Ahmad ibn 'Abdallah al-Marwazi,[1][2] known as Habash al-Hasib (Persian: حبش الحاسب, lit.
'Abyssinian calculator',[1][2][3] died c. 869[4]) was a Persian[5][2] astronomer,[6] geographer, and mathematician from Merv in Khorasan, who was the first to describe the trigonometric ratios tangent, and cotangent.
[3] Habash al-Hasib flourished in Baghdad, and died a centenarian some time between 864 and 874[2][3][7] possibly in Abbasid Samarra.
Apropos of the solar eclipse of 829, Habash gives us the first instance of a determination of time by an altitude (in this case, of the sun); a method which was generally adopted by Muslim astronomers.
[10][11] Habash al-Hasib conducted various observations at the Al-Shammisiyyah observatory in Baghdad and estimated a number of geographic and astronomical values.