Thursday, 13 October 2016

Copernicus was not right

During my parental leave I took the opportunity to learn more about areas that I normally don't have time to explore. One of these topics was history of science and in particular the changing world views that man has held throughout history.

Perhaps the largest shift in our view of the world happened when the geocentric world view was replaced by the heliocentric. Although some ancient philosophers argued for a heliocentric worldview (most notably Aristarchos of Samos), the general belief was that the earth was located at the centre of the universe and that the planets were carried round the earth on spheres, the outmost one holding the fixed stars. This framework was described in mathematical terms by Claudius Ptolemaeus in the 2nd century AD, in his astronomical work Almagest. Ptolemaeus constructed a mathematical model in which all planets orbited the earth on circles, and in addition each planet travelled on a smaller circle, an epicycle, along its trajectory around the earth. The Ptolemaic system could predict the future positions of the planets with good accuracy, and in addition harmonised well with the world view of Christianity. These two reasons contributed to the fact that the Ptolemaic system remained dominant for over a thousand years.

The first serious attack on it was delivered by Nicolaus Copernicus, who in the book  De revolutionibus orbium coelestium (1543) suggested a heliocentric system. The motivation for this was two-fold. Firstly, Copernicus did not like the fact that the ordering of the planets in the Ptolemaic systems was arbitrary and simply a convention (since both the distance to earth and the speed of the planet could be adjusted to fit the data there was in modern terms one free parameter in the solution), secondly he disapproved of Ptolemaeus use of an equant point in his system. The equant is the point from which the centre of the epicycle of each planet is perceived to move with a uniform angular speed.  In other words, to a hypothetical observer placed at the equant point, the center of the epicycle would appear to move at a steady angular speed. However, in order to account for the retrograde motion of planets Ptolemaeus had to place the equant point next to earth (not at the centre of the universe). This meant that although the Ptolemaic system was constructed from circular motion there was something asymmetric about it. In conclusion Copernicus critique was aesthetic in nature. It was not about having a good fit to the data, but an elegant model.

The point I want to make is that Copernicus was not driven by an urge to create a system that was more accurate at predicting planetary motion. In fact the initial heliocentric model made predictions that were on par with the Ptolemaic system. In addition Copernicus insisted that planetary orbits were circular (and he avoided the equant) and therefore he needed even more epicycles than the Ptolemaic system. Since the system was modified several times an exact number is difficult to come up with, but it is estimated that Copernicus initially used 48 epicycles.

This is in complete contrast with the folk science story that claims that the Ptolemaic system had to be amended with more and more epicycles in order to explain data on planetary motion. And along came Copernicus and fixed the problem and got rid all epicycles by proposing a heliocentric model.

No, Copernicus took a step in the right direction, but it was not until Johannes Kepler in 1609 discovered that planetary orbits are elliptical that epicycles could be discarded from the heliocentric model.

I'm not quite sure about the take home message of this post. But one thing that I've learnt is that the scientists that we most often associate with the scientific revolution (which by extension reduced the powers of the Church) were deeply devout and held metaphysical beliefs similar to those of Aristotle and Plato. For example Kepler was convinced that the radii of the planetary orbits could be explained by circumscribing Platonic solids within one another. And as we have seen above Copernicus thought that the equant point disturbed the circular symmetry and therefore suggested a model containing only circles.

So I guess my conclusion is this: Copernicus was not right, he was just less wrong. And I guess this applies to all scientists. We can never expect to be right, just less wrong than our predecessors.




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