Geometry of the planetary transit: (a) Motion of the planet with respect to the observer, located at infinity in the direction of x of the frame {x, y, z}. The centre of the star is denoted O, P is the periastron at reference time t_P, and J_⋆ is the angular momentum of the star projected on the plane of the sky {y, z}. (b) The transit as seen by the observer and the definition of the points {T_i}; here are only shown T₁ and T₃. (c) The trajectory of the planet and the points {T_i} (transit) and (eclipse) as seen in the orbital plane {X, Y}. Up to an angle Ω, the observer is in the direction of X. (d) View of the transit from above. The direction (ON) is the line of “nodes”, corresponding to (OX) in (a).