(1) A perturbation of the orbit of a body in the Solar System, caused by the attraction of the Moon and the Sun. (2) A perturbation of the orbit of a satellite of the Earth, caused by the attractions of the Sun and Moon. The most important perturbations are secular variations in the mean anomaly, in the right ascension of the ascending node, and in the argument of perigee. All other orbital elements except the length of the semi-major axis undergo changes having a long period.