  The galaxy pair NGC 5194/95 (M 51) is one of the closest and best known
interacting systems. Despite its notoriety, however, many of its features are
not well studied. Extending westward from NGC 5195 is a low surface brightness
tidal tail, which can only be seen in deep broadband exposures. Our previous [O
III] lambda 5007 planetary nebulae (PN) survey of M 51 recovered this tidal
tail, and presented us with a opportunity to study the kinematics of a galaxy
interaction in progress. We report the results of a spectroscopy survey of the
PN, aimed at determining their kinematic properties. We then use these data to
constrain new self-consistent numerical models of the system.
