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.