Modern Physics understands nature through theoretical mathematical equations. Newton's second law F = ma serves as an example of that kind of equation. Solutions to the equation are supposed to describe the possible course of events. Any phenomenon in the realm in which the theory applies should be describable by some solution to the equations of the theory. The orbits of planets are solutions to the equation F = ma. The required solutions are usually selected from the set of mathematically correct solutions by means of initial or boundary conditions.