Newton’s Law of Universal Gravitation shows that the objects in the solar system move according to a mathematically predictable set of rules. It shows scientifically why Kepler’s three laws of orbital motion are true, and it allows astronomers to predict the locations and motions of celestial objects. When Edmund Halley, for example, used the law to predict the seventy-six-year orbital period of a well-known comet—a prediction confirmed after Halley’s death—it marked a milestone in astronomy: the final transformation from superstition and ignorance to science and knowledge.