From: JASA 25 (March 1973): 21-24.
The year of 1973 has been designated Copernican Year in honor of the 500th anniversary of the birth of Copernicus M 1473. In keeping with this commemoration, the journal ASA offers o four-port publication of a paper presented by T. H. Leith at the 1972 Convention of the American Scientific Affiliation at York University.

Introduction

The Copernican revolution began in the first decade of the Sixteenth century in an unpublished manuscript, entitled the Commentary, by a rather obscure household physician in a bishop's palace in northern Europe. Some thirty years later the seeds of its heliocentric reformation of astronomy were to find a stony reception in the minds of two other reformers: Luther called its author a fool¹ and Melanchthon was prodded by it to remark that "wise governments ought to repress the impudence of the intellectuals ".² In 1543 there appeared in print Copernicus' full defence of his unsettling scheme, the Revolutions of the Heavenly Spheres.
  Seventy-three years thereafter the Congregation of the Index in Rome was to find its doctrines of the centrality and immobility of the Sun philosophically absurd and formally heretical, its thesis that the Earth exhibited a daily and annual motion incorrect in philosophy and erroneous in theology. Another seventeen years brings us to the condemnation of Galileo, guilty said the Holy Office of holding and defending these evidently false and scriptural beliefs in his Dialogues on the Two Great World Systems, which resulted in the banning of his great book and his spending the remaining years of his life in house arrest.

This century and a quarter, as sketched, suggests a rather unfavorable future for the relationship of the church, Protestant or Roman, to the new astronomy. The origins of the evident tensions and their context in the second half of the Sixteenth and the first half of the Seventeenth Centuries require some examination, for neither the Copernicans nor the churchmen of the time could see any necessity for the conflict. Each was convinced that it recognized clearly the proper means to a reconciliation. It was the failure to achieve their ambitious, the methods each recommended being so different and unpalatable to the other, which resulted in the impasse and left the horizons so clouded. So much would have been evident to anyone observing events that historic day in June of 1633 when Galileo was sentenced before the Congregation of the Holy Office in the convent of Santa Maria Sopra Minerva. With the benefit of nearly three hundred and fifty years of perspective we cannot, however, avoid assessing that appraisal. Astronomy was to move in directions as unforeseen by either churchman or Copernican as they were recalcitrant to the techniques prescribed by each camp for harmonizing their mutual concerns. Certainly new principles of accommodation have been offered over the long interval, often fostered by developments in other sciences as each has undergone its revolutionary modern changes. About these ton we shall make comment.

The Instrumentalist View of Astronomy

Our account must begin with some remarks on the traditions within which Copernican astronomy was to appear so revolutionary. One of these, the assumption that the astronomer's task was to employ whatever mathematical devices afforded a convenient description of the observed behavior of the heavens and reasonable predictions of future events, without any considerable regard for their correspondence to the actual state of affairs obtaining, was of long standing. It is apparent in the astronomy of the Seleucid period in Mesopotamia, which followed the conquests of Alexander, when the positions of the Sun, the Moon, and the planets at various occasions useful for astrological or calendar purposes were calculated using techniques which involved in essence the plotting of these bodies as points of light moving across the stars as across a graph paper. Nowhere do we find any indication that their motion in 3 dimensions was considered, any suggestion of a guiding model of their movements in space.³

Even earlier at the beginning of the Fourth Century B.C., the Greek philosopher Plato had developed a model of the universe by means of which he intended to illustrate the planning and design of the world but which he took to be no more than suggestive of how nature might have achieved whatever order observation
revealed. Beyond such convenient myth he would not go, for he took the senses to he incapable of revealing the truth about affairs on the Earth as in the skies.4 Examining his scheme and developing one of its ideas, a younger contemporary, Eudoxus, described the motions of each heavenly body as the resultant of clever combinations of eternal and uniform movements in circles centered upon the Earth.

In the next few decades, Callippus offered further refinements. Though these schemes introduce 3-dimensional geometrical models into astronomy, which is missing in Seleucid studies, the models are akin to these studies in spirit: apparently no attempt was made to argue that the set of inter-connected circular motions combining to move any given planet or the Sun or the Moon were real, nor was any explanation sought as to why each member of a set should have the particular angular velocity, sense, and axis of rotation suited to it. The models remained no more than useful fictions convenient in aiding our imagination as we predict the paths of the heavenly bodies across the stars.
Of course it was a remarkable feat to show that the complex motions of the heavens were apparently
solvable into components all of which are alike in being uniform, circular, and concentric. If the fit were inexact all that was necessary was to adjust the relative tilts of the imaginary axes of rotation, to revise the rates of rotation properly, or perhaps to add a further circular motion to the set, The ideal of explaining all heavenly motions as the result of uniform circular motions around the Earth could be maintained to indefinite precision. However, the technique was intended only to predict properly the paths, speeds, and directions of the heavenly bodies against the starry background. It could not predict the changing size of the Moon as it appears to a careful observer nor the great variations in brightness so noticeable on observing the planets. All require a scheme providing for changing the distances of these bodies from the Earth.

That need was fulfilled in the epicychc astronomy introduced by Heracleides of Pontus in the time of Callippus and developed further by Apollonius in Alexandria during the second half of the third Century B.C., by Hipparchus at Rhodes a century later, and by Ptolemy again at Alexandria, in the Second Century AD. Like the geocentric models, the movements of the heavenly bodies were interpreted as the product of combinations of uniform and endless circular motions except that here each body would be taken to move on a circle whose center itself moved at a different angular rate on another circle of different size, the center of which in turn might itself be circling the Earth. The Earth was now central only to the imagined shell of the stars⁵.

As with the geocentric schemes, epicyclic devices were capable of indefinite refinement to fit improved observational data and, as with those schemes, epicyclic astronomy saw itself as inventing its devices merely as convenient predictive fictions. Ptolemy took this to be a necessary evil, informing generations of his followers that the complexities of observed heavenly motion seem to defy the ability of philosophers of nature to fathom them. Their causes and their true nature, therefore, remain matters of speculation and controversy. His assessment of the limits of astronomy was still widely accepted by workers in the field in the days of Copernicus fourteen centuries later and for many years thereafter. It provided a major challenge to Copernican astronomers from their own colleagues.

Aristotelian Cosmology

Difficulties for the heliocentric scheme arose also from a source of equal antiquity, the philosophy of Aristotle. His great mind had, in the mid-Fourth Century B.C., introduced a magnificent and systematic natural philosophy which still attracted many as late as the Seventeenth Century of our era. We cannot attempt to outline it here but various aspects require some mention.

In the heavens, Aristotle employed the devices of Eudoxus and Callippus to interpret motions of the stars, the Sun, the Moon, and the planets with one major revision: these motions were now taken to arise from the combined effect of physically-real but invisible shells moving endlessly at their own angular rates and directions about the Earth. Connected ultimately to the starry sphere revolving rapidly around us once each day, the shells carrying these bodies therefore exhibited both the effects of that daily revolution and the influence of the four or five shells which gave to each its particular drifting path across the stars. From the Moon outward, our cosmos became a vast mechanism eternally carrying the heavenly bodies in their cycles about the Earth.
All of this Aristotle saw as quite rational. Only a spherical world could revolve in its own space and exhibit simple symmetry: the former was necessitated by what he took to be the impossibility of a void and the latter by the requirement that the influence of stellar motion must bear equally upon the bodies within the starry sphere in all directions. Only a finite universe could revolve within the finite period of 24 hours, infinite speeds being impossible. Only a mechanism involving a nest of transparent shells could move the heavenly bodies, give them their cyclic sidereal periods and explain how these periods increased as the bodies lay at greater distances from the starry shell, itself moved by the Unmoved Mover, and the source of motion elsewhere in the heavens.

The centrality of the Earth seemed equally necessary. Beneath the Moon, the natural motions of the elements were quite different from the eternal movements of the heavenly and transparent ether. They were linear and had a beginning and end: earth and water naturally move toward the center of the cosmos, and air and fire naturally rise toward the shell carrying the Moon. The natural place of earth is as close to the center as possible and above it should lie the successive shells of water, air, and fire. Observation appeared to confirm this except that various forces prevent the separation being perfect: the Earth he knew to be spherical and it did seem to be equidistant from the stars in all directions, water and air do lie successively above it, and the presence of fire was indicated when extraneous matter entered its realm and burned as in the ease of the aurorae, meteors, and comets.

To Aristotle the motion of the Earth seemed quite irrational. Were it to be translated through the sub-lunar region some force in contact with it would be necessary and he could find no basis for accepting its presence. Were it to rotate, an equally gratuitous force would be required for rotational motion is not natural to it, and Aristotle's physics of motion required that all movement in any event he caused by some internal or external force. Even a falling stone is moved by its potential to become more earthlike being actualized as it passes
toward its natural place in the scheme of things. Clearly he had no concept of inertial movement, of gravity, or of angular momentum.

So coherently did his arguments on these and other matters appear to fit together and to arise from premises which seemed intuitively evident to the mind that his cosmological scheme was destined to find many adherents⁶. One weakness at least is, however, apparent: Aristotle's model of the cosmos cannot explain the apparent variations in distance of the heavenly bodies from the Earth. It was revised to provide for this. In the Second Century, Theon of Smyrna suggested that these bodies he immersed eccentrically within transparent hall-bearings each rolling between concentric shells, a scheme which at once gave physical intelligibility to the epicycles which we have mentioned and carried any body to varied distances from us. A later variant moved the planets themselves along tracks between shells eccentric to the Earth. And, as late as the 1530's, Girolamo Fracastoro in a book dedicated to the same Pope as was Copernicus' Revolutions, introduced a shell of variable density between us and the Moon. This he not only utilized to explain the variable brightness of the planets and the apparent changing size of the Moon as seen in solar eclipses but to preserve the pristine simplicity of the Aristotlian heavens.

Neither the Copernicans nor the churchmen of the time could see any necessity for the conflict.
These sorts of endeavors might have been of only casual interest to most members of the small astronomical fraternity by the mid-Sixteenth Century, intent upon their predictive Ptolemaic devices rather than on speculations about the physical nature of heavenly motions, but matters were rather different among certain groups of philosophers and theologians. For these it was more important that Thomas Aquinas in the Thirteenth Century had brought Aristotelian philosophy and its attendant cosmological system, by various adjustments, into seeming accord with the generally accepted tenets of the Christian faith.⁷ Aristotle's teachings therefore played their part in forming a full-orbed Christian philosophy inclusive of both science and philosophy. If the Copernicans had to provide a serious critique of Aristotelian astronomy and its physical bases they also had to meet the challenge, flung at it by those who followed Aquinas, of indicating just how the novel heliocentric scheme might be reconciled to the teachings of Scripture in the face of their own synthesis involving a very different system.

Scientific Scepticism

A third difficulty facing Copernicus and his followers deserves comment at this time. It arises in the context of the relationship of faith and reason. For Aquinas, faith had been the necessary approach to Biblical teaching, while reason provided both the necessary route to understanding those matters on which the Bible was silent and a means to sustaining the credibility of revelation. With reason came a coherent pulling together of our experiences with nature; in this Aristotle was to prove of considerable value. However, Aquinas' analysis was called into question by the debates of the

Fourteenth Century.

One of the seminal minds of that period, Duns Scotus, carried the conclusion that propositions regarding the purposes and nature of God, the immortality of the soul, and similar doctrines were matters of faith to the point that faith became an act of will rather than intellect. We accept, then, revelation because it is prescribed by God and not because it is rational. In turn, this implied that the will of God was not constrained by the implication that His decisions must be reasonable. Rather, they are reasonable because they are willed in accordance with his nature.

To William of Ockham that implied further important ideas. If God's will, and nothing else, determines the character of the world among other things, it will then he impossible for us to use reason to lead ourselves hack to the nature of God or to His purposes. Reason ceases to support our faith and the goal of uniting philosophy and theology has proven to be a chimera. Further, Ockham believed that only particular things exist and only propositions about these deal with reality. The attributes which one finds in common among the objects and happenings of the world are merely one's concepts and have no claim to reality; they are abstractions and they lie wholly in the mind. The statements, then, which are found in science about these abstractions deal only with names which one has given to them and not with reality directly. The same problem faces the temporal sequence of events which are found in nature. When science discusses the causes of these sequences, which are not directly observed, it is reduced to guessing at the relationships which obtain. Many hypotheses may he offered, none of which can with certainty be said to be true. Our views of the world not only fail to sustain our faith but they are thoroughly fallible.

Ockham's teachings were prohibited in certain quarters such as the University of Paris, but at the new universities in Prague, Vienna, Heidelberg, and Cologne they were widely followed and carried influence far beyond their doors. The results were not entirely salutary for the advancement of science. Often there was a loss of interest in careful observation when it was concluded that the hypotheses to be derived were merely speculative. Again, it turned the attention of many to imaginary situations, such as motion in a vacuum was taken to be, which revealed only how God might have done things had He wished or to the sort of purely abstract studies such as those found at the Universities of Oxford and Paris in kinematics. The latter tendencies were reininforced by the Paris condemnations of 1277 of numerous theses suggesting that God could not have created a world or indeed a plurality of worlds, different from our own.

Jean Buridan, in the first half of the Fourteenth Century, lived under both the impact of these condenmations and the teachings of his contemporary Ockham. His writings reveal the speculative atmosphere occasioned by the former and the continual tentativeness demanded by the latter. For example, the question of whether the Earth or the heavens turned daily he left quite open: different theories may always he employed to explain what is observed. Likewise, around 1380, Nicole Oresme may be found arguing that science must remain incapable of deciding upon the motion of the Earth and that only Scriptural revelation can settle the matter.

No speculations of the years shortly before Copernicus are as startling though as those of Nicolas of Cusa, a scholar and church official who died within a decade of Copernicus' birth. In a work dealing with the limits of human knowledge and in a later note, Nicolas points out a variety of difficulties in conceiving the world in traditional and supposedly rational terms. There was the impossibility of understanding the conception of a finite and bounded spherical universe. Given the realization that the universe must be taken to be indeterminate in size, he asked what sense there was then in talking about the Earth as if it lay in the middle. Again, he suggested that when motion is perceived it requires that some reference be treated as if it were at rest; but no reference can be chosen, except abitrarily as tradition had done because we inhabit the Earth, as being absolutely at rest. Thus he concluded that both the heavens and the Earth were in motion in some manner which gave the appearance of a single revolution of the stars counter-clockwise about the Earth's axis of rotation, as we look north, every day.

In the end, Nicolas was to conclude that even the world-view of every thinker is determined by his place in time and space. Because none of these is privileged, he reasoned that it was completely impossible to arrive at a true picture of our world. On that thoroughly sceptical note, epitomizing the extremest form of
Ockhamism, he culminates the tentativist trend of numerous thinkers in the pre-Copernican world. It was this sort of attitude which Copernicus had to face for he was equally convinced of the truth of his ideas.⁸

(To be continued)