En mecánica celeste, se dice que hay resonancia orbital cuando las órbitas de dos cuerpos tienen períodos cuya razón es una fracción de números enteros simple. Ello significa que se ejercen una influencia gravitatoria regular. El efecto de la resonancia es muy conocido en física. Supongamos una niña que se columpia con un periodo de 2 segundos. Si su padre la empuja a periodos arbitrarios no causará el mismo efecto que si la impulsa cada 2 segundos, pues entonces lo hará de manera eficaz y causando el aumento de la oscilación. A esta intensificación o amplificación de la fuerza que llega a afectar de forma notable a sus movimientos se le conoce con el nombre de resonancia. Considérese que, si el período orbital de un satélite es un múltiplo exacto o una fracción del de otro satélite, el efecto gravitatorio neto de cada satélite sobre el otro será, en resumidas cuentas, un tirón o un empujón aplicado, repetidamente, en el mismo punto del movimiento cíclico. Así se amplifica el efecto. Esto tiene un doble efecto: en algunos casos estabiliza y en otros desestabiliza las órbitas.

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In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. The physics principle behind orbital resonance is similar in concept to pushing a child on a swing, where the orbit and the swing both have a natural frequency, and the other body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e., their ability to alter or constrain each other's orbits. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Jupiter's moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune. Unstable resonances with Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large Solar System bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet. A binary resonance ratio in this article should be interpreted as the ratio of number of orbits completed in the same time interval, rather than as the ratio of orbital periods, which would be the inverse ratio. Thus the 2:3 ratio above means Pluto completes two orbits in the time it takes Neptune to complete three. In the case of resonance relationships between three or more bodies, either type of ratio may be used (in such cases the smallest whole-integer ratio sequences are not necessarily reversals of each other) and the type of ratio will be specified.

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