 The next work he produced was in 1764 on the libration of the Moon, and an explanation as to why the same face was always turned to the earth, a problem which he treated by the aid of virtual work.His solution is especially interesting as containing the germ of the idea of generalised equations of motion, equations which he first formally proved in 1780.Already by 1756, Euler and Maupertuis, seeing his mathematical talent, tried to persuade him to come to Berlin, but Lagrange shyly refused the offer.