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Ocean tides are the result of gravitational interactions between the earth, moon, and sun and the waters of the earth. Earth’s gravity holds you and me and everything else on the earth’s surface in a firm grip. Although it’s a very small force, lunar gravity also acts on you and me and the earth itself. The reason we don’t “fall” straight into the moon is that there is a separate force counteracting the lunar attractive force.
In the diagram at left, the lunar force is designated FM and the offsetting force is the centrifugal force, FC, produced by the revolution of the earth about its common center of gravity with the moon. If the earth and moon were “twin planets” with equal size and mass, the common center would be a point halfway between them but because the earth is much larger and more massive than the moon, it’s actually a point lying just inside the earth.
As the image at left shows, we can regard the forces FM and FC to be in balance (FM = FC) for the earth as a whole and acting at a single point at the earth’s center. Similarly, as the earth orbits the sun it experiences solar attractive forces like those of the earth-moon system but of lesser magnitude.