Richard Feynman's fame rests to a large extent on his picaresque exploits, but he was also a theoretical physicist of some significance. The discovery of the notes for his major lecture on the motion of planets around the sun, presented in this book, allows readers an insight into the workings of his mind. The book relates how the notes came to be lost, and how they came to be found again and reconstructed. The lecture concerns the fact that, when a planet or any other body arcs through space under the influence of gravity, it traces out a specific set of mathematical curves. Feynman considers why nature chooses to trace out in the sky those - and only those - elegant geometrical constructions.