  Orbital motion of a body can be found from Newtonian equation of motion.
However, it is useful to express the motion through time derivatives of
Keplerian orbital elements, mainly if the motion is perturbed by small
perturbing force. The first set of equations for the time derivatives of the
orbital elements can be derived from the equation of motion using Lagrange
brackets. The second one by using equation of motion and perturbation
acceleration decomposed to radial, transversal and normal components. This
paper shows that the second type of the perturbation equations can be derived
from the first type using simple mathematical operations.
