# The Encyclopedia Americana (1920)/al-Battani

**AL-BATTA'NI** (Mohammed ibn Jabir ibn
Sinan), 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 ‘Mahometis
Albatenii de Motu Stellarum.’ 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".