Is it true that Ptolemaic models gave better predictions than Copernican models until 50 years after the publication of
"De revolutionibus orbium coelestium" in 1543 ?
I have recently noticed that this claim is
sometimes made by
economists. The appeal is simple. The fact that decades of effort by macroeconomists have not yielded models which give better predictions than an IS(with accelerator) LM adaptive expectations augmented Phillips curve model is frustrating. If, indeed, the same was true of the Copernican revolution, we can believe that we have made great progress.
I know that the Wikipedia is not edited, but no one has objected to this claim
"Copernicus' theory was at least as accurate as Ptolemy's but never achieved the stature and recognition of Ptolemy's theory."A serious problem with the claim of fact is that the Copernican model was not improved in the 50 years which followed 1543. In fact, it wasn't improved at all.
Rather it was replaced by Kepler's model (which yields excellent predictions). Also Tycho Brahe noted that one could reconcile the Copernican model with the belief that the Earth stood still by assuming that the sun orbits the Earth and the other planets orbit the Sun. Brahe's model can not be distinguished from Copernicus's model by astronomical observations.
I think I understand the origin of the claim. Galileo obtained evidence which proved that the Ptolomaic model was not close to correct. In 1610 Galileo observed the phases of Venus. This made it possible to determine the angle between the line from the Sun to Venus and the line from Venus to the Earth. That angle corresponded to the angle predicted by Copernicus and not to the angle predicted by any Ptolomaic model.
Note 2: In planetary astronomy, predictions are made from a model of the solar system as a whole. This important because the Ptolemaic and Copernican models of the whole solar system makes different predictions about the phases of Venus, for example, even if they made the same predictions about the angular positions of the planets. Therefore, Ptolemaic and Copernican models are not predictively equivalent in any unrestricted sense. The change in relative performance was due to the invention of the telescope not due to the relatively rapid improvement of successive models of of a new type.
This does not sound like a reasonable basis for hope for contemporary macroeconomists. It would change nothing if new data were observed which don't correspond at all to the implications of our models (IS-LM included). Many such data are available already. The economist's response to such data is that models are false by definition and the interesting question is whether they help us predict the variables of interest.
Nor is it true that further research based on the set of core assertions made by Copernicus was fruitful. The idea, old in Ptolomy's time (first written down by Plato) was that eternal motion must be circular. A hard core position was that the rate of motion around the circle must be constant and the largest circle (deferent) must be centered at the Earth (the epicycles were centered on that circle). Of course Copernicus put the Sun at the center. But he was a hard line circle guy rejecting off center deferents and insisting that motion around the deferent and epicycles had to be at a constant rate.
As noted by Kepler, the Sun is not at the center of the orbits of the planets (the center of an ellipse is halfway between the foci) but rather at a focus. Also angular momentum is constant so the speed at which planets move is not. In other words, on two of three issues which divided Copernicus from Ptolomaic astronomers, we now believe that the Ptolomaic astronomers were right. On two key points of agreement, that orbits must be made of circles and that there are deferents and epicycles, we disagree with both.
I am actually interested in whether non economists (other than Van Jacobson) make the claim that Ptolomaic predictions were better than Copernican predictions for 50 years. I suspect not. The belief seems to follow from the conviction that we have achieved a scientific revolution combined with the fact that our theories have had no empirical success.
