All the attempts that were developed in the Islamic world to revamp Ptolemaic astronomy were, in essence, motivated by the simple and straightforward requirement of keeping astronomical theory consistent with its premises. That is, all those astronomers took Ptolemy at his word that he wished to develop astronomical models all based on a universe made of Aristotelian spheres, and all spheres moved around their own centers, in place, at uniform speed. When they found the Ptolemaic models wanting, they developed their own alternative models, for which, at times, they had to develop the right mathematical theorems in order to maintain the correspondence between those models and the observations upon which the models were based in the first place.
With that attitude, they managed to introduce the feature of consistency into astronomical theory, and to subsume mathematics as a tool of that theory. These astronomers who did not tolerate the Ptolemaic transgression into elegant mathematics at the expense of the physical nature of the presupposed spheres, would not in all likelihood have tolerated a whole new heliocentric system where the very foundations of the spheres, that were still retained by Copernicus, no longer made sense in the heliocentric world. Consistency between the presuppositions, the physical nature of the spheres, and the mathematics that represented the motions of those spheres, on the one hand, and models that served as predictive models for the behavior of the planets at any time and place, on the other, became the guiding principle of Islamic astronomy at this stage. Only at a later stage, i.e. toward the middle of the sixteenth century, would mathematics take its proper role as a tool of astronomical theory.
The fact that Copernicus too launched his own research, in the Commentariolus, with the same attitude of wishing to solve the problem of the equants, simply means that at the early date of the sixteenth century, if not well before, the flow of ideas across the Mediterranean was already in full gear. And now that we can document the similarities between the works of Copernicus and those of his predecessors in the Islamic world, they only confirm the fluidity of this traffic. Once that becomes clear, one can then readdress the question of "locality" versus "essence" of Islamic science by basing the discussion on concrete examples such as the ones that are being raised here.[332] If the solution of a problem that was developed in Damascus in the middle of the thirteenth century, and still made perfect sense to someone like Copernicus who was writing within the context of the Latin world of the Renaissance, then it becomes obvious that neither the passage of time nor the cultural borders could inhibit this motion of perfectly valid solutions. So what is "local" and what is "essence" about the solutions of such problems?
These findings do not only explain the background and motivation of the Copernican works; they also explain the continuity of thought from the Middle Ages into the Renaissance time, without having to make wild assumptions about ideas being born in abstract contexts. Their sheer number and complexity, as well as their technical nature also remove the possibility of coincidental discovery, and force us to agree with Swerdlow and Neugebauer that it is no longer the problem of "if" but "when, where and in what form" did Copernicus learn of those earlier works.[333] The answer to this question promises to change our common understanding of the history of science itself, as well as change our understanding of the nature of the relationship between Europe and the Islamic world at this crucial time in history.
So far I have limited the discussion of the possible routes of contacts between Copernicus and the Islamic world to the language that Copernicus could read and into which the Arabic sources could have been translated: Byzantine Greek. But since the discovery of the Byzantine manuscript, Gr 211, of the Vatican, by Neugebauer, other hints of possible routes have come to light, mainly from the Arabic manuscripts themselves. One such manuscript, also kept at Vatican, Arabo 319, is another copy of the Tadhkira of Naṣīr al-Dīn al-Ṭūsī, in which, of course, there is a chapter that included the proof of the Ṭūsī Couple. The manuscript itself was passed on to the Vatican Library as part of the legacy of a Frenchman by the name of Guillaume Postel (1510-1581) who was a younger contemporary of Copernicus.[334]
What makes this Vatican manuscript quite unusual is the fact that it is titled "Epitome Almagesti". With Della Vida's help it was then determined that the identification was done by Postel himself. But more importantly, the manuscript also contains marginal annotations in Latin, also by Postel, that indicate his ability to read this highly technical astronomical text of Ṭūsī. He could clearly comment on it, although very briefly, which means that he understood what he was reading. The text is reasonably well preserved, especially around the chapter that included the statement and proof of the Ṭūsī Couple, and thus could have presented no material difficulty to someone who was capable of understanding its contents.
The existence of such a manuscript also indicates that there were Renaissance men who knew Arabic, and definitely knew of the contents of technical scientific texts.[335] The problem is to determine whether Copernicus himself ever came to know such men. For if he did, then it would be quite possible to assume with Willy Hartner that someone could have briefed him about the contents of such manuscripts, that is, bring him up to date on the latest in Arabic astronomy.
Such a scenario may inadvertently help solve the problem of Copernicus's indebtedness to more than one Arabic text and for which I had to speculate about the existence of a text that was written after the time of Ibn al-Shāṭir in the form of a commentary that included elements from the works of Ṭūsī, 'Urḍī, and Ibn al-Shāṭir; texts that we know have come to the attention of Copernicus. Assuming the existence of such a colleague, to whom Copernicus could go for consultation, may solve that problem: it would make the colleague the gatherer of such information from various texts, and it could make him responsible for passing it on to Copernicus.
But once it was known that Postel owned at least one technical Arabic astronomical manuscript, it was reasonable to investigate other collections and see if they also included such manuscripts that were owned by him, in order to determine the extent of Postel's own commentaries. The hope was that this kind of research would shed light on the kinds of texts such contemporaries of Copernicus were reading. We would also know if the Vatican manuscript was an exception or a unique occurrence.
The search was then enlarged to include the astronomical texts that are still preserved in other European libraries. Luckily the first step in that research was immediately rewarded by the collection of the Bibliothèque Nationale of Paris. Among the Arabic manuscripts still kept in that collection there appeared another technical text, called Muntahā al-idrāk fī taqāsīm al-aflāk (The Ultimate Grasp of the Divisions of Spheres), this time written by Abū Muḥammad 'Abd al-Jabbār al-Kharaqī (1138/9). The manuscript is definitely devoted to mathematical astronomy as is clearly indicated in the title. Furthermore, it is explicitly marked as having been owned by the same Guillaume Postel with the phrase "ex libris guilielmi postelli" clearly marked on the title page.[336] On the first flyleaf, the manuscript also states that it was bought in Constantinople in 1536, as it is clearly marked: "G. postellus Contantinopoli 1536". The year 1536 also happens to be the year that culminated the mission of the delegation that had been sent to Constantinople by the French King François I (1515-47) to negotiate a treaty with the Ottoman Sultan, Suleiman the Magnificent (1520-66). The treaty was in fact signed in that year.[337] This Postel was apparently a member of the delegation. And we know that he was charged to buy Greek books by Budé, the Librarian of François I. But apparently Postel opted to buy Arabic scientific texts instead.
332
Such issues of "locality" versus "essence", as discussed by 'Abd al-Hamid Sabra, "Situating Arabic Science: Locality versus Essence",
334
See Giorgio Levi Della Vida,
335
For a very detailed account of such men, see Dannenfeldt, "The Renaissance Humanists."
336
See the title page of Kharaqī's manuscript (Bibliothèque Nationale de France, Arabe 2499).
337
For a brief description of the conditions that led to that treaty, and the privileges it granted to the French, both commercial and military, see V. H. H. Green,