  Epimetheus, a small moon of Saturn, has a rotational libration (an
oscillation about synchronous rotation) of 5.9 +- 1.2 degrees, placing
Epimetheus in the company of Earth's Moon and Mars' Phobos as the only natural
satellites for which forced rotational libration has been detected. The forced
libration is caused by the satellite's slightly eccentric orbit and
non-spherical shape.
  Detection of a moon's forced libration allows us to probe its interior by
comparing the measured amplitude to that predicted by a shape model assuming
constant density. A discrepancy between the two would indicate internal density
asymmetries. For Epimetheus, the uncertainties in the shape model are large
enough to account for the measured libration amplitude. For Janus, on the other
hand, although we cannot rule out synchronous rotation, a permanent offset of
several degrees between Janus' minimum moment of inertia (long axis) and the
equilibrium sub-Saturn point may indicate that Janus does have modest internal
density asymmetries.
  The rotation states of Janus and Epimetheus experience a perturbation every
four years, as the two moons "swap" orbits. The sudden change in the orbital
periods produces a free libration about synchronous rotation that is
subsequently damped by internal friction. We calculate that this free libration
is small in amplitude (<0.1 degree) and decays quickly (a few weeks, at most),
and is thus below the current limits for detection using Cassini images.
