The effect causing this is said to be gravity . Newton recognized that the same effect was responsible for controlling the orbital motion of the Moon about Earth, and indeed it governs the motion of all heavenly bodies including the planets, stars and galaxies. Gravity is an interaction occurring between all massive bodies. It can be observed even on a small scale, though the apparent force is very weak. Newton presented a theory of gravitation in which he described it in terms of a gravitational force between massive objects.
G is the universal gravitational constant .
The unit vector e r points radially outward from M and the minus sign implies attraction. This theory was enormously successful, describing well the orbits of the planets and even successfully predicting the existence and location of the previously unknown planet Neptune, to account for small observed perturbations in the orbit of Uranus. However, very accurate astronomical observations have detected a few shortcomings. In particular, it has been known since 1845 that even after accounting for the perturbations of all the other planets there is a discrepancy of 43'' of arc per century in the precession of the perihelion of the orbit of the innermost planet, Mercury. Furthermore, Newton's theory has the disturbing feature that the gravitational force implies that distant objects respond instantaneously to changes in position of the masses, contrary to relativistic causality.