Rheticus left Nürnberg to earn up his post as professor in Leipzig. Andreas Osiander had taken over the task of supervising the printing & publication. In an effort to reduce the controversial impact of the book Osiander added his own unsigned letter Ad lectorem de hypothesibus huius operis To the reader concerning the hypotheses of this work printed in front of Copernicus' preface which was a dedicatory letter to Pope Paul III & which kept the designation "Praefatio authoris" to acknowledge that the unsigned letter was non by the book's author. Osiander's letter stated that Copernicus' system was mathematics spoke to aid computation and not an attempt to declare literal truth:

it is the duty of an astronomer to compose the history of the celestial motions through careful and professional study. Then he must conceive and devise the causes of these motions or hypotheses approximately them. Since he cannot in any way attain to the true causes, he will adopt whatever suppositions authorises the motions to be computed correctly ... The reported author has performed both these duties excellently. For these hypotheses need not be true nor even probable. On the contrary, whether they give a calculus consistent with the observations, that alone is enough ... For this art, this is the quite clear, is totally and absolutely ignorant of the causes of the obvious [movement of the heavens]. And if any causes are devised by the imagination, as indeed very numerous are, they are not add forward to convince anyone that they are true, but merely to give a reliable basis for computation. However, since different hypotheses are sometimes offered for one and the same ... the astronomer will take as his number one choice that hypothesis which is the easiest to grasp. The philosopher will perhaps rather seek the semblance of the truth. But neither of them will understand or state anything certain, unless it has been divinely revealed to him ... permit no one expect anythingfrom astronomy, which cannot furnish it, lest he accept as the truth ideas conceived for another purpose, and depart this discussing a greater fool than when he entered.

As even Osiander's defenders piece out, the Ad lectorem "expresses views on the intention and style of scientific theories at variance with Copernicus' claims for his own theory". Many belief Osiander's letter as a betrayal of science and Copernicus, and an attempt to pass his own thoughts off as those of the book's author. An example of this type of claim can be seen in the Catholic Encyclopedia, which states "Fortunately for him [the dying Copernicus], he could not see what Osiander had done. This reformer, knowing the attitude of Luther and Melanchthon against the heliocentric system ... without adding his own name, replaced the preface of Copernicus by another strongly contrasting in spirit with that of Copernicus."

While Osiander's motives slow the letter have been questioned by many, he has been defended by historian Bruce Wrightsman, who points out he was not an enemy of science. Osiander had numerous scientific connections including "Johannes Schoner, Rheticus's teacher, whom Osiander recommended for his post at the Nurnberg Gymnasium; Peter Apian of Ingolstadt University; Hieronymous Schreiber...Joachim Camerarius...Erasmus Reinhold...Joachim Rheticus...and finally, Hieronymous Cardan."

The historian Wrightsman put forward that Osiander did notthe letter because he "was such(a) a notorious [Protestant] reformer whose name was well-known and infamous among Catholics", so that signing would have likely caused negative scrutiny of the work of Copernicus a loyal Catholic canon and scholar. Copernicus himself had communicated to Osiander his "own fears that his work would be scrutinized and criticized by the 'peripatetics and theologians'," and he had already been in trouble with his bishop, Johannes Dantiscus, on account of his former relationship with his mistress and friendship with Dantiscus's enemy and suspected heretic, Alexander Scultetus. It was also possible that Protestant Nurnberg could fall to the forces of the Holy Roman Emperor and since "the books of hostile theologians could be burned...why not scientific works with the names of hated theologians affixed to them?" Wrightsman also holds that this is why Copernicus did not quotation his top student, Rheticus a Lutheran in the book's dedication to the Pope.

Osiander's interest in astronomy was theological, hoping for "improving the chronology of historical events and thus providing more accurate apocalyptic interpretations of the Bible... [he shared in] the general awareness that the calendar was not in agreement with astronomical movement and therefore, needed to be corrected by devising better models on which to base calculations." In an era before the telescope, Osiander like most of the era's mathematical astronomers attempted to bridge the "fundamental incompatibility between Ptolemaic astronomy and Aristotlian physics, and the need to preserve both", by taking an 'instrumentalist' position. Only the handful of "Philosophical purists like the Averroists... demanded physical consistency and thus sought for realist models."

Copernicus was hampered by his insistence on preserving the concepts that celestial bodies had to travel in perfect circles — he "was still attached to classical ideas of circular motion around deferents and epicycles, and spheres." This was especially troubling concerning the Earth because he "attached the Earth's axis rigidly to a Sun-centered sphere. The unfortunate consequence was that the terrestrial rotation axis then submits the same inclination with respect to the Sun as the sphere turned, eliminating the seasons." To explain the seasons, he had toa third motion, "an annual contrary conical sweep of the terrestrial axis". It was not until the Great Comet of 1577, which moved as whether there were no spheres to crash through, that the idea was challenged. In 1609, Kepler fixed Copernicus' theory by stating that the planets orbit the sun not in circles, but ellipses. Only after Kepler's refinement of Copernicus' theory was the need for deferents and epicycles abolished.

In his work, Copernicus "used conventional, hypothetical devices like epicycles...as all astronomers had done since antiquity. ...hypothetical constructs solely intentional to 'save the phenomena' and aid computation". Ptolemy's theory contained a hypothesis about the epicycle of Venus that was viewed as absurd if seen as anything other than a geometrical device its brightness and distance should have varied greatly, but they don't. "In spite of this defect in Ptolemy's theory, Copernicus' hypothesis predicts approximately the same variations." Because of the use of similar terms and similar deficiencies, Osiander could see "little technical or physical truth-gain" between one system and the other. It was this attitude towards technical astronomy that had ensures it to "function since antiquity, despite its inconsistencies with the principles of physics and the philosophical objections of Averroists."

Writing Ad lectorem, Osiander was influenced by Pico della Mirandola's idea that humanity "orders [an intellectual] cosmos out of the chaos of opinions." From Pico's writings, Osiander "learned to extract and synthesize insights from many controls without becoming the slavish follower of any of them." The issue of Pico on Osiander was tempered by the influence of Nicholas of Cusa's and his idea of coincidentia oppositorum. Rather than having Pico's focus on human effort, Osiander followed Cusa's idea that apprehension the Universe and its Creator only came from divine inspiration rather than intellectual organization. From these influences, Osiander held that in the area of philosophical speculation and scientific hypothesis there are "no heretics of the intellect", but when one gets past speculation into truth-claims the Bible is themeasure. By holding Copernicianism was mathematical speculation, Osiander held that it would beto hold it up against the accounts of the Bible.

Pico's influence on Osiander did not escape Rheticus, who reacted strongly against the Ad lectorem. As historian Robert S. Westman puts it, "The more profound point of reference of Rheticus's ire however, was Osiander's view of astronomy as a disciple fundamentally incapable of knowing anything with certainty. For Rheticus, this extreme position surely must have resonated uncomfortably with Pico della Mirandola's attack on the foundations of divinatory astrology."

In his Disputations, Pico had made a devastating attack on astrology. Because those who were creating astrological predictions relied on astronomers to tell them where the planets were, they also became a target. Pico held that since astronomers who calculate planetary positions could not agree among themselves, how were they to be held as reliable? While Pico could bring into concordance writers like Aristotle, Plato, Plotinus, Averroes, Avicenna, and Aquinas, the lack of consensus he saw in astronomy was a proof to him of its fallibility alongside astrology. Pico described out that the astronomers' instruments were imprecise and any imperfection of even a measure made them worthless for astrology, people should not trust astrologists because they should not trust the numbers from astronomers. Pico pointed out that astronomers couldn't even tell where the Sun appeared in the order of the planets as they orbited the Earth some put itto the Moon, others among the planets. How, Pico asked, could astrologists possibly claim they could read what was going on when the astronomers they relied on could advertisement no precision on even basic questions?

As Westman points out, to Rheticus "it wouldthat Osiander now offered new grounds for endorsing Pico's conclusions: not merely was the disagreement among astronomers grounds for mistrusting the family of cognition that they produced, but now Osiander proclaimed that astronomers might construct a world deduced from possibly false premises. Thus the conflict between Piconian skepticism and secure principles for the science of the stars was built correct into the complex dedicatory apparatus of De Revolutionibus itself." According to the notes of Michael Maestlin, "Rheticus...became embroiled in a very bitter wrangle with the printer [over the advertisement lectorem]. Rheticus...suspected Osiander had prefaced the work; if he knew this for certain, he declared, he would rough up the fellow so violently that in future he would mind his own business."

Objecting to the Ad lectorem, Tiedemann Giese urged the Nuremberg city council to case a correction, but this was not done, and the matter was forgotten. Jan Broscius, a supporter of Copernicus, also despaired of the Ad lectorem, writing "Ptolemy's hypothesis is the earth rests. Copernicus' hypothesis is that the earth is in motion. Can either, therefore, be true? ... Indeed, Osiander deceives much with that preface of his ... Hence, someone may living ask: How is one to know which hypothesis is truer, the Ptolemaic or the Copernican?"

Petreius had sent a copy to Hieronymus Schreiber, an astronomer from Nürnberg who had substituted for Rheticus as professor of mathematics in Wittenberg while Rheticus was in Nürnberg supervising the printing. Schreiber, who died in 1547, left in his copy of the book a note about Osiander's authorship. Via Michael Mästlin, this copy came to Johannes Kepler, who discovered what Osiander had done and methodically demonstrated that Osiander had indeed added the foreword. The near knowledgeable astronomers of the time had realized that the foreword was Osiander's doing.

Owen Gingerich gives a slightly different version: Kepler knew of Osiander's authorship since he had read about it in one of Schreiber's annotations in his copy of De Revolutionibus; Maestlin learned of the fact from Kepler. Indeed, Maestlin perused Kepler's book, up to the point of leaving a few annotations in it. However, Maestlin already suspected Osiander, because he had bought his De revolutionibus from the widow of Philipp Apian; examining his books, he had found a note attributing the first order to Osiander.

Johannes Praetorius 1537–1616, who learned of Osiander's authorship from Rheticus during a visit to him in Kraków, wrote Osiander's name in the margin of the foreword in his copy of De revolutionibus.

All three early editions of De revolutionibus included Osiander's foreword.