The Islamic civilization did not seem to have produced a rigorous astronomical criticism of the type that would have questioned the natural philosophical foundations of Greek astronomy themselves. Although some religiously inspired cosmologies did in fact speak to that point, yet there were no astronomers that I know of who adopted these views or sought to interpret the astronomical implications of such cosmologies. The final rejection of Aristotelian cosmology had to come late in the history of astronomy, and only after a long and arduous struggle that was initiated by modern science under conditions that were completely different from those that prevailed in the Islamic civilization.
4. Islamic Astronomy Defines Itself: The Critical Innovations
Now that we have seen the kind of reactions the encounter with Greek science has produced in Islamic civilization, we can better appreciate the context for the astronomical developments that took place, as we continue to use astronomy as a template for the other disciplines that must have experienced similar transformations. In astronomy, the reactions expressed, at all levels, ranged from simple corrections of what was thought to be a mistake in the text, as was done by al-Ḥajjāj in the case of the Almagest, to correcting the basic parameters by fresh observations, as in the case of redetermining the better values of precession and the inclination of the ecliptic among others, to critiquing the methods of observation, as was done in the case of the fuṣūl method, and finally to casting doubt on the reliability of the very foundations of the Greek astronomical tradition itself when it seemed to violate the principles upon which it was based in the first place.
All these developments, when coupled with the very watchful eyes of the competing groups we spoke about earlier, from inside the profession as well as from outside, generated a skeptic attitude toward the incoming tradition. In itself this attitude emboldened astronomers to raise deeper and deeper questions as they continued to examine this Greek tradition in light of their own research. In this environment, it becomes easy to understand why good competent astronomers could not continue to practice astronomy by simply taking the Greek astronomical tradition at its face value. They had to compete by proving that they could achieve better results than those that were achieved by the Greeks and which were being continuously criticized at the time.
This did not mean that the Greek astronomical sources were yielding such dramatically erroneous mistakes that they could no longer be used to answer simple mundane questions like casting a horoscope or the like. But it did mean that the professional astronomer, from early Abbāsid times on, could no longer survive the competition if he limited himself to such simple questions in the first place. The serious astronomers had to answer more complex questions regarding the suitability of the proposed Ptolemaic astronomical configurations in accounting for the observations on the one hand, and in embodying the prevailing cosmological system of Aristotle on the other. For them, it was no longer sufficient to find the positions of the planets at any time for purposes of casting a horoscope or some such things, but they had to know how the planets moved, what caused their motion, why do they appear to go through all sorts of irregular behavior, and how does one account for that, all within the assumption of a universe made up of spheres all moving in place at uniform speeds as Aristotle had stipulated. At this degree of seriousness, Ptolemaic astronomy was found to be desperately wanting.
With the work of Muḥammad b. Mūsā b. Shākir, during the first half of the ninth century, regarding the properties and the admissibility of the existence of the ninth sphere, the stage was set for undertaking a total overhaul of the entire Greek astronomical edifice. When it was found later on, as we have already seen with the critiques of Ibn al-Haitham, that the physical foundations of the Ptolemaic configurations did not match the mathematical representations that were offered by Ptolemy, the motivation for the overall reform of that astronomy became a matter of necessity rather than choice. Only practicing astrologers could satisfy themselves, if they so pleased, with the use of the Ptolemaic Handy Tables, for example, to calculate the planetary positions that they needed for their horoscope casting. But those astrologers themselves fell under the censoring eye of the society at large, despite their ability to continue to function, and still try to make themselves useful to that society. Even then, they too had to require better and better astronomical tables (zījes) for their craft, as the old parameters of the Ptolemaic tables were continuously corrected as time went on. Socially though, no self-respecting astronomer would admittedly want to be cast in an astrologer's garb, if he could help it. This despite the fact that some of them did. While the best of them would want to associate themselves with the critical tradition that was beginning to pick up steam from the earliest decades of the ninth century. The latter had to cast a new name for the discipline they practiced (the discipline of 'ilm al-hay'a), because they did not wish to be associated with the lesser figure of the practicing astrologer.[235]
It was this environment that motivated the research of the new Islamic astronomy. Its main mission, as was enunciated later by Mu'ayyad al-Dīn al-'Urḍī (d. 1266) of Damascus,[236] one of the most distinguished astronomers of that tradition, was to create an astronomy that did not suffer from the cosmological shortcomings of Ptolemaic astronomy, that could account for the observations just as well as Ptolemaic astronomy could do if not better, and that did not limit itself to criticizing Ptolemy only, despite all the benefits that one derived from the detailed critique of Ptolemy's mistakes. This urgent need for a higher form of scientific astronomy was almost felt by all serious astronomers whose works we have come to know only recently, and who formed a continuous tradition inaugurated toward the beginnings of the ninth century and continued well into the sixteenth century as far as we can now tell. One astronomer after another would take very seriously Ibn al-Haitham's declaration which stated that there must be an astronomical theory, or in his language astronomical configuration (hay'a), that could account for the observations conducted in the real physical world without having to represent that world with a set of imaginary lines and circles as was done by Ptolemy.[237] One could hear them all repeat: If the world was real, made up of real spheres, as was argued by Aristotle, then let it be represented by mathematical models that did not contradict that physical reality.
On the more mundane level, when it was a matter of double-checking the observations that would account for the behavior of the real physical world, or that would help establish the very observations that were to be used as the building blocks of the theoretical representation, those had to be taken seriously as well. That is why one can document several attempts to double-check the values of the basic parameters, as we have already seen, or to initiate a whole discussion about the optimal methods of observation as we have also seen, or to initiate whole new fields of refining observational instruments or inventing whole new ones when there was a need for that. These activities continued to take place as the astronomical tradition continued to grow. The studies of Khujandī's surviving works on larger, and thus more precise, instruments, or those of 'Urḍī regarding the construction of the Marāgha observatory instruments, among others, speak exactly to such concepts.[238] But what was really wrong with Ptolemaic astronomy that generated all those discussions?
238
See, for example, Louis Cheikho, "Risālat al-Khujandī fī mayl wa-'arḍ al-balad",