 9. What causes the tides, and why do they vary in time as well as from place to place in the way they do? Because Newton's answer — the gravitational action of the Sun and the Moon — was merely qualitative, it left room to question whether the Moon attracts the Earth and, if so, by how strong a force. Also left open was the question of how the inertia and viscosity of the seas and the rotation of the Earth affect the tides, a question requiring a dynamic analysis of the motions of the seas in response to solar and lunar gravity.