= Orbital perturbation
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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