ALBATEGNI, an Arabian astronomer, whose proper name is Mohammed Ib,c Jdbir Ibn Se do derived this appellation from Batan in Mesopotamia, his native town, of which he is said to have been chief. His astronomical observations extended from 877 A.D. to his death in 929, and were principally conducted at Rakkah or Aracta, on the Euphrates, and at Antioch in Syria. His principal work, Zidje Sabi, the original MS. of which is in the Vatican, was published in a Latin translation by Plato Tiburtinus at Nuremberg in 1537, under the title De Seientia Stellarum, and reprinted at Bologna in 1645. Among the unpublished works of Albategni are commentaries on the Almagest and Mahalat of Ptolemy, and a Treatise on Astronomy and Geography. Instead of the tables of Ptolemy, which were imperfect, he computed new ones; these were adapted to the meridian of Rakkah, and were long used as the best among the Arabs. Albategni gives the motion of the sun's apogee since Ptolemy's time, as well as the motion of the stars, which he estimated at 1° in 70 years. He makes the obliquity of the ecliptic 23° 35'. His determination of the length of the tropical year is more exact than that of Ptolemy, being only 2m. 26s. short. Upon his observations were founded the Alphonsine tables of the moon's motion. He first substitilted sines for chords, and also introduced into trigonometry the use of tangents and versed sines. On account of his discoveries, the chief of which is the motion of the sun's apogee, he has been called the Arabian Ptolemy, and has been placed by some at the head of Arabian astronomers.