In a parallel development, and this was also to be expected, one finds those same astronomers, using the results of the Almagest, at times, whenever they thought that those results were still valid, while at other times they would reject them completely in favor of new ideas of their own. This multiplicity of approaches to the Almagest text can only signal the very vital reactions it must have created within the receiving culture of early Islamic times. But one should also remember that at all instances this very vitality produced an Almagest text that was considerably enriched by the process.
Returning to the earlier astronomers, such as Ḥabash al-Ḥāsib (fl. ca. 850) in particular, who produced their own independent zījes (Astronomical Handbooks), that were only constructed in the tradition of Ptolemy's Handy Tables, we find that they too have also used the most recent and fully developed trigonometric functions in those works.[178]
Looking at the complete picture of that period, and after examining the scientific sources themselves, one can begin to see a process in which one finds that as soon as the Greek scientific texts were being translated, they were also being immediately updated by the currently known material and put to use in new compositions, all in order to improve the kind of science that was then produced.[179]
The second type of intervention in the text of the Almagest, had less to do with updating it mathematically or correcting its errors as we have already seen. Instead it was more like reconstructing it or re-editing it so that it would become more useful for students of astronomy. In this regard great liberties were taken with the text, feeling completely free to add to it and delete material from it, all in order to make it a more up-to-date functional text.
The best illustration of this type of intervention is also exemplified by Ṭūsī's Taḥrīr, which was mentioned before and in which one finds a new treatment of some chapters of the Almagest, such as Almagest X,7, where Ptolemy used an iterative method to compute the eccentricity of one planet and then repeated it in great detail for each of the other planets.[180] Instead of the Ptolemaic approach, Ṭūsī adopted a new technique of explaining the method in great detail in the case of one planet and then generalizing it to the others without repeating it in every case.
I mentioned the corrections Ṭūsī had introduced in the same text regarding the apparent size of the solar disk, and the counterexample of the annular eclipses that were apparently unknown to Ptolemy. I also mentioned the correction of other factual errors, including errors in the rate of precession, the inclination of the ecliptic, and the motion of the solar apogee, as well as the development of the methods of observations like the introduction of the fuṣūl method. In all those instances, we find the text of the Almagest critically reviewed and updated before it could become useful to the receiving culture. Far from being a model to be followed, although one could argue that it was in some sense, it was more like a foundation to build upon, but only after making sure that it was a safe foundation and that its errors and contradictions had been already weeded out. There were other instances in which the text of the Almagest was also found wanting, but this time for much more fundamental considerations than the ones that have been discussed so far.
By considering only the factual corrections that have been discussed so far, one could easily arrive at the conclusion that the text of the Almagest would have become functionally serviceable once those corrections were adopted. The text would have been sufficient, for example, for practicing astronomers and astrologers and no further elaborations of it would have been necessary. But with astrology and its practice facing a veritable resistance from the main intellectual centers of the society, especially the religious ones, and thus its relationship to astronomy being consciously severed by the theoretical astronomers who invented the discipline of hay'a as we have already seen, the purpose of the discipline of astronomy was apparently defined in a slightly more nuanced fashion. This purpose can best be seen if one reads the two most famous works of Ptolemy together. Those works are: the Almagest, where one finds a detailed account regarding the relationship between the observed phenomena and the construction of geometric predictive models that explained the behavior of the planets at all times, and the Planetary Hypotheses, where one would find a detailed account of the celestial spheres that were made, in a true Aristotelian fashion, responsible for the motion of those planets. By reading those two texts together, as most people did, once they became available in Arabic, some serious cosmological problems began to appear. Most of those problems focused on Ptolemy's violation of the most basic cosmological tenet of Greek astronomy: the uniform circular motion of the planets around a fixed Earth located at the center of the universe.
That the Earth was fixed at the center of the universe was undoubtedly at the core of that Aristotelian cosmology, so much so that if one did not have such an Earth one would have had to suppose the existence of such an Earth at the very center of heaviness around which everything else revolved.[181] The real challenge was to explain the apparent phenomena from within that cosmological vision, and still retain some predictability in the geometric models that described the planetary motions.
From that cosmological perspective, the Almagest failed at almost every count. While there is the Ptolemaic pretense that the universe, which was being described, was an Aristotelian universe, within which all the Aristotelian elements were to be found as the building blocks of that universe, yet at every juncture the Almagest described situations that were physically impossible when looked upon from the perspective of the Planetary Hypothesis that emphasized that Aristotelian cosmology. It is this inconsistency between the mathematical models constructed in the Almagest to account for the motion of the planets, and the physical objects those models were supposed to represent that I have so often referred to as the major problem of the Greek astronomical tradition.[182]
Because these inconsistencies are of a completely different nature than the ones that were touched upon before, and because they were a direct byproduct of the application of Aristotelian cosmology, some people have referred to them as philosophical problems. As a result they tried to read the Almagest as divorced from that same cosmology that was wholeheartedly adopted in the Planetary Hypotheses, the text that was supposed to complement the Almagest, which it followed. And yet these inconsistencies were perceived as touching the very foundation of science; in the sense that science should not harbor contradictions, as Ptolemy seems to have allowed it to do, between the physical side of the science and the mathematical representation of the same physical universe that was being described.
178
Debarnot,
179
We have already seen the similar process that took place in the field of mathematics, where we found the translator Qusṭā b. Lūqā himself deploying the algebraic technical terminology of his time within the translation of the
181
In Aristotle's words in
182
See the most recent summation of those problems in George Saliba, "Greek Astronomy and the Medieval Arabic Tradition",