What causes tides? Why is their periodicity close to 12 hours?
The Moon’s –and to a lesser extent, the Sun’s– gravitational attraction at any spot on Earth depend on the distance between that spot and the Moon –or Sun, as the case may be. At any moment in time, the attraction on the surface of the Earth closest to the Moon is larger than that over the center of the Earth, and that on the surface of the Earth farthest farthest from the Moon is smaller. To a first approximation, the attraction on the solid continents, because they’re a mostly rigid body, is the attraction on the center of mass of the Earth, which is close to the center of the Earth. The consequent difference in the attraction on the seas and the land are the cause of tides. At the aforementioned points in the Earth-Moon axis, the attraction only has a vertical component, pulling water away from the Earth (up). It is this symmetry that causes the periodicity of tides to be close to 12 hours rather than 24. At all other places, there are horizontal components that contribute to cur
