Equilibrium Tide

The equilibrium tide is the idealized tide that would form if the ocean covered the whole Earth, had no inertia, and adjusted instantly to the tide-generating force, so the water surface always matched the equipotential of the combined Earth-Moon-Sun field. It produces two bulges, one toward the perturbing body at the sublunar point and one on the far side, giving the semidiurnal pattern of the principal lunar tide M2. Real oceans depart from it because of basin geometry, the Coriolis force, friction, and resonance, so equilibrium theory predicts the constituent frequencies correctly but not their amplitudes or phases, which harmonic analysis must measure.