This chapter introduces the fundamental mystery of the night sky: the wandering planets and their unusual retrograde motion. The story of the Copernican Revolution is the story of how this puzzle was solved, and then solved again. There are many reasons to learn about the Copernican Revolution: it is one of the great human intellectual accomplishments, it has had a tremendous effect on our understanding of the universe and the place of humanity within it, and it is a story that is often misunderstood. Furthermore, the Copernican Revolution is an excellent example of how science progresses. Science is a bit like puzzle solving, like making and using a map, like cooking, or even like doing art. New scientific theories provide us with new ways of perceiving the world that may be radically different from our previous perceptions. Learning the scientific story of the Copernican Revolution will help readers to better understand the nature of science.