The purpose of the Arm Chair Astronomy series is to make available well-written essays in popular astronomy created by expert astronomers and astrophysicists. To advance the reading and understanding, Guided Notes are provided with this essay in the series. Volume Seven of the Arm Chair Astronomy Series features The Watershed, Part Four of Arthur Koestler’s The Sleepwa The purpose of the Arm Chair Astronomy series is to make available well-written essays in popular astronomy created by expert astronomers and astrophysicists. To advance the reading and understanding, Guided Notes are provided with this essay in the series. Volume Seven of the Arm Chair Astronomy Series features The Watershed, Part Four of Arthur Koestler’s The Sleepwalkers: A History of Man’s Changing Vision of the Universe, Hutchinson of London, London UK, 1959 This portion of the book deals with the life of Johannes Kepler.