tides

The movement of fluids, or stresses induced in solid objects, by a cyclical change in the net gravitational forces acting upon them. On the Earth, ocean tides are produced and modified by the daily, monthly and annual variations in the net gravitational force on the Earth exerted by the combined pull of the Sun and Moon. These variations arise from the Earth's rotation, the Moon's orbital motion around the Earth and the Earth's orbital motion around the Sun.