Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.To understand the motion of celestial bodies, the sun, planets and the visible stars has been the main motivation for the n-body problem. The first complete mathematical formulation of this problem appeared in Newton''s Principia. Since gravity was responsible for the motion of planets and stars, Newton had to express gravitational interactions in terms of differential equations. An important fact, which Newton proved in the Principia, is that celestial bodies can be modelled as point masses.