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It's tied up with conservation of energy and the way the oceans slosh about on the surface of the earth. With each orbit a small amout of energy is transfered from the earth to the moon.
Since Apollo the distance to the moon has been observed to increase by a cm per year. The earths rotation is subsequently slowing and each day is slightly longer.
Eventually a day on earth will be the same as an orbit of the moon and the moon will only ever see one face of the earth.
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