Dispelling myths and highlighting history of the heliocentric model

The truth about a problem, and in particular about a scientific one, generally has many sides and is difficult to reach. So it is with the history of the Copernican model of the solar system and how it was finally adopted. Although the textbook version is inexact in some details and certainly not complete, the same could be said about several statements Mano Singham makes in his article discussing those myths (PHYSICS TODAY, December 2007, page 48).

Discussing various opinions on the Copernican doctrine, the author claims that "in 1615 Cardinal Robert Bellarmine, a prominent persecutor of Galileo, said that 'the Earth is very far from heaven and sits motionless at the center of the world.' "

Actually, Cardinal Bellarmine, a supporter of the church's official doctrine that the Sun traveled around the motionless Earth, was not a persecutor of Galileo. Before Galileo's visit to Rome in 1615, Bellarmine had known him for almost 15 years, had viewed Jupiter's moon through Galileo's telescope in 1611, and highly respected his achievements, which the cardinal could appreciate more than most, since he had studied astronomy in Florence.

It is true that in 1616 Cardinal Bellarmine, speaking as the pope's representative, admonished Galileo to abandon the opinion that the Sun is the center of the universe and instead hold it as a hypothesis without proof and contrary to the scripture. However, since whispers of heresy and blasphemy continued to smear Galileo's name, he appealed to the cardinal for redress and received a vindicating letter of endorsement.¹

Singham also does not mention that several of Galileo's discoveries supported the heliocentric model. Galileo saw that Venus has phases similar to those of the Moon, and his discovery of spots on the Sun spoiled the idea of the perfect heavenly sphere. In addition, the moons of Jupiter provided a proof that Earth is not unique in having a satellite.

As for Tycho Brahe, his "aid" to the success of the Copernican model was actually a geocentric model in which the other planets did circle around the Sun, but together they all moved around the immobile Earth in the center of the universe.

The PHYSICS TODAY article discusses in detail the contributions of Johannes Kepler to the "great debate" but not those of Galileo. Nowhere does Singham mention Galileo's monumental work Dialogue Concerning the Two Chief World Systems, Ptolemaic and Copernican. That now famous work presents solutions to several apparent problems with heliocentricity. But the Catholic Church did its best to prevent that knowledge from being spread and eventually accepted.

The culmination was Galileo's trial. The tragic outcome is well known: Galileo was forced to sign his abjuration and condemned to house arrest for the rest of his life. As the author rightly points out, the edict against Galileo was not lifted until 1992.

Coming back to the "myth" that the church held back scientific progress for a millennium, we must remember that this behavior began long before Copernicus and his successors. History records much of the retrograde attitude of the church toward scientific progress. In the 16th century, all books in the Catholic countries of Europe had to pass censorship, according to the papal bull issued in 1515. The Roman Inquisition, reorganized in 1542, supervised all printing in Italy and in 1559 promulgated its first index of prohibited books. In 1564, after the Council of Trent, the restrictions became even more severe.

There is another "myth" presumably found in the popular version of the story about the geocentric and heliocentric models. According to Singham, ancient Greeks, being more philosophers than experimentalists, supported the idea of Earth being stationary, with planets and stars rotating around it. But that statement is not entirely true. Actually, those ideas, from Eudoxus, Aristotle, and later Ptolemy, were willingly accepted by the Christian church later. But one must not forget Archimedes, in whose book Arenarius one finds the concept of the heliocentric cosmos, proposed by Aristarchus of Samos in the third century BC and by others before him.² Aristarchus's model was well known in Europe at the beginning of the High Middle Ages but was not seriously entertained until Copernicus.

References

1. 1. D. Sobel, Galileo's Daughter: A Historical Memoir of Science, Faith, and Love, Penguin Books, New York (2000), p. 81.
2. 2. L. Russo, The Forgotten Revolution: How Science Was Born in 300 BC and Why It Had to Be Reborn, S. Levy, trans., Springer, New York (2004).

Is it Mano Singham debunking the Copernican myths, or are those post-1650 "writers making this revisionist claim"?

Following an earlier reconstruction by Dennis Danielson, Singham bases his objection to the popular mythology on his claim that the Catholic Church did not object initially to the Copernican heliocentric model and that the initial resistance came instead from the physics and astronomy communities. His largely irrelevant (to his main thesis) recounting of some historical events preceding and following Copernicus's death in 1543 makes for interesting reading but omits certain facts that support the popular historical account that most of us learned.

Although the works of Tycho Brahe and Johannes Kepler did provide much of the factual basis for heliocentricity, the true death knell of the geocentric model was sounded by Galileo's observation in 1610 of the moons circling the planet Jupiter. Like Copernicus, who avoided exposing his radical ideas to any but a few fellow astronomers until they were published posthumously in his epic De Revolutionibus, Galileo had feared opposition by the church until his friend Cardinal Barberini ascended to the papacy as Urban VIII. Regrettably, Galileo's maladroitness in court politics led his enemies to denounce him to Pope Urban, who was then currying favor with Protestant princes. The pope issued an edict requiring that Galileo recant his support of the heliocentric model.

As the popular version has it, opposition to the Copernican heliocentric model, both by religious leaders and by some outspoken academics, was based on several factors. Primarily, though, it was based on opposition to any change in the accepted Aristotelian beliefs and adherence to the status quo that has bedeviled scientific innovation to the present day.

It is important to get the history right when we are teaching physics. I feel compelled to point out, though, that in attempting to correct physicists' understanding of the history, Mano Singham's article "The Copernican Myths" actually creates a new myth: namely, the idea that physicists distort history when they present the development of the heliocentric solar system.

Singham begins with a "breezy version of the Copernicus story," supposedly the version related in numerous physics textbooks, and then informs us that, "apart from the final sentence, not much" of that version is true. He only gives one reference for the "breezy" version: the introductory text by Paul Fishbane and coauthors.¹ If we check the pages Singham references, however, almost nothing of that version can be found. On page 1, Fishbane and coauthors write, "Blind reverence for authority impedes scientific progress," but they clearly have in mind the issue of scientific, not religious, authority.

Little of Singham's version of the "myth" can be discerned in the other pages of Fishbane and coauthors referred to by Singham (pages 320 and 321). Religious authority is not mentioned at all. Fishbane and his coauthors could be faulted for their apparently derogatory use of the phrase "culturally imposed belief"—couldn't we equally well say that Paul Dirac's theory of the electron was based on a culturally imposed belief in differential equations?—but their history, brief as it is, is essentially correct.

I have checked the other introductory physics texts on my shelf, and I find even less of Singham's "breezy" picture in those books. I can only conclude that Singham's version is a straw man, an invention of Singham's. The "Copernican myths," it seems, are completely mythical.

Historical context can be useful in introductory physics as a way to motivate discussion of a topic and to provide color and promote interest. Teachers and authors should certainly strive to present history correctly. But it is a disservice to textbook authors to ascribe to them errors they did not commit. And there is no point in creating new myths in the attempt to correct the old ones.

Reference

1. 1. P. Fishbane, S. Gasiorowicz, S. Thornton, Physics for Scientists and Engineers, 2nd ed., Prentice Hall, Upper Saddle River, NJ (1996).

The persistence of myths in the history of science is demonstrated by the fact that even Mano Singham's article is not free of them. He mentions Martin Luther's alleged statement against heliocentrism in 1539 as one of the prominent voices of the Protestant opposition to Copernicus's ideas. In the most frequently quoted version of that statement, Luther is alleged to have branded Copernicus as a fool who will turn the whole science of astronomy upside down. However, historian of science Andreas Kleinert from Martin Luther University in Halle, Germany, has shown that "the famous citation from Luther's table talks is next to worthless as an historical source, that Luther never referred to Copernicus or to the heliocentric world system in all his voluminous writings, and that there is no indication that Luther ever suppressed the Copernican viewpoint."¹ Luther was not responsible for the Protestant opposition to Copernicanism, nor did he lead a crusade against it. His opinion about the heliocentric system was indifferent or ignorant but not hostile.

Reference

1. 1. Kleinert, Berichte z. Wissenschaftsgesch. 26, 101 (2003).

At the end of his historical account, Mano Singham observes, "The story of the Copernican revolution shows that the actual history of science often bears little resemblance to the popular capsule versions." In the case of the Copernican revolution, that is particularly true, because the heliocentric model of the solar system, although frequently attributed to Copernicus, is actually from an ancient Greek astronomer, Aristarchus of Samos (circa 310–230 BC).¹

In view of religious criticisms of Copernicus, it is particularly interesting to note that Aristarchus was criticized by Cleanthes the Stoic, who said that Aristarchus should be charged with impiety, as Plutarch wrote, albeit many years later.² So not only did Aristarchus anticipate Copernicus, but Cleanthes the Stoic anticipated the criticism of Martin Luther's lieutenant, Philipp Melanchthon, who, as Thomas Kuhn reports, recommended that severe measures be taken to restrain the impiety of the Copernicans.³ Most students, before taking a course in astronomy or in the history of science, don't know about Aristarchus, although they have all heard about Copernicus. Even Stephen Hawking, in A Brief History of Time (Bantam Books, 1988), makes no mention of Aristarchus of Samos.

About 100 years after Aristarchus, Seleucus the Babylonian, a major astronomer of his time, proclaimed that Aristarchus's heliocentric model was not just hypothetical but true. Seleucus, not unexpectedly, also came in for criticism because he advocated the heliocentric model.

Presentations of the history of astronomy and physics should give more recognition to these pre-Copernican astronomers; otherwise, we are simply perpetuating another myth. It would be more appropriate to emphasize that Copernicus's great and enduring accomplishment was that he got the heliocentric model moving forward again after it had been held back for 1800 years. Properly highlighting Aristarchus's contribution also serves to illustrate that science can undergo retrograde motion in its development, as indicated by the enormous length of time the Ptolemaic model held sway, despite the fact that approximately 300 years before Ptolemy, the physically more insightful model of Aristarchus had been proposed.

References

1. 1. T. Heath, Aristarchus of Samos, the Ancient Copernicus, Clarendon Press, Oxford, UK (1913); reprinted by Dover, New York (1981).
2. 2. Plutarch, On the Face in the Moon's Orb, cited in ref. 1.
3. 3. T. Kuhn, The Copernican Revolution: Planetary Astronomy in the Development of Western Thought, Harvard U. Press, Cambridge, MA (1957).

I suggest an additional myth to Mano Singham's delightful account. That is the myth that the heliocentric theory was conceived by Copernicus with no precedent. In the third century BC, Greek astronomer Aristarchus of Samos postulated the theory. He had correctly calculated the size of the Moon and its distance from Earth. He also calculated the Sun's size and its distance from Earth, but his results for the Sun were far wrong because he lacked instruments to correctly obtain an angular measurement. Nevertheless, those calculations apparently led him to the idea that Earth revolves around the Sun. Aristarchus also concluded that the fixed stars were almost infinitely far away, and he thus explained the lack of parallax in our solar circumnavigation. So he essentially had the big picture.

Copernicus mentioned Aristarchus in earlier versions of his text, but he later deleted such mention.

The article on Copernican myths was interesting in baring the tendency of physicists to rewrite their histories, but it is clear there are other myths that even Mano Singham perpetuates. In the Ptolemaic system, the planets did not move uniformly in circles about Earth. The motion of a planet was in two circles: an epicycle on which the planet moves, and a main cycle on which the center of the epicycle moves. Although both were circles, neither centered on Earth. The main cycle was centered on a point displaced from Earth, depending on the planet. Furthermore, although the motion on the cycle was uniform, it was only so (equal angles in equal time) around the equant, a point at equal distance on the other side of the center of the circular orbit as the center is from Earth.

As Julian Barbour emphasized in his brilliant book The Discovery of Dynamics (Oxford University Press, 2001), these features of the main cycles are just Johannes Kepler's first two laws, to first order in the eccentricity of the ellipse. An ellipse is a circle to first order. Earth and the equant are the two foci of the ellipse, and the uniform rotation about the equant (second focus) is Kepler's second law (equal areas in equal times about the first focus) to first order. That is, the Ptolemaic system was, in many respects, closer to our modern description of the heavens than was the Copernican, which eliminated the equant and off-center circle.

Copernicus explained one great puzzle of the Ptolemaic system. The angle of the Sun around its orbit, the angle of the epicycle center around the major cycle (circular orbit) of the inner planets, and the angle of the outer planets in their epicycle were all the same at all times.

Copernicus recognized that if one scaled all the orbits appropriately, and made the Sun rather than Earth the center, then all those cycles with identical angles disappeared, leaving the planets in much simpler orbits around the Sun. That scenario also created a solar orbit for Earth around the Sun. The collapse of the number of parts of the orbits was the great advance. In achieving it, Copernicus had established a relative scale for the whole solar system.

But with that step forward, Copernicus took at least one large one backward, from our point of view. He got rid of the baggage of the offset orbit center and the equant and thereby destroyed the ellipticity of the Ptolemaic orbits. He thus had to introduce additional epicycles to explain what the Ptolemaic system explained automatically. Had he retained the equants, the Copernican system would have been simpler, with fewer epicycles than the Ptolemaic. It was 60 years before Kepler, in positing his elliptical orbits, restored and improved on the equants.

One could even argue that the centrality of Earth in the Ptolemaic system followed naturally from observation. If Earth moved, one would expect the stars, if they were bodies at different distances from Earth, to exhibit parallax. To the naked-eye accuracy of about one minute of arc, no stellar parallax is visible. Is it more sensible to postulate that the stars are at least a million times farther away than the Sun, or that Earth does not move? The latter, as emphasized by Singham, with the dynamical laws in place at the time, seems much more sensible. Even after Isaac Newton's laws of motion and gravity made it theoretically imperative that Earth moves and not the Sun, the lack of parallax of the stars and thus the lack of any evidence that Earth moved was problematic. It took 40 years after Newton's Principia, with James Bradley's accidental discovery of aberration during his failure to measure any stellar parallax, to obtain the first experimental evidence—as opposed to theoretical prejudice—that Earth actually moved.

Singham replies: If there is one thing that a study of the history of science teaches us, it is that the exact circumstances surrounding any specific scientific revolution—such as when exactly it occurred, who caused it, what factors triggered it, and how it gained acceptance—are unlikely to be answered to everyone's satisfaction. A probe through the mists of time surrounding the events enriches one's understanding of their complexity while seemingly not getting much closer to answering basic questions.

As Thomas Kuhn said,

[Historians of science] discover that additional research makes it harder, not easier, to answer questions like: When was oxygen discovered? Who first conceived of energy conservation? Increasingly, a few of them suspect that these are simply the wrong sorts of questions to ask. . . . [A] new theory, however special its range of application, is seldom or never just an increment to what is already known. Its assimilation requires the reconstruction of prior theory and the re-evaluation of prior fact, an intrinsically revolutionary process that is seldom completed by a single man and never overnight.¹

That is particularly true of the rich history of the Copernican revolution, even though the very accessibility of the competing models of heliocentrism and geocentrism, which can be understood by any layperson, fuels the feeling that getting at the truth of how this major change in our understanding of the solar system came about should be easy. But the truth here, like the truth elsewhere, is an elusive quantity.

The letter writers have raised important questions. Was Galileo's discovery of Jupiter's moons the decisive factor in the Copernican revolution gaining acceptance or just one of the many at play? Was the lack of acceptance of Aristarchus's earlier heliocentric model an irrational, dogma-driven mistake that set back science for more than a thousand years, or was it a perfectly rational decision by his contemporaries based on the evidence available to them at the time? How important was technology to this scientific discovery? What role does aesthetics play in the acceptance of a new idea? Although these questions have no simple answers, investigating them, as the letter writers have done, can be highly rewarding and enlightening.

I am grateful to the writers for providing additional information about the events surrounding the Copernican revolution. For every letter submitted, there are likely dozens more that could have provided additional insights. The intent of my short article was not to provide the definitive account of such a major event in scientific history—an impossible task—but to make readers aware of the richness beneath the superficial stories handed down from generation to generation and to encourage teachers and students to explore the historical record more thoughtfully. I hope the contributions of the letter writers act as further stimuli to such an endeavor.

Reference

1. 1. T. S. Kuhn, The Structure of Scientific Revolutions, 3rd ed., U. Chicago Press, Chicago (1996), pp. 2, 7.