Explanation of ocean tides developed by Bernoulli in 1740.
The gravitational attraction of the Moon is largest where the Moon is exactly overhead (because it is then closest) and smallest where it is exactly at the opposite side of the Earth. At any given place on Earth the Moon's movement produces a continuous change of its gravitational effect, from largest at full moon to smallest at new moon.
The same process occurs with the gravitational attraction of the Sun. The equilibrium theory of the tides assumes that the water of the ocean reacts instantaneously to the changing magnitude of the gravitational attraction. While the assumption is incorrect (it would require that large volumes of water are moved around the Earth twice daily) its basic premise - that the tides are produced by periodic changes in the gravitational force field - is correct and a major step forward in understanding the physics of tides.