The balance between Earth and the Moon is stable.

In the photograph below, you can see an example of stable and unstable balances. The balance of the lowest rock is stable, as it will not fall down when pushed. The balance of the highest rocks, on the other hand, is unstable. Even a small touch can make them move and fall. 



Each object has its own center of gravity. As long as the center of gravity remains on top of the supporting surface, the object remains in good balance and will not fall down. When the center of gravity moves outside the supporting surface, the object becomes unstable. Eventually, it will fall down.


A wooden object falls down from a slanted metal surface when its center of gravity (the black string) moves outside its supporting surface.