This science studies the motions of the fixed stars and the planets. From the manner in which these motions take place, astronomy deduces by geometrical methods the existence of certain shapes and positions of the spheres requiring the occurrence of those motions which can be perceived by the senses. Astronomy thus proves, for instance, by the existence of the precession of the equinoxes, that the center of the earth is not identical with the center of the sphere of the sun. Furthermore, from the retrograde and direct motions of the stars, astronomy deduces the existence of small spheres (epicycles) carrying the (stars) and moving inside their great spheres. Through the motion of the fixed stars, astronomy then proves the existence of the eighth sphere. It also proves that a single star has a number of spheres, from the (observation) that it has a number of declinations,669 and similar things.
Only astronomical observation can show the existing motions and how they take place, and their various types. It is only by this means that we know the precession of the equinoxes and the order of the spheres in their different layers as well as the retrograde and direct motions (of the stars), and similar things. The Greeks occupied themselves very much with astronomical observation. They used instruments that were invented for the observation of the motion of a given star. They called them astrolabes. The technique and theory of how to make them, so that their motion conforms to the motion of the sphere, are a (living) tradition among the people.670
In Islam, only very little attention has been paid to astronomical observation. In the days of al-Ma'mun, there was some interest in it. The construction of the instrument known as the astrolabe was begun but was not completed.671 When al-Ma'mun died, the institution of astronomical observation was lost and neglected. Later on, (scholars) based themselves upon the ancient observations. These were of no use because of the change in the motions (of the stars) over the course of time. The motion of the instrument used in astronomical observations conforms only approximately to the motion of the spheres and the stars and is not absolutely exact. When a certain amount of time has elapsed, the differences are revealed.
Astronomy is a noble craft. It does not, as is generally thought, teach the real form of the heavens nor the order of the spheres, but it teaches that the forms and shapes of the spheres are the result of those motions. As one knows, it is not improbable that one and the same thing may produce two different results. Therefore, when we say that the motions produce a result, we (merely) deduce from the (existence of) what produces the result that the result exists, but the statement in no way teaches (us) the real character (of the resulting thing). Still, astronomy is an important science. It is one of the pillars of the mathematical disciplines.672
One of the best works on the subject is the Majisti (Almagest).673 It is ascribed to Ptolemy, who, as the commentators of the work have established, was not one of the Greek rulers called Ptolemy. The leading Muslim philosophers have abridged Ptolemy's work. Avicenna, for instance, did that. He inserted his abridgment in the mathematical section of the Shifa'. Ptolemy's work was also abbreviated by the Spanish philosopher Averroes, by Ibn as-Samh,674 and by Ibn as-Salt, 675 in the Kitdb al-Iqtisar. Ibn alFarghani 676 wrote an abridged astronomy. In it, he treated the subject along more easily understandable lines. He omitted the geometrical proofs.
"God taught man what he did not know." 677
Astronomical tables 678
A subdivision of astronomy is the science of astronomical tables. This is a craft based upon calculations according to arithmetical rules. It is concerned with the courses of motions peculiar to each star and with the character of that motion, fast, slow, direct, retrograde, and so on, as proven by astronomical means. This serves to show the positions of the stars in their spheres at any given time, by calculating their motions according to the rules evolved from astronomical works.
This craft follows certain norms. They constitute a sort of introductory and basic material for it. They deal with months and days and past eras. 679
It (further) follows established basic principles. They deal with apogee and perigee, declinations, 680 the different kinds of motions, and how (these things) shed light upon each other. They are written down in well-arranged tables, in order to make it easy for students. These tables are called zij. The determination of the positions of the stars at a given time by means of this craft is called "adjustment and tabulation." 681
Recent contemporary Maghribi scholars are using, as their reference work, the zij that is ascribed to Ibn Ishaq. 684 It is thought that Ibn Ishaq based his work on astronomical observations. A Jew in Sicily who was skilled in astronomy and the mathematical sciences, and who occupied himself with astronomical observation, sent (Ibn Ishaq) information on the conditions and motions of the stars he had ascertained. Thus, the Maghribis have been using Ibn Ishaq's work, because they assume that it is based upon reliable information.
Ibn al-Banna, 685 wrote an abridgment (of Ibn Ishaq's Zij) which he entitled al-Minhaj. People have been very eager to use the Minhaj, because the operations described in it are easy.
(Knowledge of the) positions of the stars in the spheres is the necessary basis for astrological judgments, that is, knowledge of the various kinds of influence over the world of man that are exercised by the stars depending on their positions and that affect religious groups, dynasties, human activities, and all events. We shall explain this later on, and we shall clarify the evidence adduced by astrologers, if God, He is exalted, wills.