= Pericentre geometry of an exterior resonance
At pericentre, <mean longitude> equals <longitude of periapsis>, so <resonant argument> $\phi=(p+q)\lambda-p\lambda_p-q\varpi$ becomes $\phi=p(\varpi-\lambda_p)$. The pericentre angle in the planet's rotating frame is $\phi/p$ modulo $2\pi/p$. For coprime $p,q$, there are $p$ distinct pericentre directions before the trajectory closes. <Resonant-argument libration> restricts these directions and supplies <resonance protection>.
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