Simple mathematical demonstration solves one of the problems of lunar motion, the regression of lunar nodes, observed more than 2000 years ago. Although their motions draw similar traces (such as retrograde motion in Ptolemy’s and Copernicus’s models of the universe) we show that celestial bodies do not rotate around their common centre of mass, but, by unified law, one around the other. Due to the rigidity of principle, regardless of the calculation of rounded up values, the range of discrepancy between predicted and observed cycle of regression is in level of magnitude of only 3.4x10-5 (due to lunar trajectory perturbation, its perceived values also on a small-scale vary cyclically). Besides the constant π and Terrestrial measure of time, the only variables used in the solution of this dual orbiting system problem are radiuses and surface accelerations of observed bodies.