Science enables us to understand and manipulate an awesome range of nature’s properties. Scientists are a lot like detectives, and the process they follow when trying to explain scientific phenomena has a good deal in common with the activities of sleuths as they try to solve mysteries. For example, an investigator might suddenly realize that his prime suspect, the person around whom his whole case has revolved, could not have committed the crime because he was actually baking a soufflé at the time it occurred. Similarly, the early natural philosophers (as investigators into natural phenomena were then called) who studied the motions of heavenly bodies made their biggest leap forward when the evidence forced them to look beyond the suspect they believed to be at the center of everything—Earth.

This chapter traces how we moved from an Earth-centered view of the universe to a Sun-centered one and how we came to understand the motion of astronomical bodies under the influence of the gravitational force. The process of this discovery initially involved the efforts of a few determined people. To unravel the mysteries that puzzled them—such as why planets appear at times to change direction on the celestial sphere or why the assumption that Earth is the center of the universe fails to predict the locations of certain bodies—they used careful observations and a willingness to question their own and others’ assumptions.

The groundwork for modern science was set down by Greek natural philosophers beginning around 2500 years ago, when Pythagoras (c. 570 b.c.e.–c. 495 b.c.e.) and his followers began using mathematics to describe natural phenomena. About 200 years later, Aristotle (384 b.c.e.–322 b.c.e.) asserted that the universe is comprehensible: It is governed by regular laws. The Greeks typically did not, however, perform experiments to test their ideas, an essential part of the scientific method used today. Nevertheless, they were among the first to leave a written record of their ideas, allowing succeeding generations to develop, criticize, and test their conclusions.

The works of these early scholars were translated into Arabic and further expanded by Persian and Islamic scholars, especially between the eighth and fifteenth centuries. Among many other things, they developed elaborate tables predicting the movements of the Sun, Moon, and planets; explored and revised the Ptolemaic model of the universe; and constructed telescopes and a variety of other astronomical devices.

Building in part upon Greek texts translated first to Arabic and then to Latin, Greek concepts of mathematics and natural philosophy were rediscovered in the West during the fifteenth century, along with related ideas shared from the East. Synthesis of these works led to the development of the scientific method of examining, explaining, testing, and predicting how things work: Science provides explanations for activities and events, and it makes predictions about things that have not yet happened or that have not yet been observed. These predictions are incredibly powerful tools that enable us to understand what we see without having to accept events on faith, or to not fear that phenomena like the force of gravity will change on a whim. Science simplifies life and takes some of the uncertainty out of the world.

Consider the topic of this chapter, our understanding of gravity. Until the seventeenth century, when Isaac Newton (1642–1727) made the conceptual leap that the force holding Earth in orbit around the Sun is the same force that holds us onto Earth, these two effects were considered to be separate and unrelated. Once Newton connected these forces and then wrote an equation to describe the behavior of Earth and of falling objects, people had, for the first time, the ability to reliably predict trajectories of projectiles and other objects moving under the influence of Earth’s gravitational attraction. In the early twentieth century, Albert Einstein (Chapter 14) spearheaded discoveries that plumbed the depths of gravitational behavior even further, leading to equations that make even more accurate predictions about the motion of objects. The fundamentals of gravity we study here lead to our understanding of its effects on the universe, including how it causes stars and planets to form (Chapter 5), enables stars to shine (Chapter 10), and holds galaxies together (Chapter 15), among many other things.