Arzachel (Al-Zarqali, Abu Ishaq Ibrahim ibn Yahya) (1028–1087)
Arzachel was the foremost Spanish Arab astronomer of his time. He carried out a series of observations at Toledo and compiled them as his famous Toledan Tables, correcting geographical data from Ptolemy and Al-Khwarizmi. The Tables were translated into Latin in the twelfth century. Arzachel was the first to prove conclusively the motion of the aphelion relative to the stars, measuring it as 12.04" per year – remarkably close to the modern value of 11.8". He also invented a flat astrolabe, known as a safihah, details of which were published in Latin, Hebrew, and several European languages. Copernicus, in his De Revolutionibus Orbium Clestium expresses his indebtedness to Arzachel and Albategnius and quotes their work several times.