because a gravitational field weakens with distance, the moon's gravity exerts a slightly stronger pulling force on the side of the earth facing the moon, and a slightly weaker force on the opposite side. the moon thus tends to "stretch" the earth slightly along the line connecting the two bodies. the solid earth deforms a bit, but ocean water, being fluid, is free to move much more in response to the tidal force, particularly horizontally. as the earth rotates, the magnitude and direction of the tidal force at any particular point on the earth's surface change constantly; although the ocean never reaches equilibrium—there is never time for the fluid to "catch up" to the state it would eventually reach if the tidal force were constant—the changing tidal force nonetheless causes rhythmic changes in sea surface height.