Resonant centres of apsidal resonance distributed in the inclination-eccentricity space (black dots) and libration zones obtained by analysing phase portraits (shaded areas). The level curves of the motion integral Σ₂ are shown as background and the resonant width is measured along the isoline of Σ₂. There are eight libration zones, denoted by numbers from 1 to 8. Evidently, the distribution of libration zones is symmetric with respect to the line of i = 90°.