  Tidal dissipation of kinetic energy, when it is strong enough, tends to
synchronize the rotation of planets and moons with the mean orbital motion, or
drive it into long-term stable spin-orbit resonances. As the orbital motion
undergoes periodic acceleration due to a finite orbital eccentricity, the spin
rate oscillates around the equilibrium mean value too, giving rise to the
forced, or eccentricity-driven, librations. Both the shape and amplitude of
forced librations of synchronous viscoelastic planets and moons are defined by
a combination of two different types of perturbative torque, the tidal torque
and the triaxial torque. Consequently, forced librations can be tidally
dominated (e.g., Io and possibly Titan) or deformation-dominated (e.g., the
Moon) depending on a set of orbital, rheological, and other physical
parameters. With small eccentricities, for the former kind, the largest term in
the libration angle can be minus cosine of the mean anomaly, whereas for the
latter kind, it is minus sine of the mean anomaly. The shape and the amplitude
of tidal forced librations determine the rate of orbital evolution of
synchronous planets and moons, i.e., the rate of dissipative damping of
semimajor axis and eccentricity. The known super-Earth exoplanets can exhibit
both kinds of libration, or a mixture thereof, depending on, for example, the
effective Maxwell time of their rigid mantles. Our approach can be extended to
estimate the amplitudes of other libration harmonics, as well as the forced
libration in non-synchronous spin-orbit resonances.
