The Encyclopedia Americana (1920)/al-Battani

AL-BATTA'NI, Arab astronomer and mathematician: b. 850; d. 929. He began his astronomical observations at Rakka in 878 and continued them for over 40 years. In pure mathematics he also made important investigations. He used the sine of an angle instead of the chord of double the angle, computed a table of cotangents and formulated several propositions in spherical trigonometry. Plato of Tivoli translated his astronomical works under the title &lsquo;Mahometis Albatenii de Motu Stellarum.&rsquo; This work made him known to Europeans. This work was edited by C. A. Nallino in Arabic and Latin and published at Milan (1899). Al-Battani corrected numerous errors of astronomers; he gave the length of the tropical year as 365 days, 5 hours, 46 minutes, 24 seconds; too short by 2 minutes, 26 seconds, and he stated the obliquity of the ecliptic as 23° 35' instead of 23° 51' 20".