Zenith Tidal Force

The zenith tidal force is the tide-generating force at the point on Earth directly beneath the perturbing body, the sublunar or subsolar point, where the difference between the body’s pull on that point and on Earth’s center reaches its maximum outward value along the line to the body. The tide-generating force is the gradient of the gravitational field, so it falls off as the inverse cube of distance, not the inverse square. In equilibrium theory this force raises the near-side tidal bulge, while an almost equal force on the far side raises the antipodal bulge, together giving the two daily highs of the semidiurnal tide. Its lunar value is about 2.2 times the solar value.