The Copernican Revolution

Compiled from several websites

From http://phyun5.ucr.edu/~wudka/Physics7/Notes_www/node41.html

The 16th century finally saw what came to be a watershed in the development of Cosmology. In 1543 Nicolas Copernicus published his treatise De Revolutionibus Orbium Coelestium (The Revolution of Celestial Spheres) where a new view of the world is presented: the heliocentric model.

It is hard to underestimate the importance of this work: it challenged the age long views of the way the universe worked and the preponderance of the Earth and, by extension, of human beings. The realization that we, our planet, and indeed our solar system (and even our galaxy) are quite common in the heavens and reproduced by myriads of planetary systems provided a sobering (though unsettling) view of the universe. All the reassurances of the cosmology of the Middle Ages were gone, and a new view of the world, less secure and comfortable, came into being. Despite these ``problems'' and the many critics the model attracted, the system was soon accepted by the best minds of the time such as Galileo

Copenicus' model, a rediscovery of the one proposed by Aristarchus centuries before (see Sect. 2.4), explained the observed motions of the planets (eg. the peculiar motions of Mars; see Fig. 2.13) more simply than Ptolemy's by assuming a central sun around which all planets rotated, with the slower planets having orbits farther from the sun. Superimposed on this motion, the planets rotate around their axes. Note that Copernicus was not completely divorced from the old Aristotelian views: the planets are assumed to move in circles around the sun (Fig. 3.3).

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**Figure 3.3:** The page in Copernicus' book *De Revolutionibus Orbium Coelestium* outlining the heliocentric model.
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It must be noted that Copernicus not only put forth the heliocentric idea, but also calculated various effects that his model predicted (thus following the steps outlined in Sect. 1.2.1). The presentation of the results was made to follow Ptolemy's Almagest step by step, chapter by chapter. Copernicus' results were quite as good as Ptolemy's and his model was simpler; but its predictions were not superior (since the planets do not actually move in circles but follow another - though closely related - curve, the ellipse); in order to achieve the same accuracy as Ptolemy, Copernicus also used epicycles, but now in the motion of the planets around the Sun. The traditional criticisms to the heliocentric model he answered thusly,

To the objection that a moving Earth would experience an enormous centrifugal force which would tear it to pieces, Copernicus answered that the same would be true of, say, Mars in the Ptolemaic system, and worse for Saturn since the velocity is much larger.
To the question of how can one explain that things fall downwards without using the Aristotelian idea that all things move towards the center, Copernicus stated that that gravity is just the tendency of things to the place from which they have been separated; hence a rock on Earth falls towards the Earth, but one near the Moon would fall there. Thus he flatly contradicted one of the basic claims of Aristotle regarding motion.
To the objection that any object thrown upward would be ``left behind'' if the Earth moves, and would never fall in the same place, Copernicus argued that this will not occur as all objects in the Earth's vicinity participate in its motion and are being carried by it.

Copernicus was aware that these ideas would inevitably create conflicts with the Church, and they did. Though he informally discussed his ideas he waited until he was about to die to publish his magnum opus, of which he only printed a few hundred copies. Nonetheless this work was far from ignored and in fact was the first (and perhaps the strongest) blow to the Medieval cosmology. His caution did not save him from pointed criticisms, for example, Luther pointed out (from his Tabletalk)

There was mention of a certain new astrologer who wanted to prove that the Earth moves and not the sky, the Sun, and the Moon. This would be as if somebody were riding on a cart or in a ship and imagined that he was standing still while the Earth and the trees were moving . So it goes now. Whoever wants to be clever must agree with nothing that others esteem. He must do something of his own. This is what that fellow does who wishes to turn the whole astronomy upside down. Even in these things that are thrown into disorder I believe the Holy Scriptures, for Joshua commanded the Sun to stand still and not the Earth.

The Pope Paul III was not very critical, but his bishops and cardinals agreed with Luther and the model was condemned by the Church.

The heliocentric model was eventually universally accepted by the scientific community, but it spread quite slowly. There were several reasons for this, on the one hand there certainly was a reticence to oppose the authority of the Church and of Aristotle, but there was also the fact that the heliocentric model apparently contradicted the evidence of the senses. Nonetheless the model became better known and was even improved. For example, Copernicus' version had the fixed stars attached to an immovable sphere surrounding the Sun, but its generalizations did and assumed them to be dispersed throughout the universe (Fig. 3.4); Giordano Bruno even proposed that the universe is infinite containing many worlds like ours where intelligent beings live.

In fact it was Bruno's advocacy of the Copernican system that produced one of the strongest reactions by the Church: Bruno advocated not only the heliocentric model, but denied that objects posses a natural motion, denied the existence of a center of the universe, denying even the Sun of a privileged place in the cosmos. Bruno was executed by the Inquisition in 1600.

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**Figure 3.4:** The heliocentric model of Thomas Digges (1546-1595) who enlarged the Copernican system by asserting that the stars are not fixed in a celestial orb, but dispersed throughout the universe.
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The slow progress of the heliocentric model was also apparent among part of the scientific community of the time; in particular Tycho Brahe, the best astronomer of the late 16th century, was opposed to it. He proposed instead a ``compromise'': the earth moves around the sun, but the rest of the planets move around the Earth (Fig. 3.5). Brahe's argument against the Copernican system was roughly the following: if the Earth moves in circles around the Sun, nearby stars will appear in different positions at different times of the year. Since the stars are fixed they must be very far away but then they should be enormous and this is ``unreasonable'' (of course they only need to be enormously bright!)

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**Figure 3.5:** Brahe's model of the universe: a central Earth around which the sun moves surrounded by the other planets [From the *Compendio di un trattato del Padre Christoforo Borro Giesuita della nuova costitution del mondo secondo Tichone Brahe e gli altri astologi moderni* (Compendium of a treatise of Father Christoforo Borri, S.J. on the new model of the universe according to Tycho Brahe and the other modern astronomers) by Pietro della Valle, Risalah- i Padri Khristafarus Burris Isavi dar tufiq-i jadid dunya.
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From http://www.skyscript.co.uk/copernicus.html

The Sun-centred or heliocentric theory of the solar system is usually associated with the 16th century Polish astronomer Nicholas Copernicus (1473-1543). It is often called 'the Copernican System', just as the Earth-centred 'Ptolemaic System' is named after Ptolemy. Yet neither Ptolemy nor Copernicus invented the systems named after them. Both formulated coherent mathematical frameworks to explain ideas that were 'in the air' during their lifetimes. The concept of a Sun-centred solar system was known to the ancient Greeks. It predates Copernicus by nearly two millennia and can be traced back several centuries before Ptolemy's pronouncement that the Earth stood fixed and motionless at the centre of the universe.

Ionians and Pythagoreans

In the earliest Greek cosmological systems, the Earth was envisaged as a shield-like disc floating on water and surrounded by the mythical streams of Ocean. This is the view presented around 800 BC in Homer's iliad and Odyssey. During the 6th century BC, the earliest schools of Greek philosophy emerged. Among them was the Ionian school, inspired by Thales of Miletus (c.624-c.547 BC), who began looking for explanations of the origin and nature of the universe that did not involve the supernatural intervention of the gods of Olympus. The essential ingredients of what later became the Ptolemaic system were first formulated by the Ionian philosophers. Anaximander (c.610-c.547 BC) made the first attempt to explain planetary motion as some kind of mechanism; Anaximenes (fl. 550 BC) suggested that the planets were carried in transparent crystal spheres in their orbits around the Earth; Anaxagoras (c.500-c. 428 BC) postulated an outer sphere of the 'Prime Mover' that was analogous to mind and reason.

Other schools of thought emerged alongside the Ionian philosophers with different theories to account for the origin of the universe and to explain its workings. The most influential of these was the Pythagorean Brotherhood. Even during his own lifetime Pythagoras (c.580-490 BC) was a legendary figure. His most enthusiastic followers declared that he was the son of Apollo. Astronomers in the Pythagorean tradition moved steadily towards an understanding of the solar system. Pythagoras himself is credited with advancing the idea that the Earth is a spherical globe. His pupil Philolaus (fl.500 BC) was the first to visualise it moving through space and simultaneously rotating on its axis. Philolaus took the first step towards a heliocentric theory with his conception of a mystical 'central fire' around which all the celestial bodies, including Earth and Sun, were said to revolve.

Herakleides (c. 380-c.310 BC) developed a hybrid theory half-way between the geocentric and heliocentric systems in which the inferior planets Mercury and Venus were in orbit around the Sun while the Sun itself and the superior planets Mars, Jupiter and Saturn were in orbit around the Earth. Finally Aristarchus of Samos (c.310-c.250 BC), the last of the Pythagorean astronomers, reasoned that the motions of the celestial bodies could all be explained by assuming that the Sun rather than the Earth stood at the centre.

Aristarchus' heliocentric theory was not widely accepted. Greek astronomy of the 3rd century BC was preoccupied with developing an accurate method of predicting planetary movements but the heliocentric theory in itself offered no practical solutions. Furthermore, the concept of a moving Earth seemed to contradict the evidence of the senses as well as breaking every law of physics as then understood. Yet the idea never quite went away. A century after Aristarchus, Hipparchus (c. 190-c.127 BC), generally acknowledged as the greatest astronomical observer of the ancient world, investigated the heliocentric theory. He rejected it because the instruments and techniques at his disposal were not advanced enough to detect any scientific evidence for a moving Earth. But even when Ptolemy wrote his great compendium of astronomy the Almagest around 150 AD, he too felt obliged to present arguments 'proving' that the Earth was stationary at the centre of the universe.

1) Classical geocentric (Ptolemaic) system
2) System of Herakleides (4th century BC) and Tycho Brahe (16th century AD)
3) Heliocentric system of Aristarchus and Copernicus.

Plato and Aristotle

The persistence of the Earth-centred theory from the time of the ancient Greeks down to the 16th and 17th centuries can be attributed to the tremendous influence of Plato and Aristotle on Greek, Arabic and European philosophy.

Plato (c.427-c.347 BC) taught that behind the world of physical appearances was an archetypal world of forms and ideas. The everyday world was no more than a shadow of the Ideal world; sensory impressions were simply illusions. Consequently, Plato had little interest in the facts and figures of empirical science. His pure world of ideas could only be approached intuitively - through allegory, poetry or myth. This was the spirit in which his pronouncements on the nature of the celestial bodies were made. He declared that the Earth, which possessed a divine essence or 'world soul', was perfectly spherical in shape. It stood fixed and motionless at the centre of creation. The orbits of the Sun, Moon and planets around it were perfectly circular; they moved along their orbits at perfectly uniform speeds, never speeding up or slowing down.

The idealised Platonic universe of spheres and circles came to be regarded as axiomatic. The principle of circular motion was inviolable because the heavens were believed to be perfect and immutable. The messy processes of generation and decay, death and rebirth, prevailed only in the earthly 'sub-lunar' regions. Later generations of astronomers and mathematicians were saddled with the task of devising a theory of planetary motion that could explain such awkward anomalies as retrograde motion and the apparent variations in the brilliance of the planets, yet remain within the prescribed requirements of a fixed central Earth with circular planetary orbits and uniform speeds.

Aristotle (c.384-c.322 BC) was a disciple of Plato but later rejected his abstract idealism in favour of a more pragmatic approach, insisting that all speculation must be based upon observation, analysis and systematic research. He left a vast body of written works that provided a first foundation for several modern sciences. Classical astrology - from the Greeks to the 17th century masters - owes its theoretical foundation to the doctrines of Aristotle. In astronomy Aristotle followed Plato's geocentric concepts, but unlike Plato he attempted to explain the laws that made the planets physically revolve.

Plato's pupil Eudoxus (c.400-c.355 BC) had devised a model of planetary motion in which the planets revolved in several spheres, one nestling within another and all rotating independently. This construct explained all the variations of planetary motion by combinations of simple circular movements, thus demonstrating that Plato's conception of circular motion and uniform speeds was mathematically possible - and mathematical truth was close to the Ideal in Platonic thought. Aristotle took Eudoxus' scheme literally as a working model of the universe and set about improving it in the light of contemporary physics. He attempted to explain retrograde motion with a system of 'working' and 'neutralising' spheres. While this idea never really caught on, it led eventually to the development of the elaborate theory of planetary epicycles or wheels-within-wheels, a theory first proposed by Appolonius of Perga (fl. 200 BC), developed by Hipparchus and perfected by Ptolemy (c. 100-178 AD). The Earth-centred 'Ptolemaic' universe, founded upon the doctrines of Plato and Aristotle, remained the last word in astronomical theory for 1500 years.

An Arabic Interlude

With the collapse of the Roman Empire in 476 AD, knowledge of classical science was lost to Europe for several centuries. The Church eventually established the monastic system as a means of promoting learning but the philosophers of pre-Christian times were regarded with suspicion by the Church fathers. In early Christendom, astronomy regressed into Old Testament fundamentalism.

In 622 AD the Prophet Mohammed launched his holy war against the infidel. Within a century, the Islamic Empire extended eastwards across northern India to the borders of China, and westwards across Asia Minor, north Africa and - with the Arabic conquest of Spain and Sicily - into western Europe. Alexandria, the centre of classical learning, fell to the Arabians in 642. When the period of military expansion was over, Islamic scholars became enthusiastic students of classical philosophy. Many important manuscripts were translated from Greek into Arabic. In the world of medieval Islam, Aristotle and Ptolemy were the supreme authorities in matters of natural science and astronomy.

In 999 AD, Gerbert of Aurillac, the most accomplished mathematician, musician, astronomer and classical scholar in Europe, ascended to the papal throne as Sylvester II, known as 'the magician Pope' to his contemporaries. His papacy, at the symbolic date 1000 AD marks the turning point of the European 'dark ages'. Contact was established with Arabic centres of learning in Spain, where Muslim, Christian and Jewish scholars congregated. During the following centuries, Hebrew and Arabic versions of ancient Greek texts were translated into Latin and began to circulate in Europe. The works of Aristotle were translated from around 1200. The translation of Ptolemy's Almagest into Latin in 1175 re-vitalised European astronomy. King Alfonso X of Castile (122-184) commissioned new astronomical tables calculated according to Ptolemy's theory with Arabic mathematical refinements. Completed in 1252, the Alphonsine Tables remained the best astronomical tables available in Europe for the next three centuries. The complexities of the Ptolemaic system exasperated King Alfonso however. When the intricacies of epicycles, deferents and equants were explained to him Alfonso 'the Wise' is said to have remarked that if the Almighty had consulted him on the matter, he would have recommended something a little simpler...

The Copernican Revolution

The heliocentric theory proposed by Aristarchus also found its way into Europe through translations of Arabic texts. It was discussed in learned circles as it had been in ancient Greece, but the Ptolemaic system gave the only explanation of planetary motion that could be put to practical use. The heliocentric theory remained speculative because observing instruments were not good enough to detect any evidence for it. In an attempt to rectify this, Regiomontanus (1436-76) set up the first European observatory, which he established at Nuremburg in 1471.

Regiomontanus (Johann Muller) was born at Konigsburg in Prussia. He was a child prodigy who went to Leipzig university at the age of 11, published his first almanac at the age of 12 and at age 15 was casting horoscopes for the Hapsburg Emperor Frederick III. Regiomontanus became famous as a mathematician. He made important advances in spherical trigonometry and developed the system of astrological house division that bears his name. He also wrote a celebrated summary of the Almagest but grew dissatisfied with Ptolemy's explanation of planetary motion and impatient with his fellow astronomers' unquestioning acceptance of it. He was interested in the alternative theories of Herakleides and Aristarchus, but realised that better observational data would be needed before any advances could be made. He also recognised the limitations of contemporary astronomical instruments and with the help of a wealthy patron equipped his observatory with improved instruments of his own design. Regiomontanus' preparations for what could have become a major reform of astronomy were cut short by his untimely death at the age of 40. Yet his influence played its part in shaping the ideas of Doctor Copernicus.

Born at Torun on the border between Prussia and Poland, Copernicus studied astronomy and mathematics at the university of Cracow, then went to Italy where he studied canon law at Bologna and medicine at Padua, finally obtaining his degree at Ferrera in 1503. His principal teachers in astronomy, Brudzevsky at Cracow and Novara at Bologna, both described themselves as students of Regiomontanus. The heliocentric theory that came to be associated with the name of Copernicus was a regular topic of discussion and debate during his student years.

Copernicus was not particularly interested in observing the sky but he was devoted to Pythagorean mathematics. He believed that the harmony of the universe revealed itself through the perfect geometry of planetary orbits. A technical imperfection in the Ptolemaic scheme forced him to formulate his Sun-centred theory. Ptolemy had been a little devious in the matter of uniform planetary speeds. In his system each planet would appear to move at a constant rate (as Plato decreed it should) only if it could be seen from a hypothetical point in space called its equant. Most philosophers were content to accept this device but it irritated the perfectionist Copernicus. He concluded that the only way to 'save the phenomena' of perfect circles and uniform speeds was to place the Sun at the centre of the solar system and let the planets revolve around it, as Aristarchus had suggested. Since Copernicus assumed that the orbits of the planets are circular his scheme still needed epicycles to make it work, but the simulation was precise. For the first time, tables of planetary motion could be calculated from heliocentric principles. Furthermore, these tables proved more accurate than those based on the Ptolemaic system.

Copernicus was reluctant to commit his theory to print. Around 1512 he wrote the Commentariolus ('brief commentary') in which he outlined the new system. This was circulated in manuscript form amongst a few selected scholars. By 1530 he had completed the text of his major work De Revolutionibus ('On the Revolutions of the Heavenly Spheres') but he kept the manuscript locked away and made no plans to publish it. This was not through fear of religious persecution as is often supposed. At least during the early part of the 16th century, a climate of intellectual tolerance prevailed in Europe. Cardinal Schonberg, a close advisor to three successive popes, urged him to publish but Copernicus had no desire to draw attention to himself. He suspected that he would be ridiculed as his ideas became known outside rarefied academic circles.

It was the Protestant astronomer Georg Joachim von Lauten (15 14-74), known as Rheticus, who persuaded Copernicus to publish. Rheticus was professor of mathematics and astronomy at the university of Wittenberg, the newly-established centre of Protestant learning. Although Martin Luther and other Protestant theologians argued against the heliocentric theory Rheticus was given permission to visit Copernicus, in Catholic Frauenburg, in order to discuss it. After lengthy negotiations he obtained Copernicus' permission to publish De Revolutionibus and it finally appeared in print in May 1543. Copernicus died within a few hours of receiving the first copy.

The book's initial impact was negligible. Few people bothered to read it. Of those who did, most regarded the Copernican system as a useful calculation device rather than a serious theory of the structure of the solar system. The literally earth-shaking implications of the Copernican revolution did not begin to emerge until the work of Galileo and Kepler at the beginning of the 17th century.

From http://es.rice.edu/ES/humsoc/Galileo/Images/Port/copernicus1.gif

The first speculations about the possibility of the Sun being the center of the cosmos and the Earth being one of the planets going around it go back to the third century BCE. In his Sand-Reckoner, Archimedes (d. 212 BCE), discusses how to express very large numbers. As an example he chooses the question as to how many grains of sand there are in the cosmos. And in order to make the problem more difficult, he chooses not the geocentric cosmos generally accepted at the time, but the heliocentric cosmos proposed by Aristarchus of Samos (ca. 310-230 BCE), which would have to be many times larger because of the lack of observable stellar parallax. We know, therefore, that already in Hellenistic times thinkers were at least toying with this notion, and because of its mention in Archimedes's book Aristarchus's speculation was well-known in Europe beginning in the High Middle Ages but not seriously entertained until Copernicus.

European learning was based on the Greek sources that had been passed down, and cosmological and astronomical thought were based on Aristotle and Ptolemy. Aristotle's cosmology of a central Earth surrounded by concentric spherical shells carrying the planets and fixed stars was the basis of European thought from the 12th century CE onward. Technical astronomy, also geocentric, was based on the constructions of excentric circles and epicycles codified in Ptolemy's Almagest (2d. century CE).

In the fifteenth century, the reform of European astronomy was begun by the astronomer/humanist Georg Peurbach (1423-1461) and his student Johannes Regiomontanus (1436-1476). Their efforts (like those of their colleagues in other fields) were concentrated on ridding astronomical texts, especially Ptolemy's, from errors by going back to the original Greek texts and providing deeper insight into the thoughts of the original authors. With their new textbook and a guide to the Almagest, Peurbach and Regiomontanus raised the level of theoretical astronomy in Europe.

Several problems were facing astronomers at the beginning of the sixteenth century. First, the tables (by means of which to predict astronomical events such as eclipses and conjunctions) were deemed not to be sufficiently accurate. Second, Portuguese and Spanish expeditions to the Far East and America sailed out of sight of land for weeks on end, and only astronomical methods could help them in finding their locations on the high seas. Third, the calendar, instituted by Julius Caesar in 44 BCE was no longer accurate. The equinox, which at the time of the Council of Nicea (325 CE) had fallen on the 21st, had now slipped to the 11th. Since the date of Easter (the celebration of the defining event in Christianity) was determined with reference to the equinox, and since most of the other religious holidays through the year were counted forward or backward from Easter, the slippage of the calendar with regard to celestial events was a very serious problem. For the solution to all three problems, Europeans looked to the astronomers.

Nicholas Copernicus (1473-1543) learned the works of Peurbach and Regiomontanus in the undergraduate curriculum at the university of Cracow and then spent a decade studying in Italy. Upon his return to Poland, he spent the rest of his life as a physician, lawyer, and church administrator. During his spare time he continued his research in astronomy. The result was De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orbs"), which was published in Nuremberg in 1543, the year of his death. The book was dedicated to Pope Paul III and initially caused litle controversy. An anonymous preface (added by Andreas Osiander, the Protestant reformer of Nuremberg) stated that the theory put forward in this book was only a mathematical hypothesis: the geometrical constructions used by astronomers had traditionally had only hypothetical status; cosmological interpretations were reserved for the philosophers. Indeed, except for the first eleven chapters of Book I, De Revolutionibus was a technical mathematical work in the tradition of the Almagest.

But in the first book, Copernicus stated that the Sun was the center of the universe and that the Earth had a triple motion[1] around this center. His theory gave a simple and elegant explanation of the retrograde motions of the planets (the annual motion of the Earth necessarily projected onto the motions of the planets in geocentric astronomy) and settled the order of the planets (which had been a convention in Ptolemy's work) definitively. He argued that his system was more elegant than the traditional geocentric system. Copernicus still retained the priviledged status of circular motion and therefore had to construct his planetary orbits from circles upon and within circles, just as his predecessors had done. His tables were perhaps only marginally better than existing ones.

The reception of De Revolutionibus was mixed. The heliocentric hypothesis was rejected out of hand by virtually all, but the book was the most sophisticated astronomical treatise since the Almagest, and for this it was widely admired. Its mathematical constructions were easily transferred into geocentric ones, and many astronomers used them. In 1551 Erasmus Reinhold, no believer in the mobility of the Earth, published a new set of tables, the Prutenic Tables, based on Copernicus's parameters. These tables came to be preferred for their accuracy. Further, De revolutionibus became the central work in a network of astronomers, who dissected it in great detail. Not until a generation after its appearance, however, can we begin point to a community of practicing astronomers who accepted heliocentric cosmology. Perhaps the most remarkable early follower of Copernicus was Thomas Digges (c. 1545-c.1595), who in A Perfit Description of the Coelestiall Orbes (1576) translated a large part of Book I of De Revolutionibus into English and illustrated it with a diagram in which the Copernican arrangement of the planets is imbedded in an infinite universe of stars

The reason for this delay was that, on the face of it, the heliocentric cosmology was absurd from a common-sensical and a physical point of view. Thinkers had grown up on the Aristotelian division between the heavens and the earthly region, between perfection and corruption. In Aristotle's physics, bodies moved to their natural places. Stones fell because the natural place of heavy bodies was the center of the universe, and that was why the Earth was there. Accepting Copernicus's system meant abandoning Aristotelian physics. How would birds find their nest again after they had flown from them? Why does a stone thrown up come straight down if the Earth underneath it is rotating rapidly to the east? Since bodies can only have one sort of motion at a time, how can the Earth have several? And if the Earth is a planet, why should it be the only planet with a moon?

For astronomical purposes, astronomers always assumed that the Earth is as a point with respect to the heavens. Only in the case of the Moon could one notice a parallactic displacement (about 1°) with respect to the fixed stars during its (i.e., the Earth's) diurnal motion. In Copernican astronomy one now had to assume that the orbit of the Earth was as a point with respect to the fixed stars, and because the fixed stars did not reflect the Earth's annual motion by showing an annual parallax, the sphere of the fixed stars had to be immense. What was the purpose of such a large space between the region of Saturn and that of the fixed stars?

These and others were objections that needed answers. The Copernican system simply did not fit into the Aristotelian way of thinking. It took a century and a half for a new physics to be devised to undegird heliocentric astronomy. The works in physics and astronomy of Galileo and Johannes Kepler were crucial steps on this road.

There was another problem. A stationary Sun and moving Earth also clashed with many biblical passages. Protestants and Catholics alike often dismissed heliocentrism on these grounds. Martin Luther did so in one of his "table talks" in 1539, before De Revolutionibus had appeared. (Preliminary sketches had circulated in manuscript form.) In the long run, Protestants, who had some freedom to interpret the bible personally, accepted heliocentrism somewhat more quickly. Catholics, especially in Spain and Italy, had to be more cautious in the religious climate of the Counter Reformation *, as the case of Galileo clearly demonstrates. Christoph Clavius, the leading Jesuit mathematician from about 1570 to his death in 1612, used biblical arguments against heliocentrism in his astronomical textbook.

The situation was never simple, however. For one thing, late in the sixteenth century Tycho Brahe devised a hybrid geostatic heliocentric system in which the Moon and Sun went around the Earth but the planets went around the Sun. In this system the elegance and harmony of the Copernican system were married to the solidity of a central and stable Earth so that Aristotelian physics could be maintained. Especially after Galileo's telescopic discoveries, many astronomers switched from the traditional to the Tychonic cosmology. For another thing, by 1600 there were still very few astronomers who accepted Copernicus's cosmology. It is not clear whether the execution of Giordano Bruno, a Neoplatonist mystic who knew little about astronomy, had anything to do with his Copernican beliefs. Finally, we must not forget that Copernicus had dedicated De Revolutionibus to the Pope. During the sixteenth century the Copernican issue was not considered important by the Church and no official pronouncements were made.

Galileo's discoveries changed all that. Beginning with Sidereus Nuncius in 1610, Galileo brought the issue before a wide audience. He continued his efforts, ever more boldly, in his letters on sunspots, and in his letter to the Grand Duchess Christina (circulated in manuscript only) he actually interpreted the problematical biblical passage in the book of Joshua to conform to a heliocentric cosmology. More importantly, he argued that the Bible is written in the language of the common person who is not an expert in astronomy. Scripture, he argued, teaches us how to go to heaven, not how the heavens go. At about the same time, Paolo Antonio Foscarini, a Carmelite* theologian in Naples, published a book in which he argued that the Copernican theory did not conflict with Scripture. It was at this point that Church officials took notice of the Copernican theory and placed De Revolutionibus on the Index of Forbidden Books * until corrected.

Galileo's Dialogue Concerning the Two Chief World Systems of 1632 was a watershed in what had shaped up to be the "Great Debate." Galileo's arguments undermined the physics and cosmology of Aristotle for an increasingly receptive audience. His telescopic discoveries, although they did not prove that the Earth moved around the Sun, added greatly to his argument. In the meantime, Johannes Kepler (who had died in 1630) had introduced physical considerations into the heavens and had published his Rudolphine Tables, based on his own elliptical theory and Tycho Brahe's accurate observations, and these tables were more accurate by far than any previous ones. The tide now ran in favor of the heliocentric theory, and from the middle of the seventeenth century there were few important astronomers who were not Copernicans.

Notes
[1]A daily rotation about its center, an annual motion around the Sun, and a conical motion of its axis of rotation. This last motion was made necessary because Copernicus conceptualized the Earth's annual motion as the result of the Earth being embedded in a spherical shell centered on the Sun. Its axis of rotation therefore did not remain parallel to itself with respect to the fixed stars. To keep the axis parallel to itself, Copernicus gave the axis a conical motion with a period just about equal to the year. The very small difference from the annual period accounted for the precesion of the equinoxes, an effect caused by the fact that the Earth's axis (in Newtonian terms) precesses like a top, with a period of about 26,000 years. (Copernicus's ideas about this precession were more cumbersome and based on faulty data.)

Sources
Edward Rosen, Copernicus and the Scientific Revolution (Malabar, FL: Krieger, 1984) is a useful, if eccentric biography of Copernicus with a collection of documents concerning his life. There are two modern, reliable translations of De Revolutionibus: Edward Rosen, tr. On the Revolutions , vol. 2 of Complete works (London: Macmillan, 1972-; issued separately, Baltimore: Johns Hopkins Press, 1978); A. M. Duncan, tr., On the Revolutions of the Heavenly Spheres (London: David & Charles; New York:Barnes & Noble, 1976). The best account of the Copernican revolution is Thomas S. Kuhn, The Copernican Revolution (Cambridge: Harvard University Press, 1957). For the different receptions of De Revolutionibus, see Robert S. Westman, "Three Responses to the Copernican Theory: Johannes Praetorius, Tycho Brahe, and Michael Maestlin," in The Copernican Achievement, ed. Robert S. Westman (Berkeley and Los Angeles: University of California Press, 1975), pp. 285-345. On Galileo's Copernicanism, see Stillman Drake, "Galileo's Steps to Full Copernicanism and Back, Studies in History and Philosophy of Science, 18 (1987): 93-105; and Maurice A. Finocchiaro, "Galileo's Copernicanism and the Acceptability of Guiding Assumptions," in Scrutinizing Science: Empirical Studies of Scientific Change, ed. Arthur Donovan, Larry Laudan, and Rachel Laudan (Dordrecht Kluwer, 1988), pp. 49-67.