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Astronomy

Of the 146 books written by al-Bīrūnī, 95 are devoted to astronomy, mathematics, and related subjects like mathematical geography. He lived during the Islamic Golden Age, when the Abbasid Caliphs promoted astronomical research, because such research possessed not only a scientific but also a religious dimension: in Islam worship and prayer require a knowledge of the precise directions of sacred locations, which can be determined accurately only through the use of astronomical data.

In carrying out his research, al-Biruni used a variety of different techniques dependent upon the particular field of study involved.

His major work on astrology is primarily an astronomical and mathematical text; he states: "I have begun with Geometry and proceeded to Arithmetic and the Science of Numbers, then to the structure of the Universe and finally to Judicial Astrology, for no one who is worthy of the style and title of Astrologer who is not thoroughly conversant with these for sciences." In these earlier chapters he lays the foundations for the final chapter, on astrological prognostication, which he criticises. In a later work, he wrote a refutation of astrology, in contradistinction to the legitimate science of astronomy, for which he expresses wholehearted support. Some suggest that his reasons for refuting astrology relate to the methods used by astrologers being based upon pseudoscience rather than empiricism and also to a conflict between the views of the astrologers and those of the orthodox theologians of Sunni Islam.

He wrote an extensive commentary on Indian astronomy in the Taḥqīq mā li-l-Hind mostly translation of Aryabhatta's work, in which he claims to have resolved the matter of Earth's rotation in a work on astronomy that is no longer extant, his Miftah-ilm-alhai'a (" Key to Astronomy "):

he rotation of the earth does in no way impair the value of astronomy, as all appearances of an astronomic character can quite as well be explained according to this theory as to the other. There are, however, other reasons which make it impossible. This question is most difficult to solve. The most prominent of both modern and ancient astronomers have deeply studied the question of the moving of the earth, and tried to refute it. We, too, have composed a book on the subject called Miftah-ilm-alhai'a (Key to Astronomy), in which we think we have surpassed our predecessors, if not in the words, at all events in the matter.

In his major astronomical work, the Mas'ud Canon, Biruni observed that, contrary to Ptolemy, the Sun's apogee (highest point in the heavens) was mobile, not fixed. He wrote a treatise on the astrolabe, describing how to use it to tell the time and as a quadrant for surveying. One particular diagram of an eight-geared device could be considered an ancestor of later Muslim astrolabes and clocks. More recently, Biruni's eclipse data was used by Dunthorne in 1749 to help determine the acceleration of the Moon, and his data on equinox times and eclipses was used as part of a study of Earth's past rotation.