Rather than solving Newton's equations for gravity (with initial conditions determined by the masses of the bodies, their position and velocity) he simply calcu

Rather than solving Newton's equations for gravity (with initial conditions determined by the masses of the bodies, their position and velocity) he simply calculated the acceleration and a point in time .. advances time by a small bit, changes velocity (based on acceleration) and location (based on the velocity) by a small bit and then repeats (computersare good at doing the same thing iteratively). This is using a computer to numerically estimate differential calculus.