Next (or in the opinion of some, first) in importance stand the memoirs on celestial mechanics, in which he proved himself a worthy successor to Pierre-Simon Laplace. The most important of these are his memoirs "Sur les inégalités séculaires des moyens mouvements des planètes", "Sur la variation des constantes arbitraires dans les questions de mécanique", both published in the "Journal" of the École Polytechnique (1809); "Sur la libration de la lune", in "Connaissance des temps" (1821), etc.; and "Sur le mouvement de la terre autour de son centre de gravité", in "Mémoires de l'Académie" (1827), etc. In the first of these memoirs, Poisson discusses the famous question of the stability of the planetary orbits, which had already been settled by Lagrange to the first degree of approximation for the disturbing forces. Poisson showed that the result could be extended to a second approximation, and thus made an important advance in planetary theory. The memoir is remarkable inasmuch as it roused Lagrange, after an interval of inactivity, to compose in his old age one of the greatest of his memoirs, entitled "Sur la théorie des variations des éléments des planètes, et en particulier des variations des grands axes de leurs orbites". So highly did he think of Poisson's memoir that he made a copy of it with his own hand, which was found among his papers after his death. Poisson made important contributions to the theory of attraction.