Resonant forcing of Mercuryʹs libration in longitude

The period of free libration of Mercuryʹs longitude about the position it would have had if it were rotating uniformly at 1.5 times its orbital mean motion is close to resonance with Jupiterʹs orbital period. The Jupiter perturbations of Mercuryʹs orbit thereby lead to amplitudes of libration at the 11.86 year period that may exceed the amplitude of the 88 day forced libration determined by radar. Mercuryʹs libration in longitude may be thus dominated by only two periods of 88 days and 11.86 years, where other periods from the planetary perturbations of the orbit have much smaller amplitudes.