Sunday, February 17, 2008

Arabian Astronomers

From our perspective just beyond the cusp of the millennium, it is easy to disparage Ptolemy for insisting that the earth stood at the center of the solar system. But we often forget that we live in a unique age, when images of the earth and other planets are routinely beamed from space. These stunning pictures of our cosmic neighborhood have become so familiar to us that commercial TV networks wouldn’t think of elbowing aside this or that sitcom to show the images to the viewing public.

Informed as we are with “the truth” about how the solar system works, we wonder how Ptolemy’s complicated explanation could have been accepted for so long. There is no doubt that his model of the solar system was wrong, but, wrong as he was, his book contained the heart and soul of classical astronomy and survived into an age that had turned its back on classical learning. During the early Middle Ages, Ptolemy’s work remained unread in Europe, but his principal book found its way into the Arab world, and in 820 it was translated into Arabic as Almagest (roughly translatable as The Greatest Book). The circulation of Ptolemy’s work renewed interest in astronomy throughout Arabia, with centers of learning being established in both Damascus and Baghdad.

Abu ‘Abd Allah Muhammad Ibn Jabir Ibn Sinan Al-battani Al-harrani As-sabi’, more conveniently known as al-Battani (ca. 858–929), became the most celebrated of the Arab astronomers, although it took many years before his major work, On Stellar Motion, was brought to Europe in Latin (about 1116) and in Spanish translation (in the thirteenth century). Al-Battani made important refinements to calculations of the length of the year and the seasons, as well as the annual precession of the equinoxes and the angle of the ecliptic. Moreover, he demonstrated that the Sun’s apogee (its farthest point from Earth) is variable, and he refined Ptolemy’s astronomical calculations by replacing geometry with sleek trigonometry.

Another Arab, Al-Sûfi (903–986), wrote a book translated as Uranographia (in essence, Writings of the Celestial Muse), in which he discussed the comparative brilliance of the stars. Like the scale of the Greek astronomer Hipparchos, the system of Al-Sûfi rated star brightness in orders of magnitude. Relative star brightness is still rated in terms of magnitude.

Arab astronomers like Al-Sûfi also contributed star maps and catalogues, which were so influential that many of the star names in use today are of Arab origin (such as Betelgeuse, Aldebaran, and Algol), as are such basic astronomical terms as azimuth and zenith.

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