The Ptolemaic two-sphere model, with its highly accurate predictive capability, remained state of the art through to the 1500s. It was at this point that Christian schoalrs discovered flaws with the system. Copernicus began what we now view as a paradigm shift—termed the Copernican revolution—by proposing a heliocentric system, in which the earth and other planets revolved around the Sun in circular orbits. This novel concept was recognized as an improvement upon the Ptolemaic system, even though the predictive capability of Copernicus’ system was not superior to that of Ptolemy’s. Copernicus’ ideas gained a significant boost with the concept of elliptical, rather than circular orbits. This piece of the puzzle fell into place when Johannes Kepler (1571–1630) published On the Motion of Mars. This seminal paper demonstrated that Kepler’s mathematical model of elliptical orbits matched the extremely accurate astronomical data measured by the Danish astronomer Tycho Brahe. This major advance gradually swept away the concept of circular orbits that had marred Copernicus’ original idea. Not only did elliptical orbits match the planetary position data better, but they also provided a mathematical description of the velocity changes of the planets along their orbits.
The next step forward in the Copernican revolution was provided by Galileo Galilei (1564–1642), who used a telescope of his own design to provide supporting evidence for heliocentrism. Copernicus’ original heliocentric model now had empirical evidence to back it up. Galileo also investigated inertia and acceleration. The concept of inertia described the tendency of an object to either remain at rest or to continue at a constant speed travelling in a straight line unless an external force was applied.[1]
The advances of Copernicus, Kepler and Galileo were impressive, but they stopped short of an explanation for why the planets orbited the sun. This final piece of the puzzle—the capstone of the Copernican revolution—was to be put in place by Isaac Newton.
[1] Feynman, Leighton, & Sands, 2010, p. 9~1.