Ok I have had this problem for many years I have still not come to a satisfactory conclusion. I have asked my physics teacher and others I still am confused. Imagine two particles revolving about one another. They attract one another via gravity (gravity ONLY for simplicity) and remain in equilibrium. From a third distant observer that does not affect the system, the two particles continue to revolve about a common center and do not collide with each other. This is where I get confused: Let's observe as from one of the particles, in other words, analyze the system relative to one of the particles. Now we can observe the gravitational force towards one another, but yet the distance between is constant as time goes on. What, in terms relative to one of the particles, is preventing the two from colliding? How is the system revolving about the center, without an external observer? To the system, the system is not revolving about a center. Another way to put it, what is the difference between a revolving system and a non-revolving system when viewed within the system. I made the problem simple by only considering two particles, not a binary star system for example, which is where my confusion started. This problem assumes that these particles are spherically symmetrical.