Perturbations, in astronomy, are irregularities in the simple motion of one heavenly body about another, caused either by the action of other bodies or by irregularities in the shape or density of the primary body. The subject of perturbations has been specially studied with reference to the moon. [Lunae Theory.] This investigation was commenced by Newton, who devised the method known as the variation of parameters for its treatment. A perturbed body does not move in a true ellipse or in any simple curve, but it may be considered to have an elliptical orbit whose axes are slowly changing in length and direction while its forms and positions in space are constantly being altered by the disturbing influence of other bodies. In the case of the moon the sun's attraction is the chief element of disturbance, but the planets have some effect as well, and these perturbations introduce enormous difficulties in the calculation of her path. The planets influence each other's motions, and it was by noticing the perturbations of Uranus, and finding the direction in which some disturbing body must lie, that Adams and Leverrier were led to the discovery of the planet Neptune.