Much attention has been given to Newton's argument for Universal Gravitation in Book III of the "Principia". Newton brings an impressive array of phenomena, along with the three laws of motion, and his rules for reasoning to deduce Universal Gravitation. At the centre of this argument is the famous "moon test". Here it is the empirical evidence supplied by the pendulum and Huygens' results which drive Newton's argument. This paper explores Newton's argument while paying close attention to the role the pendulum plays in the argument.