Perturbation, in astronomy, a disturbance in the orbit or motion of a heavenly body. In the solar system perturbations affect the planets, their satellites, and comets. Over long periods of time, perturbations may affect the size, shape, or position of the orbit of a heavenly body. Over shorter periods, perturbations may affect the position of a body in its orbit, causing it, for example, to be sometimes ahead and sometimes behind where it would otherwise be.

Gravitational attraction is a main cause of perturbations. In the solar system, for example, the primary motion of planets and comets in their elliptical orbits is due to the sun. Perturbations are due to the attraction of the various other members of the system for each other. Perturbations are also caused by non-gravitational factors. For example, the earth's atmosphere acts as a perturbing force on the orbits of artificial satellites.

The effects of perturbations can be very noticeable. Halley's Comet has been as much as 2.5 years late because of the perturbations caused by the planets. The existence of the planets Neptune and Pluto was foretold because of the perturbing effect they had on the orbit of the planet Uranus.