Abu Ishaq Ibrahim ibn Yahya al-Zarqali, known in the West as Arzachel, was a Spanish Arab. He was the foremost astronomer of his time.

Al-Zarqali carried out a series of astronomical observations at Toledo (Arabic Al Tulaytalah) and compiled them in what is known as his famous Toledan Tables. He corrected the geographical data from Ptolemy and Al-Khwarizmi. Specifically, Al-Zarqali corrected Ptolemy’s estimate of the length of the Mediterranean sea from 62 degrees to approximately correct value of 42 degrees. The Toledo Tables were translated into Latin in the Twelfth century.

Al-Zarqali was the first to prove conclusively the motion of the Aphelion relative to the stars. He measured its rate of motion as 12.04 seconds per year, which is remarkably close to the modern calculation of 11.8 seconds. Al-Zarqali invented a flat astrolabe which is known as Safihah. Its details were published in Latin, Hebrew and several European languages.

Copernicus in his famous book ‘De Revolutionibus Orbium Clestium‘ expresses his indebtedness to al-Battani (albategnius) and Al-Zarqali (Arzachel) and quotes their work several times. Beer and Madler in their famous work Der Mond (1837) mention a surface feature of the moon after Al-Zarqali (Arzachel). It is a plain in the eighth section more than sixty miles in diameter and is surrounded by rows of mountains rising like terraces to heights of 13,000 feet above the interior region. It also includes several hills and craters and a prominent cleft by the side of the base of the western mountainous wall.