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An orbital resonance is a stable configuration of the orbits of two bodies orbiting the same third, such that they follow a pattern, e.g., their orbital periods being related by two small integers. Kinematically, such an orbital relationship is termed commensurability, which can develop if the forces of gravity during their closest encounters tend to draw them toward the pattern, and the pattern can remain stable if gravity provides negative feedback to any move away from it. Such a resonance, with a simple integer ratio between the periods is called a mean-motion resonance (MMR). The orbital periods of Pluto and Neptune have a ratio of 3:2. The orbital periods of three of Jupiter's moons (Io, Europa, and Ganymede) form a Laplace resonance, which is an orbital resonance of three bodies (orbiting a fourth) with a ratio of 4:2:1.
A mean-motion resonance tends to increase the eccentricity of the orbits, and in some cases can eventually similarly increase their orbital inclination (between their orbital planes), in which case it is termed an inclination-type resonance (or inclination resonance). Resonances only affecting eccentricity are termed eccentricity-type resonances (or eccentricity resonances).
A secular resonance is a resonance of orbits not on each circuit, but on a longer-term (secular) pattern. It might show up as a pattern in the orbits' precessions. The term is seen in analysis of the orbits of asteroids and other minor planets.