There are several features that distinguish the works of this remarkable astronomer who apparently spent his professional life working as a timekeeper at the central Umayyad mosque of Damascus. Although we do not know much about the details of his job description as a muwaqqit (timekeeper), his works, both the extant as well as the non-extant, lead us to assume that in his "spare time" he indeed managed to develop one of the most successful attempts at overhauling Greek astronomy. Not only did Ibn al-Shāṭir profit from the astronomers who preceded him with their own attempts to reform Greek astronomy, but he managed to produce some remarkable conceptual shifts of his own in the way astronomy was to be perceived and practiced.
To start with, Ibn al-Shāṭir went back to the very foundations of astronomy and insisted on resolving the very first problem of Greek astronomy: a choice between an eccentric and an epicyclic model. To him that choice was definitely limited, for one could not in any way justify the use of eccentrics. To his mind eccentrics represented a clear violation of the Aristotelian principle of the centrality of the Earth, which up till his time at least made perfect sense within the overall universal cosmology of Aristotle. On that count he insisted that all of his models would adhere to these principles, and would be strictly geocentric. Furthermore, all of his models also shunned the eccentrics completely.
That left him with the problem of epicycles, which, as we have seen, had to be used in all the other planets except the sun. On that round he had some very original remarks to make, and as far as I can tell he was the first, and probably the only one, to insist on making them. First he made the general observation that the sizes of some of the fixed stars were in fact much larger than the sizes of the largest planetary epicycles. Second, when it came to the nature of the epicycles themselves, he tossed the ball back to Aristotle's court and to the court of his followers. Aristotle and his followers had insisted that the epicycles were not permissible because they would introduce a center of heaviness around which the sphere of the epicycle would move, and thus constitute an element of composition in the celestial domain, which was supposed by Aristotle to have been fully made of the simple element ether. Here Ibn al-Shāṭir wondered how could that be, when everyone knew that the stars which were carried by spheres that were made of the same element ether emitted a light we can all see, while no such light was emitted by the spheres themselves? How could one part of the sphere, where the star was located, emit all that light, while the other part remain dark, or transparent, and still be called a simple sphere? How could the sphere and the star that it carried be both made of the same simple element ether and have such divergent appearances? Once it was admitted that such phenomena existed, and there was no way to deny something that everyone could verify for himself, it became obvious to Ibn al-Shāṭir that at least the lower celestial spheres of Aristotle, by which the stars and the planets were supposed to be moved, had to admit some kind of composition. Only the uppermost sphere, the one beyond the eighth sphere that was only responsible for the daily motion of the whole, but carried no stars of its own, that sphere could remain as simple as Aristotle would want it to be.
That reasoning allowed him to conclude that the composition introduced by the planetary epicycles should at least be as acceptable as the composition, which was already implied by the existence of the fixed stars and planets that everyone could obviously see in the skies.
Having "solved" the problem of the epicycles in this fashion, he then went ahead and systematically banished all eccentric circles from his models. Instead he substituted epicycles for the eccentricities in each case, in a manner very similar to the application of the Apollonius theorem where the epicyclic model could easily replace the eccentric one. As a result, he then managed to produce a set of models that were all unified by their strict geocentrism. And in order to achieve that throughout he used a combination of two well-known principles that had already been used: the Apollonius equation and 'Urḍī's Lemma. The latter allowed him to adjust for the Ptolemaic equant by adding yet another epicycle, which was used by 'Urḍī in the model for the upper planets.
And because all his models were geocentric and used the same two "principles" to solve the equant problem, they also managed to expunge from Ptolemaic astronomy the variety of approaches that were adopted by Ptolemy in his quadripartite model structures—different models for the Sun, the Moon, the upper planets, and Mercury. With the exception of the Mercury model, all the other models of Ibn al-Shāṭir had identical constructions but whose representation of the planetary motions were simply manipulated by the sizes and speeds of the various epicycles he had to deploy. In the case of Mercury, he only introduced an additional use of the Ṭūsī Couple at the last step, but continued to use in it the two other principles just mentioned. This procedure was also followed some two centuries later by Copernicus, and for the same planet: Mercury.
One additional advantage resulted from this systematic use of geocentricity, which was to come in handy later on during the European Renaissance: the unification of all the Ptolemaic geocentric models under one structure that lent itself to the simple shift of the centrality of the universe from the Earth to the sun, thus producing heliocentrism, without having to make any changes in the rest of the models that accounted well for the Ptolemaic observations resulting from the equant. As we shall see later on, it may not have been entirely accidental that Copernicus ended up relying so heavily on the works of Ibn al-Shāṭir when he used, among other things, a lunar model that was identical to that of Ibn al-Shāṭir, and used the same Ṭūsī Couple, in the same fashion as was done by Ibn al-Shāṭir, in order to account for the motion of Mercury.
And despite common legends that claim that Copernicus was attempting to get rid of the equant,[262] by adopting Ibn al-Shāṭir's techniques, and just shifting the direction of the line that connected the sun to the Earth, he could in fact retain the observational value of the equant, without having to assume, as was done by Ptolemy, the existence of a sphere that could move uniformly around an axis that did not pass through its center.
In addition to the flexibility of Ibn al-Shāṭir's models, and his full control of the mathematics that allowed him to adjust his models so that they would fit the observations, Ibn al-Shāṭir also made another unprecedented step. He was the only one of the astronomers in the Islamic domain who seems to have devoted a whole book (Ta'līq al-arṣād, meaning Accounting for Observations) to this particular relationship between observations and the construction of predictive models that could satisfy those observations. The book seems to be unfortunately lost, and thus we may never know the extent of his theorizing in this regard. But it is extremely significant that he did undertake the writing of such a book.
And even if we have to assume that Ta'līq al-arṣād is lost to us, we still have some inkling about the methods and contents of that book from a few instances where such approaches have been followed in his surviving works, and in particular, in his Nihāyat al-sūl fī taṣḥīḥ al-uṣūl (The Final Quest Regarding the Rectification of [Astronomical] Principles). In this last book, we are explicitly told that Ibn al-Shāṭir had conducted his own observations in order to determine the apparent sizes of the two luminaries.[263] And we are also told that with those new results, which varied considerably from the values given by Ptolemy, he managed to construct a new model for the sun which was also at variance with the Ptolemaic model. In essence, this work demonstrates quite clearly Ibn al-Shāṭir's ability to construct theoretical models that were based on observational results, just as was done by Ptolemy, but without committing the inconsistencies of Ptolemy. It is in such instances that the centrality of Ibn al-Shāṭir's work can best be appreciated, and his relationship to Copernican astronomy can be better understood.
262
This legend was elegantly stated and rebutted by Otto Neugebauer in 1968 ("On the Planetary Theory of Copernicus",
263
See G. Saliba, "Theory and Observation in Islamic Astronomy: The work of Ibn al-Shāṭir of Damascus",