Comparison of the position differences between the coupled and uncoupled trajectories (blue) and the position differences between the classical uncoupled model (i.e., with once-per-orbit libration) and fully locked (i.e., no libration) trajectories (orange). Only the final ten orbits of the nine years and four months have been plotted. Note the different units. Panel a: in-plane projection of position differences. Panel b: out-oſ-plane projection of position differences.