When considering whether the arguments for a heliocentric model were good enough to justify belief, it’s important to view the figures from Ptolemy to Galileo as a collective trajectory. However, in terms of strict logical proof, the evidence offered for heliocentrism was compatible in principle with a suitably modified geocentric model.
Copernicus presented a mathematical heliocentric model that could reproduce planetary motion at least as well as Ptolemy’s geocentric model, but more uniformly, without the separate, disconnected systems each planet required under geocentrism. Despite criticizing Ptolemy’s model for its number of moving parts, Copernicus’ model still retained many epicycles and eccentrics — he removed only the equant, and on definitional grounds (violating uniform circular motion) rather than physical ones. Most importantly, Copernicus lacked a replacement physics: why does the earth move, why do objects fall toward the center of the earth if the earth is not at the center of the cosmos, and why do we not feel the earth’s movement?
Galileo’s observations in the Sidereus Nuncius did much for the Copernican revolution by dismantling specific Aristotelian intuitions, but they did not specifically entail heliocentrism:
- Craters on the moon undermined the Aristotelian notion that the heavens were perfect and unblemished
- The moons of Jupiter showed that the earth’s moon was not unique, making earth less special and a heliocentric arrangement more plausible
- The observation of stars invisible to the naked eye undermined the assumption that human perception gives a complete picture of the heavens These observations, however, could all be accommodated by a geocentric model — they undermined Aristotelian physics, but did not directly support heliocentrism over geocentrism.
Given that the geocentric model had been the authoritative account for over 1300 years (and popular for even longer in antiquity), the burden of proof lay on heliocentrism to offer strict logical grounds for preferring it. The Copernican-Revolution, up through Galileo, could not meet this burden — mainly due to the lack of a physical theory to replace Aristotelian physics and back up Copernicus’ mathematical models and Galileo’s observations. This was not provided until Newton’s work on gravity. Strict empirical proof was not available until the Foucault pendulum (1851).