The New International Encyclopædia/al-Battani, Mohammed ibn Jabir ibn Sinan
AL-BATTA'NI, Mohammed ibn Jabir ibn Sinan, known as Albategnius (so called from Battan in Mesopotamia) (c.850-929). An Arab chief, one of the most famous astronomers and mathematicians of his race. His first astronomical observations were made at Rakka (877-878), and extended over a period of more than forty years. He also made several important contributions to pure mathematics. He used the sine of an angle in place of the chord of double the angle (an idea that had occurred to Aryabhatta), computed a table of cotangents, and formulated certain propositions in spherical trigonometry. His astronomical works were first made generally known to European scholars through a translation by Plato of Tivoli, under the title Mahometis Albatenii de Motu (or Scientia) Astrorum. This work has been edited in Arabic and Latin by C. A. Nallino (Milan, 1899). Al-Battani corrected numerous errors of Ptolemy, whom, in general, he followed: e.g. he gave the obliquity of the ecliptic as 23° 35' instead of 23° 51' 20". He also gave the length of the tropical year as 365 days, 5 hours, 46 minutes, 24 seconds; too short by 2 minutes, 26 seconds, but an improvement upon that of Hipparchus, who gave 365¼ days - 1/300 day, which was too short by 4 minutes, 48 seconds.