One of the most difficult problems of celestial mechanics was the interpretation of irregularities of motion of Jupiter and Saturn. Kepler observed a decreasing mean motion for Saturn and an increasing one for Jupiter. Newton's first treatment of this phenomenon was fundamentally qualitative. Euler derived, from the inverse-square law of gravitation, differential equations for the perturbations. Subsequently, he introduced an analytical method known as the variation of orbital elements., Vladimír Štefl., and Obsahuje seznam literatury