Spin-axis precession period of a planet orbited by a single satellite. The period is given by Eq. (42) and represented here in unit of the characteristic period T = 2π∕p. For ε >90°, the spin-axis precession frequency is negative. Each panel corresponds to a given value of the satellite’s mass parameter η (see Eq. (43)), as labelled in the top right corner. Some level curves are highlighted in black. The singular point S₁ and the ridge line a = r_F are indicated in red.