Ptolemy’s Almagest was the crowning achievement of ancient astronomy. Through careful observations and mathematical ingenuity, Ptolemy built on the work of his predecessors to develop an accurate description of the motions of all the celestial objects. Furthermore, Ptolemy’s model was fairly consistent (apart from the equant, and thus the departure from uniform circular motion) with the general physical description of the world given by Aristotle. This was important because any contradiction with known physics would have to be explained.
As we shall see in module 3, incorporating Kepler’s model of heliocentric planetary orbits into a greater physical description led to one of the most drastic revisions of our understanding of nature that has ever occurred, with a result that hardly resembled Aristotelian physics. Furthermore, although the new physics described the same phenomena, it did so more accurately. But want of greater accuracy in describing the planets’ orbits was not what initially led Copernicus to his revolutionary proposal, and Copernicus’ aim was not to eventually bring about the downfall of Aristotelian physics. In fact, his goal was the contrary: Copernicus had hoped to save Aristotelian physics from Ptolemy.
The main problems that Copernicus saw in the Ptolemaic model were: that Ptolemy had relinquished uniform circular motion when he introduced the equant; and, while the model could be used to predict the positions of celestial objects with reasonable accuracy, it failed in other respects to provide a self-consistent description of physical reality. In particular, in order to accurately describe the Moon’s orbit, Ptolemy had to make its epicycle (or eccentric) so great that the apparent angular diameter of the Moon as it orbits the Earth would have to change far more drastically than it actually does. Therefore, when Ptolemy came to describe the distance to the Moon, he had to use a different epicycle.
Such internal inconsistencies in the Ptolemaic model were not considered a problem as long as the aim was only to find an accurate empirical model, and the physical inconsistencies were mostly overlooked in favour of “saving the appearances.” The epistemic focus of astronomy had shifted: in contrast to the rational inquiry of those who followed Thales in searching for causes that would necessitate and thereby explain the phenomena, the Alexandrian astronomers had concentrated their efforts on preparing a careful record of the positions of celestial objects, and developing a mathematical description to match those observations. It would be more than a millennium before the tide would shift back, through Copernicus, to a more hypothetical mode of inquiry that sought to explain the phenomena as naturally arising, in principle, within a self-consistent physical theory. As we shall see in the next two modules, it was the right combination, in the 16th and 17th centuries, of rational thought, creativity and perseverance towards a logically consistent physical theory to account for the evidence, and meticulous data collection along with an equal amount of rigour in constructing a model to precisely describe those data, that finally led to an accurate description of planetary orbits—a description which revolutionised human understanding of the world we live in.
cosmiCave.org 