A Direct Method of Deriving Perturbation Equation of Motion

A direct method of deriving perturbation equation of motion is proposed. Namely, the variations in the orbital elements of an instantaneous ellipse in the sense of two-body problem under the action of three components of the perturbing force are directly derived by means of vector differentiation, with a sharp geometric image and explicit mechanical significance. The derivation of the two orbital elements, and , is especially more concise and objective than in the traditional method.