An orbital perturbation is a departure from a specified reference orbit caused by additional forces or corrections. In celestial mechanics, the reference is often a two-body Kepler orbit; other bodies, nonspherical gravity, drag or relativistic terms modify it.
An orbit specified by osculating orbital elements matches the instantaneous position and velocity with a Keplerian ellipse or hyperbola, while its orbital elements evolve under the perturbation. Some changes oscillate and partly cancel; secular changes accumulate over many periods. A small force need not imply a small long-time effect, especially near a resonance.
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