Phase portrait of Colombo’s top Hamiltonian when the planet has the minimum orbital inclination required for going from an obliquity ε = 0° to ε > 90° over a single libration. The top and bottom panels show the same phase portrait using two sets of coordinates. In the bottom panel, the obliquity ε is the tilt from the z-axis and the resonance angle σ is the polar angle measured in the xy-plane. The parameters are and , respectively. The Cassini states are labelled and showed with coloured dots. The thick black level is the separatrix of the resonance.