Introduction

As the Overview for this Module indicates, the Scientific Revolution began with a new idea that was supposed to do a better job of explaining the apparent motion of the planets. That idea took hold of Nicolaus Copernicus as a young adult going to university, and became the focus of his life’s work. The book describing Copernicus’ heliocentric model, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), was published just before he died in 1543.

Copernicus’ heliocentric theory was not immediately embraced by astronomers, and nor should it have been. The Copernican model

  • provided no more accurate a description of apparent planetary motion than Ptolemy’s model had done;
  • was just as complex, having roughly as many deferents and epicycles as the Ptolemaic model (e.g., the fixed Sun was not really at the centre of the Earth’s orbit: the centre of Earth’s circular orbit was a point orbiting an epicycle on a Sun-centred deferent);
  • some of the basic ideas that motivated Copernicus’ work are not actually correct; and
  • even though heliocentrism actually is correct, at the time there were important theoretical barriers to a heliocentric theory that needed to be resolved before any such model could fit into a comprehensive physical theory.

Historically, the Copernican model was important because it set the ball rolling in the right general direction, so to speak, inspiring a later generation to continue pursuing a heliocentric hypothesis. However, the long road to general acceptance would require five important things:

  1. more precise data on the positions of the planets (Brahe),
  2. the discovery of an accurate empirical model to describe the observations (Kepler),
  3. the discovery of a number of phenomena that would disprove much of the rationale which had been thought to favour geocentrism (Galileo),
  4. new rationale demonstrating that, according to observations we commonly make in the world around us, a heliocentric orbit for the Earth is physically realistic (Galileo), and
  5. the synthesis of a comprehensive physical theory which would take into account the rationale and empirical evidence that had been developed by Galileo, and would explain Kepler’s empirical model as a formal consequence (Newton)

Aristotelian physics and the Ptolemaic model were finally abandoned only after Galileo had shaken foundational assumptions that people had long thought to be justified, showing that what people had previously thought of as true was really just a shadow on the wall that is our sky, observed by individuals who are shackled to the Earth and see the world from a single vantage point in space and time. After that, Newton threw the final blow with his universal law of gravitation, which explained terrestrial phenomena such as the tides, as well as why the Earth and all the planets should follow elliptical orbits around the Sun. Thus, Newton finally explained the accurate empirical model previously discovered by Kepler. In this module, we will explore the details of these events.