Spiru Haret's Theorem

The present paper, dedicated to the 140th anniversary of the birthday of the Romanian mathematician, mechanicist, and astronomer Spiru Haret (1851-1912), is an attempt to state as a theorem his famous result concerning the well-known problem on the invariability of the major axes of planetary orbits, related to the stability of the solar-planetary system. "Spiru Haret's theorem" is to be naturally added to the logical succession of theorems with respect to this problem known as "Laplace-Lagrange theorem" and "Poisson's theorem". Using his own method to explain mathematically the phenomenon of secular acceleration of Moon's "mean motion", Spiru Haret gives a criterion for separating the higher order gravitational perturbations from every nongravitational perturbation in the motion of natural or artificial celestial bodies, criterion to which the author of the present paper intends to call the attention.