= Spin-orbit resonance
{title2=$\langle\omega\rangle/n=p/q$}
A prograde <spin-orbit resonance> relates mean spin <angular velocity> to <mean motion> by a ratio of small positive <integers>, $\langle\omega\rangle/n=p/q$: the body makes $p$ turns in $q$ <orbital periods>. <Synchronous rotation> is $1:1$; <Mercury>'s $3:2$ <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. https://arxiv.org/pdf/1209.1580[Rodríguez et al. (2012): spin-orbit coupling]{ref}.
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