Side view of a flattened star. The primed coordinates are the Cartesian coordinates used to describe the plane of the sky, as seen from Earth. (Y′, Z′) are the axes of the sky plane, and X′ the line of sight. The non-primed coordinates are linked to the star, and can be obtained by rotating the primed coordinates around the Y′ axis so that the rotation axis Z is at an inclination angle i from the line of sight. α is the angle between the normal n to the surface at an arbitrary point on the stellar surface and the line of sight. We recall that μ = cos α (cf. Eq. (8)). The star is shaded based on a 3D rendering of its shape, and not on physical effects such as gravity or limb-darkening. Other figures such as Figs. 12 and 13 show such effects.