Nicolaus Copernicus

Nicolaus (or Nicholas) Copernicus (February 19, 1473 - May 24, 1543) was an astronomer and mathematician who developed a heliocentric (Sun-centered) theory of the solar system. He was also a church canon, governor and administrator, a jurist, astrologer and a doctor. He is generally considered to be Polish, but of German origins, although there is some debate on the subject among ethnic nationalists (see Copernicus' nationality). His theory about the Sun as the center of the solar system, turning over the traditional geocentric theory (that wanted the Earth to be its central star), is considered one of the most important discoveries ever, and is the fundamental starting point of modern astronomy. His theory affected many other aspects of human life.

Biography

Copernicus was born in 1473 in the city of Torun (German Thorn), in Royal Prussia, a province of Poland. He was ten years of age when his father, a wealthy businessman and copper trader, died. Little is known of his mother, Barbara Watzenrode, but she appears to have predeceased her husband. His maternal uncle, Lucas Watzenrode, a church canon and later the Prince-Bishop of Warmia, raised him and his three other siblings after the death of Copernicus' father. His brother Andreas became canon in Frombork. A sister, Barbara, became a Benedictine nun and the other sister, Katharina, married a businessman and city councillor, Barthel Gertner.

In 1491 Copernicus entered University of Krakow, and here he met astronomy for the first time, thanks to his teacher Albert Brudzewski. This science soon fascinated him, as his books (now in Uppsala's library) show. After four years and a brief stay in Torun, he moved to Italy, where he studied law at the university of Bologna. His uncle financed his education and wished for him to become a bishop as well. However, while studying canon and civil law, he met his teacher Domenico Maria Novara da Ferrara, a famous astronomer. He followed his lessons and became a disciple and assistant.

The first observation Copernicus made in 1497 together with Domenico Novara, are recorded in De revolutionibus orbium caelestium.

In 1497 his uncle was ordained the bishop of Warmia (Ermland) and Copernicus was named a canon in the Frombork cathedral, but he waited in Italy for the great Jubilee of 1500, so he went to Rome, where he could observe a lunar eclipse and where he gave some lessons of astronomy or math (unfortunately nothing of this remains to us).

He would have then visited Frombork only in 1501. As soon as he reached this town, he asked and obtained permission to return to Italy to complete his studies in Padua (with Guarico and Fracastoro) and in Ferrara (the town of his teacher Novara, with Bianchini), where in 1503 received his doctoral degree in canon law. It has been supposed that it was in Padua that he gained access to those passages of Cicero and Plato about the opinion of Ancients on the movement of the Earth, having the first intuition of his theory. His collection of observations and ideas on the theory started in 1504.

Having left Italy at the end of his studies, he came to live and work in Frombork. Some time before his return to Warmia, he had received a position at the Collegiate Church of the Holy Cross in Wroclaw, Silesia, which he resigned a few years prior to his death, when he progressively became ill.

Copernicus worked for years with duke Albert of Prussia on monetary reform and published some studies about the value of money; as a governor of some parts of the Duchy, he administered and dealt out justice, taxes and a cadastrian-like activity. It was at this time that Copernicus came up with one of the earliest iterations of the theory now known as Gresham's Law. During these years he he also travelled extensively on government business and as a diplomat, on the behalf of the prince-bishop of Warmia.

In 1514 he made his "Commentariolus" available to his friends.

In 1536 his work was already in a definitive form, and some rumours about his theory had reached the scientists of all Europe. From many parts of the Continent, Copernicus received invitations to publish it, but he felt quite apprehensive of persecution for his revolutionary work by the establishment of the time. The cardinal Nicola Schonberg of Capua wrote him for a copy of his manuscript, and this made Copernicus, who saw in this a certain nervousness of the Church, even more frightened of eventual reactions.

Copernicus was still completing his work (even if he was not convinced to publish it), when in 1539 Georg Joachim Rheticus, a great mathematician at Wittenberg, directly arrived in Frombork . Philipp Melanchthon had arranged with several astronomers for Rheticus to visit and study with them. Rheticus became a disciple of Copernicus' and stayed with him for two years, in which he wrote a book, Narratio prima, in which he included the essence of the theory.

In 1542, in the name of Copernicus, Rheticus published a treatise on trigonometry (later included in the second book of De revolutionibus). Under the strong pressure from Rheticus, and having seen that the first general reception of his work had not been favorable, Copernicus finally agreed to give the book to his close friend Tiedemann Giese[?], (the bishop of Chelmno[?] in Culmer Land), to be delivered to Rheticus for printing at Nuremberg.

Legend says that the first printed copy of De revolutionibus was put in Copernicus's hands the same day of his death, so that he could say goodbye to his opus vitae. He - allegedly - awoke from his stroke induced coma, looked at his book, and died peacefully.

Copernicus was buried in the Frombork Cathedral.

The Copernican Heliocentric System

Copernicus' major theory was published in the book De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres") in the year of his death 1543, even though he had arrived at it several decades earlier.

This book marks the beginning of the shift from a geocentric (and anthropocentric[?]) universe with the Earth at its center. Copernicus held that the Earth is another planet revolving around the fixed sun once a year, and turning on its axis once a day. He arrived at the correct order of the planets and explained the precession of the equinoxes correctly by a slow change in the position of the Earth's rotational axis.

His theory, unfortunately, still had some serious defects, among them circular as opposed to elliptical orbits and epicycles, that made it no more precise in predicting ephemerides than the then current tables based on Ptolemy's model.

The system nevertheless had a large influence on scientists such as Galileo and Kepler, who adopted, championed and, in Kepler's case, improved the model. Galileo's observation of the phases of Venus produced however the first observational evidence for Copernicus' theory.

The Copernican system can be summarized in seven propositions, as Copernicus himself had collected them in a Compendium of De revolutionibus... that was found and published in 1878:

1. Orbits and celestial spheres do not have a unique center.
2. The center of the Earth is not the center of the Universe, but only the center of the mass and of lunar orbit.
3. All the planets move along orbits which center is the Sun, therefore the Sun is the center of the World.
4. The distance between the Earth and the Sun, compared with the distance between the Earth and the fixed stars, is very little.
5. The daytime movement of the Sun is only apparent, and represents the effect of a rotation that the Earth makes in the 24 hours around its axis, always parallel to itself.
6. The Earth (together with its Moon, and just like the other planets) moves around the Sun, so the movements that the Sun seems making (his apparent moving during daytime, and his annual moving through the Zodiac) are nothing else than effects of the Earth's real movements.
7. These movements of the Earth and of the other planets around the Sun, can explain the stations, and all the particular characteristics of the planets' movements.

These propositions represent the exact contrary of what the dominant geocentric propositions stated.

Earlier theories

Much has been written about earlier heliocentric theories. Philolaus (4th century BC) was one of the first to suppose a movement of the Earth, probably inspired by Pythagoras's theories on a spherical Globe.

Aristarchus of Samos developed some theories by Heraclides Ponticus (already talking about a revolution of our planet on its axis) and, adding his own studies on distances and dimensions of Sun and Earth, had a quite sufficient idea of a heliocentric system. Unfortunately, his work about his heliocentric hypothesis did not survive, so we can only speculate about what led him to his conclusions. It is notable that, according to Plutarch, a contemporary of Aristarchus accused him of impiety for "putting the Earth in motion".

Copernicus cited Aristarchus and Philolaus in an early manuscript of his book which has survived, stating: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion." For reasons unknown he crossed out this passage before publication of his book.

De Revolutionibus Orbium Coelestium

De Revolutionibus Orbium Coelestium[?] (1543), dedicated to the Pope Paul III, is divided into 6 books.

The first book contains a general vision of the heliocentric theory, and a summarized exposition of his idea on the World.

The second book is eminently theoretical and reports the principles of spherical astronomy and a list of stars (as a basis for the arguments developed in the following books).

The third book is mainly dedicated to the apparent movements of the Sun and to related phenomena.

The fourth book contains a similar description of the Moon and its orbital movements.

The fifth and the sixth book contain the concrete exposition of the new system.

The Lutheran philosopher Osiander is believed to have added an anonymous preface that the whole work was only a simple hypothesis, implying that it might only be fantastic speculation. But when, reading the work, Copernicus' belief appeared instead as a certain conviction, the book was censored and his theory fought by traditional doctrines. (Doubts have been advanced regarding this volunteer addition in order to let the theory have a wider circulation before a foreseeable reaction - see text here [1] (http://www.dartmouth.edu/~matc/readers/renaissance.astro/1.1.Revol)).

The book was put on the Index librorum prohibitorum (Index of Forbidden Books) in 1616 by the Roman Catholic Church, which prosecuted the followers of this "heretic" theory. Galileo saved himself with the famous abiura[?], Giordano Bruno was condemned to the stake in Rome.

A few years after his death, Erasmus Reinhold developed the Prutenischen Tafeln (Prussian Tables), based on Copernicus' observations. Reinhold's Prussian Tabels were used as a basis for the calendar reformation by Pope Gregory XIII. The tables were also used by sailors and sea explorers, who during fourteenth and fifteenth century used the Table of the Stars by Regiomontanus.

Copernicus and Copernicanism

Copernicus' theories have an extraordinary relevance in the history of human knowledge, and many authors suggest that only Euclidean geometry, or Charles Darwin's Evolutionism, or Newton's physics could have a similar influence on human culture in general and on science in particular. Quite obviously, Copernicus cannot be regarded only under a directly or merely scientifical, technical point of view.

Many meanings have been seen in his theory, apart from his properly scientific value. It has been said that his work represented a break in the relationships between science and religion, between dogmatism[?] and freedom of scientific investigation. His figure is often compared with Galileo.

Another figure that had to deal with the ruling culture and its dogmatic absolutism was Giordano Bruno, who studied Copernicus' work in depth. Bruno extended the meaning of Copernicus' heliocentrism to the whole universe; postulating that the universe is filled with infinitely many stars just like our Sun and surrounded by planets just like our Earth. This was a rejection of Ptolemy's cosmogony, where the universe was surrounded, closed by something, perhaps a sort of spherical envelope, that could render it a closed space (or, other suggested, a comprehensible scheme).

Of course, Copernicanism was very far from official acception in the dominant culture. And even farther from the actually ruling religious influence on science was the following conclusion that an infinitive reality rendered de facto impossible the hypothesis of an external "engine", an entity (God) that from outside could give a soul, a power and a life to the World and to Human beings. No transcendence, the most evident inspiring theme of philosophy at that time, could find an explanation in such a cosmic system, none of the most basic dogmas of Christianism (but of other religions too, the same way) could be compatible with such a revolutionary theory. The Catholic Church consequently fought this new scientific and philosophic mentality by prosecuting its followers ("Galileo's affair" was re-examined by theologists and only in 1992 Pope John Paul II stated that "science has a legitimate freedom in its own sphere"). It has to be recalled that science was submitted to religion, and that mathematicians and astronomers were considered as having neither philosophical nor theological relevance (orthodox philosophy coincided in practice with official theology) allegedly because they had no studies in theology. As a final consequence, the Church could have accepted scientific theories in these fields only after consensus by theologists. And these matters were properly studied, at the time, by natural philosophy[?]. Transcendence was central in revealed religions[?], and in Aristotle's preminent position in official doctrines. Not differently, therefore, and presumedly for the same reasons, Luther and Philipp Melanchthon too opposed a heliocentric hypothesis.

Besides, having weakened the importance of transcendence, Copernicanism opened a way to immanence and immanentism[?], which remained and developed in modern philosophy. Given that immanentism is the logical foundation of subjectivism, that finds inside the Man the principles that rule thought, history and reality, some find that Copernicanism demolished the foundations of medieval science and metaphysics, therefore giving a start to a general movement that would have brought modern thought to rebel against the objectivism and the authoritarism of traditional thought.

One of the consequences of Copernicanism (that some describe as influenced by neo-platonism) was that scientific laws must not necessarily coincide with appearance: Aristotle's system was effectively "demonstrated" by the personal experiment of anyone practically observing the movements in the sky (and one of the weakest points of geocentrism was in fact the question of "retrograde motion" of some planets - which now we know is an optical illusion). Now, a theoretical logical scheme could bring to results which did not need to be confirmed by appearance. Yet, Francis Bacon still kept on a more Aristotelian line, developing his "true induction" and his related empirism (Bacon however observed that not necessarily the planets' motion had to be in perfectly circular orbits), even if, by his famous metaphor of the bee, he proposed that a philosopher (a term that, as said, could include scientists) should keep in a wise avoidance of extreme positions of senses and reason.

Copernicus' innovation has been quite unanimously defined as a real revolution (despite the unwanted calembour). By some it was indicated as "the" revolution ([2] (http://www.anselm.edu/homepage/dbanach/timel.htm)). Immanuel Kant, for instance, caught the symbolic character of Copernicus' revolution (of which he put in evidence the trascendental rationalism) underlining that, in his vision, human rationality was the real legislator of the phenomenical reality; Copernicanism was in a winning opposition against the scientific and philosophical Aristotelism[?], a quite subjective position (in a Kantist sense) meant to fight against the ruling dogmatism. More recent philosophers too have found in Copernicus a still valid and valuable philosophical meaning, properly used to describe the position of the modern man in front of cultural traditions. A so-called Homo Copernicanus was then by some described like that modern man whose central themes are to be found in ordinary human problems, as a general cultural reference.

Discussion

It could be useful to investigate what induced Copernicus to jettison the long-established doctrine of the geocentric universe.

To explain the reasons Nicholas Copernicus might have had, in the early sixteenth century, for developing a cosmology so radically different from the prevailing Ptolemaic geocentric description of the universe, it is helpful to gain some understanding of Ptolemy's theory in this regard. An acquaintance with Ptolemaic cosmology, which had been the accepted model of the universe since the second century BC, sets the scientific context in which Copernicus worked. A more complete understanding of what motivated Copernicus' study of astronomy and mathematics also takes into account the cultural, religious, and social contexts of life in Europe, and particularly Prussia and Poland, at the time.

Copernicus was well educated. At the University of Cracow, which he attended in 1491 and 1492, Copernicus studied both mathematics and astronomy in common with all university students of that time. There is evidence that his interest in these subjects continued after he had left Cracow. Colin Russell, in chapter 2 of the book, The Rise of Scientific Europe 1500 — 1800, mentions that mathematical and astronomical texts were found in Copernicus' luggage when he moved from Cracow to study canon law at Bologna (Russell, 1999, p38). If these texts described the theory of the universe then current, they would have illustrated Ptolemy's model, which, although handling each planet's circular motion individually, was the first model of the universe to explain some of the eccentric behaviour of the planets. Ptolemaic cosmology was based on the earlier work of Aristotle and Eudoxus and maintained that all planetary motion, and the motion of the Moon, the Sun, and the stars was circular, around a stationary Earth.

In Copernicus' view, an accurate mathematical description of the universe was a technical problem that Ptolemy's explanation failed to satisfy. An accurate calculation of the astronomical year was important to a clergyman, like Copernicus, allowing him to forecast properly the various festivals that comprised the liturgical calendar. The mathematical confusion that Copernicus says caused him to develop an alternative to the geocentric model (Wallis, 1939, p.507), derived from an inadequate reconciliation of the Aristotelian model and amendments to it by Ptolemy.

Another failing of the Ptolemaic system, which Copernicus corrected in his heliocentric model, was the problem of precession. This characteristic of the Earth's movement is apparent only with observation over long periods of time. Although Copernicus' revolutionary text, de Revolutionibus orbium coelestium, contains only 27 of his own astronomical observations, Copernicus did consider empirical evidence important. Russell thinks that an observation that Copernicus had made in 1497 of the star Aldebaron, that did not coincide with predictions made by Ptolemy, might have provided further evidence of weaknesses in the Ptolemaic system — further undermining its authority for Copernicus (Russell, 1999, p.50).

The Almagest, Ptolemy's treatise on astronomy, mathematics, and geography, was written 150 years before Christ putting its authorship in the classical period. Apart from his quotation of Virgil in de Revolutionibus, discussing the characteristics of relative motion, there is evidence that Copernicus was acquainted with ideas espoused by other classical authors (Russell, 1999, p.51). Some of the ideas expressed by Philalaus[?] (5th century BC), Heraclides[?] (4th century BC), and Aristarchus (3rd century BC) discuss cosmological models that have the Earth in motion. Heraclides' description of the revolutions of Mercury and Venus around the Sun might have led Copernicus, Russell thinks, to consider that the other planets, including the Earth, did the same (Russell, 1999, p.51).

The Ptolemaic geocentric model was complicated and inconsistent in Copernicus' estimations. Copernicus' mathematical experience engendered in his thought a desire for a simpler and more elegant model of the universe, more worthy, perhaps, of its maker. The heliocentric universe of Copernicus accomplished this aim by dispensing with individual explanations for the motion of each planet to make its observed behaviour conform to observation, and replacing them with a description that applied to all the planets, including the Earth.

Elegance was a consequence of the overall simplicity of Copernicus' cosmology and much of this seeming simplicity resulted from his retention of circular orbits for the planets around the central Sun. Not that Copernicus might have looked elsewhere for a description of planetary orbits, because, according to Butterfield, Copernicus had "almost an obsession for circularity and sphericity" (Butterfield, 1957, p.31). Copernicus therefore had no choice but to use the eccentrics, epicycles, and equants of Ptolemaic cosmology, to which he added three kinds of motion he used to describe the observed behaviour of the Earth:

• Annual motion — the yearly orbit around the Sun
• Daily rotation — the motion around a tilted axis that results in day and night
• Precession — the axial wobble mentioned earlier that explains why the position of the fixed stars seems to change over long periods of time.

Until 1543, the year that Copernicus died, and the year in which his de Revolutionibus was published, and for many years afterwards, Copernicus' description of the motion of the Earth was not ratified by empirical evidence. In his unauthorized and anonymous preface to de Revolutionibus, Andreas Osiander was technically correct when he made reference to "the hypothesis of this work" (Prefaces to de Revolutionibus). Its consistency with the observed behaviour of the universe however, in a time before the telescope made more detailed observation and the gathering of more accurate measurements practicable, gave the Copernican model its strongest support. Not much more than a century later, Kepler had certainly despatched the circular orbits of the planets and replaced them with ellipses, but the Copernican heliocentric universe was still intact.

In his own preface to his work, dedicated to Pope Paul III, Copernicus took care to point out that his motives for developing a cosmology that included a moving, rather than a stationary, Earth, were inspired by his dissatisfaction with the mathematical and astronomical descriptions of the geocentric model, and were not intended to defy the written Word. "Mathematics", he says, "is written for mathematicians;" (Prefaces to de Revolutionibus). Copernicus seems to have been benefited in the bishops who were his superiors in the church - Johann Dantiscus[?] (1485 - 1548) and Tiedmann Giese (1480- 1550). Both preferred, at least initially, to promote tolerance of differing views within the church rather than open discord, and both encouraged Copernicus' publication of his scientific beliefs. However, Russell considers that previously lenient attitudes in Chelmno[?], where Copernicus carried out much of his work, had begun to change and might have contributed to Copernicus' isolation in the last years of his life (Russell, 1999, p.57). For orthodox Catholics, the Copernican model of the universe might have seemed too radically different from the geocentric model, sustained as it was by its agreement with many scriptural references. They might not have been ready to change to an understanding of the Bible as a source only of moral and spiritual, rather than scientific, wisdom.

This last idea, the separation of scientific from spiritual knowledge, first advanced by Joachim Rheticus (1514 -1576), who was instrumental in the publication of de Revolutonibus in Nuremberg in 1543, went against the efforts St. Thomas Aquinas had made, nearly three hundred years earlier, to unify reason and revelation. Russell points out, however, that in spite of his loyalty to the Catholic Church, Copernicus had a greater loyalty to what he considered to be truth — his explanation of the structure and behaviour of the universe (Russell, 1999, p, 59).

Copernicus was aware of the neo-Platonic ideas prevalent in Renaissance Europe. There is written evidence of this interest in a copy of Ficino's translation of Plato's works owned by Copernicus (Russell, 1999, p.52). In these works, as in the ideas of other classical authors, Copernicus found what he considered to be solid authority for his emphasis of the circular motion of the planets around the Sun — the centre of the universe. The fact, as far as Copernicus was concerned, that the Sun, a distinctive element in classical thought, held the central and most important position in the universe, gave added credence to his cosmology. His reverence for the sun can be seen in the most famous passage of de Revolutionibus:

"In the centre of all rests the Sun. For who would place this lamp of a very beautful temple in another or better place then this from which it can illuminate everything at the same time? As a matter of fact, not unhappily do some call it the lantern; others, the mind, and still others, the pilot of the world. Trismegistus calls it a 'visible God'; Sophocles' Electra, 'that which gazes upon all things.' And so the Sun, as if resting on a kingly throne, governs the family of stars which wheel around." (Wallis, 1939 p.526)

In this discussion of Copernicus' reasons for discarding such a long-held belief as the geocentric cosmology of Ptolemy, we can see that the Copernican revolution was simmering against a background revolution of theological thought — the Reformation. Neo-Platonic and classical ideas formed the intellectual environment in which Copernicus worked. Although not holding ordained office within the Catholic Church, Copernicus was devout and unwilling to be openly defiant of the Church's teaching, but, in common with supporters of the Reformation, Copernicus was criticising orthodox theory and belief. His reasons for doing so lay in his dissatisfaction with the inadequacies of the geocentric model, in his strong belief in the truth of the solution to the problem that he developed, its elegance and relative simplicity, and its coincidence with observation and with the classical ideals to which he had subscribed since his youth.

Sources referred to:.

RUSSELL, C. A., 1999. Copernicus and his revolution. In: D. C. GOODMAN, C. A. RUSSELL, eds. The Rise of Scientific Europe 1500-1800. Bath, UK: Hodder & Stoughton, 33-61.

WALLIS, C. G. (1939). Great Books of the World. In: D. C. GOODMAN, C. A. RUSSELL, eds. The Rise of Scientific Europe 1500-1800. Bath, UK: Hodder & Stoughton, 33-61.

BUTTERFIELD, H. (1957). The Origins of Modern Science, 1300-1800. In: D. C. GOODMAN, C. A. RUSSELL, eds. The Rise of Scientific Europe 1500-1800. Bath, UK: Hodder & Stoughton, 33-61.

Preface to COPERNICUS' de Revolutionibus orbium coelestium (1543). In: Great Books of the World, vol.16, (1952). Chicago: Encyclopaedia Britannica Inc., 505-509. In: C. A RUSSELL, ed. Science in Europe 1500 - 1800, Volume 1, A Primary Sources Anthology. Exeter, UK: Polestar Wheatons Ltd.

Christoph Hartknoch, the director of the Torunian Gymnasium in 1684, added an illustration of the 'famous Mathematicus' in his book Alt-und Neues Preussen, Von den Staedten und Schloessern. Der beruehmte Mathematicus Nicolaus Copernicus (About the Cities and Castles ).

Quotes

Goethe:

"Of all discoveries and opinions, none may have exerted a greater effect on the human spirit than the doctrine of Copernicus. The world had scarcely become known as round and complete in itself when it was asked to waive the tremendous privilege of being the center of the universe. Never, perhaps, was a greater demand made on mankind - for by this admission so many things vanished in mist and smoke! What became of our Eden, our world of innocence, piety and poetry; the testimony of the senses; the conviction of a poetic - religious faith? No wonder his contemporaries did not wish to let all this go and offered every possible resistance to a doctrine which in its converts authorized and demanded a freedom of view and greatness of thought so far unknown, indeed not even dreamed of."

Copernicus:

For I am not so enamored of my own opinions that I disregard what others may think of them. I am aware that a philosopher's ideas are not subject to the judgement of ordinary persons, because it is his endeavor to seek the truth in all things, to the extent permitted to human reason by God. Yet I hold that completely erroneous views should be shunned. Those who know that the consensus of many centuries has sanctioned the conception that the earth remains at rest in the middle of the heaven as its center would, I reflected, regard it as an insane pronouncement if I made the opposite assertion that the earth moves.

For when a ship is floating calmly along, the sailors see its motion mirrored in everything outside, while on the other hand they suppose that they are stationary, together with everything on board. In the same way, the motion of the earth can unquestionably produce the impression that the entire universe is rotating.

See also: inferior planet, superior planet

Nationality speculations

Copernicus was Polish, although there is some debate on the subject among ethnic nationalists (see Copernicus' nationality).

External links:

• Portraits of Copernicus: [3] (http://www-groups.dcs.st-andrews.ac.uk/~history/PictDisplay/Copernicus), [4] (http://www.frombork.art.pl/Ang10.htm)
• The Copernican Universe from the De Revolutionibus (http://es.rice.edu/ES/humsoc/Galileo/Images/Astro/Conceptions/copernican_universe.gif)
• The front page of the De Revolutionibus (http://www.hao.ucar.edu/public/education/sp/images/derevolutionibus)
• The text of the De Revolutionibus (http://www.dartmouth.edu/~matc/readers/renaissance.astro/1.1.Revol)
• A link collection (http://www.geometry.net/cgi-bin/988/988.cgi?q=copernicus)
• A java applet about Retrograde Motion (http://www.flex.com/~jai/astrology/retrograde)
• Copernicus in Bologna (http://www.bo.astro.it/dip/Museum/italiano/sto1_08) - in Italian