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.