Schematics of the coordinate system. Main: 2D projection of the system, illustrating the observed planet centre (x_p,y_p), the orthogonal distances from the stellar spin-axis and equator ( – respectively), and the projected obliquity (λ); the stellar pole is indicated by a star. Insets a) and b) are adapted from Fabrycky & Winn (2009), to illustrate the 3D observer-oriented and orbit-oriented reference frames, respectively. Inset a): illustrates the projected obliquity in relation to the orbital inclination (i_p), stellar inclination (i_⋆), and the normals to the orbital plane (n_p) and stellar rotation (n_⋆). Inset b): (XYZ)′′ obtained after a rotation of the observer-oriented frame about the x axis by π/ 2−i_p, illustrates the 3D obliquity (ψ) and the azimuthal angle (Ω′ – not to be confused with the angular rotation denoted earlier as Ω).