


Newton, at the end of the 17th century, understood that the movement of the planets, as well as the movement of the moon, is due to the gravitational force and that this same force is also the cause of "weight" on Earth (the apple).
Two objects of masses M1 and M2 attract each other with a force proportional to the masses and in inverse proportion to the square of the distance between them. The Kepler equations are a consequence of this expression. When we consider more than two bodies, each object is submitted to the forces of the other bodies, and it is not possible anymore to directly find the movement of these objects. Even the 3body problem is very complicated. Thus, the theory of the movement of the moon when the Sun and the Earth are present is an entire life's work  that of CharlesEugène Delaunay, whose work is still useful now for computer simulations. 