A prograde spin-orbit resonance relates mean spin angular velocity to mean motion by a ratio of small positive integers, : the body makes turns in orbital periods. Synchronous rotation is ; Mercury's spin-orbit resonance is nonsynchronous. The relevant combination of spin and orbital angles must stay bounded around a preferred orientation, not merely have a near-integer frequency ratio. For an elongated body on an eccentric orbit, the companion's gravitational torque on the permanent shape can sustain the resonance while tidal dissipation permits capture. [ref].
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