 Resonant argument

A resonant argument is a slow angular combination whose libration diagnoses resonance. For an exterior $j:(j-1)$ eccentric resonance it can be $\varphi =j\lambda -(j-1){\lambda }_p-\varpi$.

A resonant argument is an integer combination of orbital angles whose slow variation can diagnose mean-motion resonance. Its libration, rather than circulation through every angle, characterizes trapping in the resonance.