Time variation of the libration action of an orbit trapped and dragged by a moving resonance. The bar slowed from Ω_p = 60–|$30 \, {\rm Gyr}^{-1}$| in approximately |$7.5 \, {\rm Gyr}$| (slowing rate η = 0.002) and the orbit was initially placed at |$J_{\rm s}= 550 \, {\rm kpc}^2 \, {\rm Gyr}^{-1}$|⁠. The libration action calculated along the contours of the time-frozen Hamiltonian in the comoving frame |$\widehat{H}$| (blue) is better preserved than that in the static frame |$\bar{H}$| (black).