Although the Principia has been called 'one of the most inaccessible books ever written'7 (Newton intentionally made it difficult so that he wouldn't be pestered by mathematical 'smatterers', as he called them), it was a beacon to those who could follow it. It not only explained mathematically the orbits of heavenly bodies, but also identified the attractive force that got them moving in the first place - gravity. Suddenly every motion in the universe made sense. At