deviations between the actual orbit of a celestial body and an assumed reference orbit; also the forces that cause deviations between the actual and reference orbits. Perturbations, according to the first meaning, are usually calculated as quantities to be added to the coordinates of the reference orbit to obtain the precise coordinates.

Gravitational influences ("tugging" and "pulling") of one astronomical body on another. Comets are strongly perturbed by the gravitational forces of the major planets, particularly by the largest planet, Jupiter. These perturbations must be allowed for in orbit computations, and they lead to what are known as "osculating elements" (which means that the orbital element numbers change from day to day and month to month due to continued perturbations by the major planets, so that an epoch is necessarily stated to denote the particular date that the elements are valid.