(1028–1087), Latinized as Arzachel, was a leading Arab mathematician and the foremost astronomer of his time. He flourished in Toledo in Castile, Al-Andalus (now Spain). The Arzachel crater on the Moon is named after him. Astronomy Instruments Combining theoretical knowledge with technical skill, he excelled at the construction of precision instruments for astronomical use. He constructed a flat astrolabe that was 'universal,' for it could be used at any latitude, and he built a water clock capable of determining the hours of the day and night and indicating the days of the lunar months. Al-Zarq?l? 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