button02.gif (624 bytes)Side Effects -- Special Phenomenon caused by Tidal Forces

s4.gif (481 bytes) Rotation of the Earth is Slowing Down?

The tidal forces that apply upon the Earth not only result the tides, but also affect the Earth's rotation.  It seems that, the gravitational forces from the Moon causes the distortion on the Earth's ocean ( the bulge ).  However, the bulge not only contains a mass of ocean, but also a little bit of ground.  As the Earth spin under the tidal bulge, a significant amount of fiction is generated since the ocean (bulge) and the Earth's body impede each other.  Consequently, the existence of this frictional force slow down the individual revolution of the Earth.


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s4.gif (481 bytes) Orbit of the Moon is Enlarging

Since the bulges contains a lot of mass and the friction with the ocean beds drags the bulges easward out of the direct Earth-Moon line, the gravity pulls the Moon forward in its orbit (illustrated in the above diagram).  This speeding enlarges the orbit of the Moon.

s4.gif (481 bytes) Tidal Coupling in the Earth-Moon System by Angular Momentum

The consequence of the enlarge in Moon's orbit because of slowing down of the Earth's rotation actually is an example of conservation of angular momentum.   Angular momentum is the amount of spin motion an object has.

L = mvr

In general, the tidal coupling of the orbital and rotational motion occur at the same time.  The period of rotation for two objects and the orbital period would become the same because of this tidal coupling.

Refer to our Earth-Moon System, the Earth is slowly decreasing its rotational period and eventually the Earth and the Moon ill have exactly the same rotational period, which is equal to the orbital period.  Simultaneously, the separation between the Earth and the Moon will slowly increase to conserve the angular momentum for the whole system.