Secular variation of the mean motion ratio between Ariel and Umbriel (violet curve), obtained using t_α,2 = 4.2 Gyr and assuming that the moons were locked into the 2:1 resonance. The two values of Q_U,2 were computed from Eq. (12) at the epoch of the escape from the resonance and at the current epoch, respectively. During the mean motion resonance, we accounted for the extra dissipation factor ℱ ≈ 2.28. The violet dot indicates the current value of n₂/n₃, while the violet dashed line is the continuation of the evolution given by resonance locking without capture into the 2:1 resonance.