Arzachel
Abu Ishaq Ibrahim ibn Yahya al-Zarqali
Abu Ishaq Ibrahim ibn Yahya al-Zarqali known in the West as Arzachel, was a Spanish Arab. He was a leading Arab mathematician and the foremost astronomer of his time. He flourished in Toledo in Castile, now Spain.
Arzachel crater on the Moon is named after him.
Al-Zarqali carried out a series of astronomical observations at Toledo (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. Combining theoretical knowledge with technical skill, he excelled at the construction of precision instruments for astronomical use.
He constructed a flat astrolabe – known as Safihah - that was 'universal,' for it could be used at any latitude, its details were published in Latin, Hebrew and several European languages. Also, he built a water clock capable of determining the hours of the day and night and indicating the days of the lunar months.
Al-Zarqali also wrote a treatise on the construction of an instrument (an equatorium) for computing the position of the planets using diagrams of the Ptolemaic model. This work was translated into Spanish in the 13th century by order of King Alfonso X in a section of the Libros del Saber de Astronomia entitled the "Libros de los laminas de los vii planetas."
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.
By Rania ElSaadawy