlimb, xlimb] segment is negligible in determining the transit radius, and the corresponding limb opening angle ψ.
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