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8. Tidal friction and the phase delay
In the absence of friction the phase shift between the variation of the driving forces and the steady-state forced oscillations is zero below resonance and equals half-period over resonance. In the first case the tidal bulges are oriented along the earth-sun (or earth-moon) line, while in the second -perpendicularly to this line. In both cases the net torque of the gravitational force exerted on the earth equals zero.
The tidal friction destroys the symmetric configuration of the tidal bulges, making them deviate from the line earth-moon or from the perpendicular line. Due to this deviation a retarding torque appears, giving rise to a gradual slowing down of the earth's axial rotation. Simultaneously the moon's orbit gradually expands. Such a heavenly evolution eventually synchronizes the axial rotation with the orbital motion of both celestial bodies coupled by mutual gravitation.
Theoretical background and further details of the phenomenon can be found in the manual A Dynamical Picture of the Oceanic Tides available in the PDF format.
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The Oceanic Tides – Section 8 (of 8)