Fig. 1: Schematic representation of "diurnal" variation of tides raised on Europa by Jupiter: Part (a) shows the epicyclic motion of Jupiter over one orbital period (in four steps, starting with pericenter at left) viewed in a reference frame rotating at the average orbital angular velocity. The tidal amplitude is greatest at pericenter, etc. Part (b) shows the primary tidal component, which is of constant magnitude, aligned with the mean direction of Jupiter. If Europa rotates synchronously, the primary component produces no tidal stress on the body; If Europa rotates non-synchronously, the primary component can potentially produce substantial stress. The variability of the tide over an orbit is can be described by two fourier components, traveling waves moving counterclockwise (c) and clockwise (d). Due to dissipation, these traveling components may lag slightly in phase from the nominal orientations shown here. Such a lag can be represented mathematically by addition of very small components 45 dge.behind each wave as shown by the gray bulges.