Phase plane for the normal form Hamiltonian obtained by restricting Eq. (22) to first-order terms in the eccentricity and retaining the full secular effect of Jupiter’s J₂ harmonic in Eq. (24) (along with Eq. (21), of course). The full blue curve is obtained by integrating Hamilton’s equations with Eq. (30) as initial conditions, except for q₃ = 0 and P₃ as in Eq. (31). These are used also for the dashed blue curve, where Eq. (25) and the second-order terms of Eq. (22) are also taken into account.