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1. Global picture of tidal forces and the force at a fixed point
The tidal forces describe the differential effects of gravitation of the moon (and the sun) near the earth. The tidal force exerted on a body at an arbitrary point over the earth equals the vector difference between the gravitational pull of the moon (or the sun) exerted on the body at this point, and the gravitational force that would have been experienced by this body at the centre of the earth. Details and the theoretical background can be found in the manual A Dynamical Picture of the Oceanic Tides. The simulation below shows tide-generating forces in the equatorial plane at different points over the earth's surface.
The tidal forces tend to stretch the earth along the line moon-earth (or sun-earth) and squeeze it in the perpendicular directions. This pattern of forces is coupled to the position of the moon (or the sun) with respect to the earth. The system of forces rotates as a whole as the moon (or the sun) makes its apparent daily path around the earth. Click here, or on the "Start" button of the control panel to observe the daily variation of the system of tidal forces. (Click also here to see the applet.)
At an arbitrary fixed point in the equatorial plane of the earth (or near the earth), the tidal force is represented by a rotating vector. To observe clearly this behavior, click here, or check the corresponding checkbox on the control panel. (Click also here to see the applet.) At any fixed point in the equatorial plane over the earth, the tidal force vector rotates uniformly making two full revolutions during one revolution of the moon around the earth. That is, the angular velocity of this rotation is twice the angular velocity of the moon's apparent motion (of the earth's axial rotation). The tidal force is directed vertically up twice during a day: at the moment when the moon (or the sun) occurs for this equatorial point at zenith and at nadir. The force is directed vertically down also twice a day. This happens when the moon (or the sun) occurs for this point at the horizon.
The Oceanic Tides – Section 1 (of 8)