The myth concerning circular orbits

In this posting we will examine the myth that the Copernican revolution was hampered by its insistence that the orbits be circles.

To understand the reasons behind this we need to look at the work of an influential, but often unrecognized, player in the Copernican revolution, the astronomer Tycho Brahe (1546-1601). He is considered the greatest naked-eye astronomer. He lived just before the invention of telescopes and the accuracy, scope, and reliability of his observations had enormous impact on the field.

It is interesting that Brahe, like most astronomers at that time, rejected Copernicus’ ideas of a moving Earth. He could not accept the arguments for the Earth’s motion, seeing that as creating more problems than solving them. In fact, he developed his own system (called the Tychonic system) that was mathematically equivalent to the Copernican system, but had the Earth as a stationary center. (p. 202. Most of the material in this post is from Thomas Kuhn’s book The Copernican Revolution and page numbers are from that book.)

But despite his opposition to a heliocentric system, his data provided two major benefits for the Copernican model. They got rid of some erroneous old data that plagued all the earlier models because they were so hard to fit within any mathematical framework. That helped to remove some of the anomalies that the Copernican system could not explain. And they generated new precise data that provided the kinds of puzzles that enabled Copernican convert Johannes Kepler (1571-1630) to come up with the idea that the motions of the planets were not based on circular motion (as Copernicus and Ptolemy and Brahe had all assumed) but elliptical.

In the folk history that surrounds the Copernican revolution, the introduction of elliptical orbits is rightly recognized as a crucial development that helped in its eventual acceptance, but the pre-Keplerian astronomers are wrongly criticized for insisting on circular motion. It is implied that they did this because of aesthetic considerations and because of slavish adherence to the authority of the Greek philosophers and so forth. We quoted in a previous post the introductory physics textbook Physics by Fishbane, Gasiorowicz, and Thornton which gently chided Copernicus for his seemingly rigid and dogmatic adherence to some aspects of orthodoxy. “Unfortunately, Copernicus continued to insist on describing all motions with circles and, because the true motions of the planets about the Sun are not circles, epicycles continued to be needed in the Copernican description to accommodate the observations of planetary motion.” (p. 320, emphasis added.)

This is an unfair characterization.

The reasons for assuming circular motions were quite reasonable, when seen through the eyes of the people at that time. Since there was no notion of force or gravity then, you needed to have an explanation of motion. As the motions of the stars and planets were regularly repeating, it was not unreasonable to assume they had the simplest repeating motion, which was that of an object going around in a circle. In the case of circular motion the dynamical questions could also be finessed away by saying that the object could be considered as “doing the same thing” or “moving in the same way” all the time or with no variation in its motion. This made it easier to think of this kind of motion as ‘natural’ in some way, that once it had been set in motion it would continue, and did not require any further explanation. (p. 245)

If you had more complicated motions like ellipses and the like, that would have meant that the speeds and positions of the planets were constantly changing and this required a dynamical theory of motion that simply did not exist at that time. Recall that just introducing the idea of a moving Earth created all kinds of new unsolved problems for the existing physical theories of that time. Adding non-circular motions would have compounded those problems even further and provided even greater grounds for rejection of Copernican ideas.

So it was eminently sensible of Copernicus to stick with circular motions (assuming that he had even considered alternatives) and we should refrain from imposing harsh retrospective judgments on the people involved simply because we have the benefit of hindsight. Copernicus had to work with what he had.

Kepler’s innovative idea of elliptical orbits enabled the Copernican model to dispense with the cumbersome and complicated epicycles. But in order to justify this non-circular motion and make it more worthy of acceptance, Kepler had to introduce his own theories of motion, which included an inverse-square law and an anima motrix (p. 214). These innovations led to Kepler’s laws of planetary motion, which led in turn later to Newton’s successful theories of motion and gravitation.

If all these matters had been left only to astronomers, Copernicus’ ideas would probably have achieved a quiet victory. But that was not to be. As the significance of his ideas reached beyond the community of mathematical astronomers, a religious reaction ensued, and it is this that we will examine in a future posting. The nature of this religious reaction has some surprising features that run counter to the folk history surrounding the Copernican revolution, and has interesting parallels with the current efforts to oppose evolution.

Those things will be explored in future postings.

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