Equilibrium theory of tides

Summary

The equilibrium theory of tides assumes that the ocean surface adjusts at once to the tide-generating potential. Then the surface is always a level surface of the total potential, gravity plus tidal. It goes back to Newton's Principia. It predicts two tidal bulges and the right periods and relative sizes of the main tides. It does not predict the actual times and heights of tide at a coast, which depend on the dynamics in the Laplace tidal equations.