By 1500, however, over 1,300 years of astronomical observation in Europe and the Arab world had raised serious problems for the earth-centred cosmology of Ptolemy (c. AD 150) and earlier Greeks. For while common sense, logic, and the Physics of Aristotle were all in agreement that the spherical earth lay at the centre of the universe, and that the nine planet-and-star-carrying 'Spheres of Heaven' rotated around it, a growing number of anomalies in the motions of the planets were making their movements harder to account for, or predict, with certainty.

For medieval and Renaissance astronomy stood firmly upon the two classical Greek axioms that, assuming the heavens to be perfect, then (a) the planets must rotate around the earth in perfect spheres, and (b) they must move at perfectly uniform velocities. Yet even the Greeks had been aware that actual observed planetary motions displayed complex accelerations, decelerations, and loopings. These periodic backwards-moving loops, or retrogrades, had been 'explained' by an increasingly elaborate system of epicycles, in which circles rotated around circles to simulate the loops that were actually observed in the night sky. By 1500, however, more and more epicyclic tinkering was becoming necessary to account for all the observed motions of the celestial bodies.

This is what led Copernicus to his theory that the sun might well be at the centre of the cosmos, with the earth, moon, planets, and stars rotating about it. He appears to have come to this conclusion about 1510-14, as a result of a great deal of calculating, and wrote up his 'model' in a manuscript treatise, Commentariolus, which was circulated amongst mathematical cognoscenti. Over the next 30 years he perfected his 'heliocentric' or sun-centred cosmological model, which finally appeared as De Revolutionibus Orbium Coelestium ('On the Revolutions of the Spheres of Heaven'), Nuremberg, 1543.

As he was an instinctive conservative, with a profound admiration for the classical Greek writers and astronomers, the last thing his book was designed to become was provocative or polemical. Instead, he saw himself as respectfully correcting the ancients, not overturning them. Indeed, the structural format for his work mirrors in many ways that of Ptolemy's Almagest (c. AD 150), with the exception that he used geometry to demonstrate a sun- rather than earth-centred system. In his 'Preface', moreover, he goes out of his way to proclaim his respect, drawing attention to the fact that Greek writers, such as Ecphantus and Philolaus, had discussed sun-centred models before him. Then, as his book progresses, he builds up an incremental, geometrically-based scheme to show the plausibility of a heliocentric universe.

On the other hand, as Copernicus was all too well aware, he explained the planetary retrograde loops at the expense of throwing out both common-sense experience and much of the physics of Aristotle: the principal system of physics of his day. For surely, as his critics would later point out, if the earth was spinning on its axis and hurtling around the sun, then would not everything fly off into space? Would not a permanent gale engulf the flying earth, and would not stones lose their 'heaviness' and, instead of falling down to earth, shoot off into the air? Yet none of these things could be seen in nature, as the earth appeared fixed, stable, and stationary. And all of this, moreover, derived not from any kind of slavish adherence to the Bible, but from daily, practical experience, and the best Aristotelian science of 1540. And this apparent outrage against common-sense physics is no doubt what made Copernicus cautious about publishing his magnum opus sooner, and it is hardly surprising that, when he did so, the acerbic Martin Luther spoke of him as 'that fool who would turn astronomy upon its head'. (Yet 'fool' was mild in the extreme compared to what Luther called the Pope and his Roman Catholic apologists. Besides, Luther made no claim to any special technical knowledge of astronomy.)

De Revolutionibus consists of six books, each subdivided into 'Chapters' or sections. They are very technical and mathematical in content, presupposing on the part of the reader an intimate familiarity with Greek astronomy, especially of Ptolemy's Almagest. Book I begins by setting out basic terms, describing the heavens, and discussing the nature of motion. And in Chapter 10, Copernicus sets forth in detail his heliocentric theory, complete with an illustration on page 9 verso [overleaf]. Book II deals with the basic geometry of the risings and settings of the planets, and the arcs they describe across the sky. Book III looks at the precession of the equinoxes, the length of the solar year, and their effects. Book IV analyses the complex orbit of the moon, and deals with the mechanism of lunar and solar eclipses and the prediction of eclipses. Book V addresses the very complex orbits and motions of the five classical planets: Mercury, Venus, Mars, Jupiter, and Saturn. Book VI further analyses the motions of the planets, and in particular their orbital planes, eccentricities (unequal motions around a centre), and obliquities (tilts) to the ecliptic (the sun's apparent path through the zodiac).

De Revolutionibus was published in Nuremberg in 1543, the same year in which Copernicus, back in Frombork, died of a stroke. It is incorrect, however, to suggest that he only published on his deathbed to escape punishment for heresy, for his heliocentric cosmology was coming to be well known across Europe well before his final illness: in fact, at a time when he was still undertaking high-profile medical consultations. Indeed, his ideas had long been spreading by word of mouth, and on 1st November, 1536, the great Churchman Nicholas Schönberg, Cardinal of Capua, had written a most laudatory letter to Copernicus, from Rome (reproduced in the printed Preface pages of De Revolutionibus), praising his profound astronomical learning. Cardinal Schönberg clearly knew all about Copernicus's heliocentric system by 1536, for in his letter not only does he outline it in some detail - specifically mentioning the central sun, moving earth, and stationary stars - but also offered to send a scribe to copy out his manuscript for the Cardinal to read, and no doubt enjoy, in Rome. Then in 1540 Copernicus's Wittenberg, Lutheran, disciple Joachim Rhaeticus published his Narratio Prima ('First Account') of Copernicus's theory, following it with a second edition in 1541. (Indeed, it is even possible to speculate whether twenty years previously Copernicus's Polish friend Bishop Johannes Dantiscus discussed the outlines of his heliocentric theory when on a diplomatic mission to the Court of King Henry VIII in England!)

The man who saw De Revolutionibus through the press in Nuremberg in 1543 was another Wittenberg Lutheran disciple, Andreas Osiander. Osiander added a 'Preface' - probably unauthorised - to the work, suggesting that the heliocentric model was not of necessity to be taken as literally true, but could be seen as a 'phenomenon-saving' model to facilitate the easier computation of the planetary motions. Osiander's 'Preface' has provoked much debate amongst twentieth-century scholars. Yet we must bear in mind that the whole of orthodox earth-centred Ptolemaic cosmology was also full of 'phenomenon-saving' devices: epicycles, equants, spheres rolling within spheres, and so on, in an attempt to make the observed heavens conform to the principles of Greek philosophical geometry. Quite simply, the Copernican 'phenomenon-saving' device was part of that same time-honoured mathematical practice: how to simplify the heavens by hypothesising a spinning, flying earth, while at the same time retaining our experience of all things being apparently rock-solid. The book, moreover, was even dedicated to Pope Paul III, although there is no evidence to suggest that Copernicus requested papal permission for the dedication beforehand. But there was no backlash, no condemnation, and no burnings. De Revolutionibus even went peacefully through a second edition in Basel, Switzerland, in 1566, and a third in Protestant Amsterdam in 1617. [The Wadham College Library copy, however, is from the original Nuremberg first edition of 1543.]

De Revolutionibus remained in open and unrestricted circulation across Europe for the next 77 years. Astronomers found it fascinating; the Lutheran Danish astronomer Tycho Brahe - a warm admirer of the Roman Catholic Polish Canon, who saw him as something of a fellow-member of the 'northern European Renaissance' - tried in the early 1580s to test Copernicus's theory by physical observation, using instruments of exquisite accuracy. But even these were insufficiently precise to detect a seasonal motion of the earth from star displacements or parallaxes.

Copernicus had supporters across Europe, and perhaps most especially in Tudor England. The Oxonians Thomas Digges, Thomas Harriot, Sir William Lower, William Gascoigne, the Welshman Dr Robert Recorde (also jointly Oxford and Cambridge), John Greaves, John Wilkins, the young Sir Christopher Wren, and Robert Hooke were all Copernicans, between the 1550s and 1650s. And there were many eminent Cambridge supporters, and, after its foundation in 1597, many Gresham College, London, Professors were also won over by the Copernican argument. Indeed, the young Emmanuel College, Cambridge, graduate Jeremiah Horrocks made crucial observations from the village of Much Hoole, Lancashire, in the 1630s which were wholly congruent with Copernicus's theory, and would influence the early Royal Society (meeting in its prototypal form in Wadham College before its formal inauguration at Gresham College, London, in 1660), and the Principia Mathematica of Sir Isaac Newton, 1685.

De Revolutionibus was not condemned by the Vatican until after Galileo's first brush with the Inquisition in 1616. In 1620 the order was given that the book had to be 'corrected', while Copernicus himself was acknowledged as a respected astronomer. And this condemnation would almost certainly never have taken place had not Galileo abrasively argued the still unproven Copernican hypothesis to be the truth, and had he not subsequently presumed upon his friendship with Pope Urban VIII in 1632, thereby precipitating his own trial and the further condemnation of Copernicus!

Although Copernicus's heliocentric theory had long been accepted on other scientific grounds, the decisive physical proofs for the earth's motion from stellar parallax displacements would not become available for almost 200 years after the Polish Canon's death. And this could not take place until precision telescopic measurements became possible with instruments that were vastly more accurate than Tycho's had been in 1580. Then in 1728 the Balliol College and Oxford Savilian Professor of Astronomy, the Revd Dr James Bradley, announced to the Royal Society his detection of a tiny seasonal movement of the stars with his precision zenith sector telescope. He called this movement the 'aberration of light', and explained it as being caused by the earth, in its annual orbit around the sun, moving for part of the year into the light of the star Gamma Draconis, and for the other part moving away from it. A light displacement that could only have occurred if the earth were orbiting the sun.

And then in 1838, using even better instruments, Friedrich Wilhelm Bessel at the Königsberg Observatory (Germany) and Wilhelm Struve at the Pulkowa Observatory (St Petersburg) detected a tiny six-monthly parallax displacement of the stars 61 Cygni and Vega respectively. Motions which, once again, could only be explained if the earth were orbiting the sun in space.

Yet neither Harriot, Galileo, Horrocks, Wilkins, Hooke, nor any other of the early Copernicans could have imagined how many years would have to pass before the heliocentric theory could be physically proven. For no one in 1543 had any idea how utterly vast the universe was, or how far away from the earth even the nearest stars were, and how much technological innovation would be necessary before these distances could be measured. So to somehow blame or ridicule early sceptics of Copernicanism for being ignorant, blinkered, or backward is absurd. For vital scientific evidences were lacking in the sixteenth century.

Even so, Copernicus's De Revolutionibus was without doubt one of the most far-reaching and formative books of modern science.