Many astronomy books today seem to simply say, "This is how it is." We want instead to offer explanations that draw as much as possible on simple, everyday experiences. Throughout the book we have made heavy use of analogies, along with carefully designed illustrations to make those analogies more concrete. An additional aim throughout this text is to explain how astronomers have come to their understanding of our universe. Some astronomy textbooks maintain brevity by omitting most of the mathematics, but we feel that math is essential for understanding many of the methods used by astronomers. We have therefore included the essential mathematics in a number of places. -- Preface.