Orbit (in blue) of a single particle in the simulation illustrating the libration, or oscillation of the angular momentum through its guiding radius (red curve), on a period of ∼2 Gyr. The particle is driven rapidly outwards during the intense episode of the bar growth at 2 − 3 Gyr. On a longer timescale, despite a significant oscillation (≈2 kpc) of the guiding radius in a non-axisymmetric bar potential, the mean guiding radius (green line) shows only a small drift due a slow evolution of the bar (see Fig. 14). These periodic oscillations of the angular momentum (libration) thus can be misinterpreted as migration (churning), while the net motion of stars in a rigid or slowly evolving barred potential, in fact, is close to zero.