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Fig. 1


Right: 3D representation of the problem. The yellow sphere is the unit sphere centred on the star. P marks the direction of the orbital angular momentum vector Lp and S that of the stellar spin Ls. The dashed circle passing through points A and S gathers all the points making an angle Ψ with P. It is represented in the top left. Top left: circle of the unit sphere gathering all the points at colatitude Ψ with respect to the orbital angular momentum vector of the planet. A is the point facing the observer; S is the point corresponding to the direction of the stellar spin. A and S are projected on the diameter of this circle perpendicular to the line of sight onto A and S; φs is then . Bottom left: projected plane, as seen by the observer. The previous dashed circle is now a dashed horizontal line, on which A and S are the projections of A and S along the direction of the line of sight. The arc PR defines an angle Ψ, while the projected spin-orbit angle β is , marked in red.

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