Abstract
Newton's famous mountain projectile diagram, reproduced in countless science books, is analysed in detail - mathematically, graphically, and historically - perhaps for the first time. A study of the relationship between this diagram from Newton's On the System of the World and the written texts on projectile motion in this book and in the Principia reveals an ambiguity in Newton's presentation. We propose an explanation of the ambiguity based on an exploration of its roots in Newton's thought.