But when the Aristotelian vision conceived of the concept of change in the world around us through a process of generation and corruption, and that generation and corruption itself was in turn dependent on the motion of the celestial bodies, then it came in conflict with the fundamental religious principle which in turn perceived the Aristotelian cosmological vision as the founding vision of astrological theory. The implications of such a conflict can be very serious indeed. For to think of human activity as directly influenced by the action of the celestial spheres, as some astrologers would in fact interpret Aristotle to say, meant that one could relieve the individual of his religious obligations, or at best relieve him of the consequences of his actions. It is in that context that astrology became the Achilles heel of Greek thought in general, and had a detrimental influence on the discipline of astronomy to which it was very closely related in the Greek tradition.
In order to avoid charges of not heeding religious precepts, astronomers working in the environment of Islamic civilization had two choices to make:
Either disregard the religious authorities and continue to associate their discipline with astrology as was done in the Greek sources before, or redefine the subject of their discipline as seeking to know the positions of the planets without having to make any comments as to the astrological significance of those positions. For those who took the second option, their discipline became then willfully restricted to the empirical pursuit of planetary positions just because the problem of the determination of such positions was in itself a challenge that needed to be met. Of course, they also opted to support their work with the religious pronouncement that urged man to study the natural phenomena as signs of God's creation and indicators of the existence of God himself.[267]
Whichever justification they used for their discipline, the net result remained the same: they attempted to construct mathematical models, that were true to cosmological presuppositions, in this instance Aristotelian presuppositions, and yet were capable of predicting the true positions of the planets. At the same time, they willfully avoided the religious and astrological implications of that Aristotelian cosmology. In essence, they trimmed Aristotle down to their needs.
Defined in this fashion, astronomy no longer looked like its Greek counterpart, although it resembled it in many varied ways. As far as the computational part, and the mathematical calculations that connected the observed phenomena to the predictive models, the two astronomies were more or less the same. The only difference was that the later astronomers in the Islamic domain benefited from the passage of time to correct the flawed astronomical parameters that were embodied in the Greek tradition. But the most important difference lied in the purpose of the two astronomies: The Greek tradition needed to determine the position of the planets so that it could predict their influence on the world of change in the sublunar region, while Islamic astronomy restricted itself to the same description of the behavior of the planets, with the utmost accuracy they could muster, and yet refrain from asking about the planet's influence on the sublunar region in general or the human behavior in particular. It is in this environment that the new discipline Him al-Hay'a (science of astronomy) was born. And as such, of course it had no Greek equivalent. Its authors were fully aware of that, and for that reason restricted themselves to calling it by its newly coined name, which meant literally "the science of the configuration [of the world]."
Once the purpose of astronomy was conceptually redefined, then the pursuit of astronomical research was fully condoned within Islamic civilization. This did not mean that astrology was finally excluded from the social domain. In fact there are plenty of sources that speak to the contrary and some even attest to its flourish and its specific widespread acceptance within the political circles where it continued to guide the actions of potentates and their cohorts by the dictates of the planetary positions. But expurgating astronomy from astrological practice meant that astronomy itself could flourish among the religious elite who saw in it a complementary discipline to their own, and thus felt at ease with it, especially when this specific new astronomy began to direct its attention to the critique of Greek astronomy. This critical feature of 'ilm al-hay'a marked the discipline from its very inception in the ninth century. In fact all the alternative planetary theories that we know from the Islamic domain were articulated in texts that identified themselves as hay'a texts. And since hay'a simply meant "configuration [of the world]", it meant that those texts were necessarily restricted to this descriptive aspect of astronomy, and never ventured as far as supplying actual tables that could be used for the actual determination of the positions of the planets as was done by the Almagest, for example. In that regard, the hay'a texts looked more like Ptolemy's Planetary Hypotheses than either the Almagest or the Handy Tables.
And because of this newfound purpose of astronomy it could afford to keep its distance from the ancient Greek tradition, and took the full freedom to subject the latter to the strictest criticism whenever criticism was found necessary. It was after all Muḥammad b. Mūsā b. Shākir (c. 850), one of the most zealous sponsors of the translations of Greek scientific texts, who offered to make sense of the Ptolemaic attempt to account for two basic motions: (1) the daily rotation of the eighth sphere that produced the variations of day and night and (2) the motion of precession which was most observed by the sliding position of the vernal equinox. Working from the Greek philosophical precept that all celestial motions are produced by specific movers, in this instance individual spheres, those two motions then had to be accounted for by two separate spheres, since it was inconceivable that the same sphere could move in two separate motions at the same time, while still in place. In order to resolve the problem, Ptolemy assigned the daily motion to the eighth sphere of the fixed stars, and then added another concentric ninth sphere to account for the precession motion. One could reverse the order and assign the precession to the sphere of the fixed stars and then ascribe the daily motion to the ninth sphere. The order was not the problem.
Rather, for Muḥammad b. Mūsā b. Shākir,[268] the problem lay in the fact that the two last spheres, which were supposed to carry those motions, were concentric. And that arrangement in particular presented an important physical problem. For how could any sphere move another, if both spheres were concentric, and if both spheres were made of the same element ether that did not allow such properties as friction, dragging, and the like? By understanding ether in the strict Aristotelian sense, in that it was a simple element that did not have any of the features of the sublunar elements like heaviness, lightness, etc., it was then impossible for two spheres, made of this same element, to force each other's motions if they shared the same center. Muḥammad b. Mūsā b. Shākir had no difficulty accounting for one sphere forcing another eccentric sphere to move along with it, for that did not require physical friction and the like. But to him, "it was in no way possible" to have a ninth sphere whose motion would necessitate the motion of the eighth. As far as we can tell, Muḥammad b. Mūsā b. Shākir had no real solution to this predicament, but he definitely had a real objection to the Ptolemaic arrangement. And his objection was strictly philosophical in that it depended completely on the definition of the element ether.
267
After all it was even Ptolemy himself who had stated in the introduction of the