Relationship between the IP system and the co-orbital system (prograde Sun). It is materialized by the three angles u_S, i_S, and Ω_S. The xy-plane is the IP. The a_π vector is the projection of the Sun's acceleration onto the instantaneous SBO plane. The θ angle is the polar angle which positions a_π from the radial direction. The z-component of the orbital solar angular momentum vector, L = Ln̆, is L_z = L cos i_S.