An orbital resonance occurs when orbital frequencies satisfy an approximately integer relation, allowing perturbations to accumulate coherently.
A mean-motion resonance has for small integers . Its slow resonant angle combines mean longitudes with apsidal or nodal longitudes.
In an exterior mean-motion resonance, the resonant small body orbits outside the perturbing planet and therefore has lower mean motion. A exterior resonance has .
A first-order mean-motion resonance has . For small eccentricity its leading eccentric resonant term is linear in eccentricity.
A resonant argument is a slow angular combination whose libration diagnoses resonance. For an exterior eccentric resonance it can be .
Resonant-argument libration is bounded oscillation of a resonant argument about a stable equilibrium value instead of circulation through all angles.
The libration amplitude is the maximum angular displacement of a librating resonant argument from its equilibrium value.
Resonance protection confines conjunctions to orbital phases that avoid close encounters even when the osculating orbits geometrically cross.
Resonance capture occurs when slow orbital migration carries a body into a resonance and the resonant argument begins to librate, so resonant torque can balance the migration torque.
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Orbital resonance occurs when two orbiting bodies exert regular, periodic gravitational influence on each other due to their orbital frequencies being related by a ratio of small integers. This situation can lead to significant effects on their orbits, including stabilization or destabilization, changes in orbital shape, and alterations in orbital inclination. In a simple example of orbital resonance, if one object completes two orbits in the same time that another object completes one orbit, they are said to be in a 2:1 resonance.