All terrestrial tides are caused by an imbalance between the centrifugal forces operating on the Earth and the changing gravitational fields of the Moon, Sun, and planets. The oceanic tide is thus a manifestation of the same phenomenon as that causing internal (‘solid’ Earth) tidal effects, particularly in the outer, liquid core of the Earth. The tides are seen at the surface mostly as resonance effects between the tidal components of similar harmonic frequencies as the Earth’s free oscillation periodicities.