Over eons the periods of rotation (spin) and revolution (orbit) of a celestial body can become inextricably linked by gravitational forces.  Astronomers call this process "gravitational coupling" and the link a "spin-orbit resonance".

Our own Moon provides a striking example of a such a resonance.  After millions of years it has become "tidally locked" to Earth so that essentially the same half of the lunar surface always faces us!  This means that, each time it orbits Earth, the Moon rotates only once, making the "lunar day" (a single lunar rotation) a month long!  Such a "1-to-1" spin-orbit resonance is not uncommon, though fractional resonances are also possible.  For example, Mercury has a "3-to-2" resonance with the Sun.  This causes the weird Mercurian day where the Sun at times literally reverses direction in Mercury's sky!

Back here closer to home, the "gravitational coupling" that constrains the Moon similarly continues to affect Earth.  In billions of years it will slow Earth's rotation so that the same side of our home planet always faces the Moon!  It is estimated that, when this happens, both the day and the month will be about 50 current Earth-days long!

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