 In the first place, Galileo restricts the possible class of causes of the tides to mechanical interactions, and so rules out Kepler’s attribution of the cause to the moon. How could the moon cause the tides to ebb and flow without any connection to the seas? Such an explanation would be an invocation of magic or occult powers. Thus, for Galileo, the only conceivable (or maybe plausible) physical cause for the regular reciprocation of the tides is the combination of the diurnal and annual motions of the Earth. Briefly, as the Earth rotates around its axis, some parts of its surface are moving along with the annual revolution around the sun and some parts are moving in the contrary direction. (In the same way that a point near the top of a car’s wheel is rotating in the same direction as the car is moving, while a point near the ground is rotating toward the rear.) In the frame of the fixed stars, this creates accelerations and retardations of the Earth’s surface, and since the terrestrial waters are not attached to the surface, they slosh back and forth as their basins speed up and slow down. Hence the tides. Moreover, since the Earth’s diurnal and annual rotations are regular, so are the tidal periods. Local differences in tidal flows are due to the differences in the physical conformations of the basins in which they occur (for background and more detail, see Palmieri 1998). Albeit mistaken, Galileo’s commitment to mechanically intelligible causation makes this is a plausible argument. One can see why Galileo thinks he has some sort of proof for the motion of the Earth, and therefore for Copernicanism. 