  Tidal torques can alter the spins of tidally interacting stars and planets,
usually over shorter timescales than the tidal damping of orbital separations
or eccentricities. Simple tidal models predict that, in eccentric binary or
planetary systems, rotation periods will evolve toward a "pseudosynchronous"
ratio with the orbital period. However, this prediction does not account for
"inertial" waves that are present in stars or gaseous planets with (i)
convective envelopes and (ii) even very slow rotation. We demonstrate that
tidal driving of inertial oscillations in eccentric systems generically
produces a network of stable "synchronization traps" at ratios of orbital to
rotation period that are simple to predict but can deviate significantly from
pseudosynchronization. The mechanism underlying spin synchronization trapping
is similar to tidal resonance locking, involving a balance between torques that
is maintained automatically by the scaling of inertial mode frequencies with
the rotation rate. In contrast with many resonance locking scenarios, however,
the torque balance required for synchronization trapping need not drive mode
amplitudes to nonlinearity. Synchronization traps may provide an explanation
for low-mass stars and hot Jupiters with observed rotation rates that deviate
from pseudosynchronous or synchronous expectations.
