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Angular Momentum

            The face of the Moon has a universal recognition.  The reason the Moon shows one constant face towards the Earth is because one rotation about its axis is equal to the time it takes for it to revolve once about the Earth.  Both rotation and revolution are 27.3 days.  The reason the Moon has a locked rotation with the Earth is due to tidal effects.  There is a great tidal force upon the Moon from Earth’s gravitational pull.  Due to this strong force, the Moon has slowed its spin until it had the same period of rotation as one revolution around the Earth (Moons and Rings 1991, 26).     

One of the observable gravitational effects of the Moon upon the Earth are ocean tides.  The Moon’s gravity also exerts a force on the Earth’s surface and atmosphere but are not as easily detectible.  The tidal movement of water on the ocean floor causes friction.  This friction, which is most notable in the shallow seas such as the Bering Sea, has a profound effect on the period of rotation about the axis of the Earth.  The tidal friction act as brakes on the movement of the Earth and is actually slowing it down (Meissner 44).  Each day is 25E-9 seconds longer than the previous day.  Since the spinning of the Earth is slowing, the distance between the Earth and Moon must increase.  Every year the Moon is four centimeters further away from the Earth.  The deacceleration of the Earth causes the Moon to move further away because of the Law of Conservation of Angular Momentum.  Angular momentum is the product of angular velocity and mass.  According to this law, if the Earth loses angular momentum, the Moon must gain the angular momentum.  So while the Earth’s spinning slows, the Moon increases its distance from the Earth.  While the days are increasing by an infinitesimal fraction of a second per day, the long term effects are profound.  In a thousand million years, a day can be three hours longer (Hecht 264).

One of the more commonly known laws is that change is undeniable.  With the changing orbit of the Moon, it follows that it will not be in orbit forever.  Eventually the Moon will extend it’s distance from the Earth until it can go no further.  Then the tidal effects of the Sun will pull the Moon closer to the Earth.  The Roche Limit is the boundary where tidal forces disturb the stability of a planetary body.  The boundary of the Roche Limit “extends two and a half times the radius of a planet from it’s center” (Moons and Rings 1991, 27).   When the Moon falls within this limit, the stress caused by the Earth’s force of gravity will break apart the Moon and it will be no more.