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For other meanings, see Arzachel (disambiguation)

Al-Zarqali (In Arabic أبو أسحاق ابراهيم بن يحيى الزرقالي ),(in full Abu Ishaq Ibrahim ibn Yahya Al-Zarqali, rendered as Arzachel in Latin Europe; Azarquiel in Spanish and Italian), (1028–1087 CE), was a leading Arab mathematician and the foremost astronomer of his time. He flourished in Toledo in Andalusia, now Spain. 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-Zarqali corrected Ptolemy's geographical data, specifically the length of the Mediterranean Sea. He was the first to prove conclusively the motion of the aphelion relative to the fixed background of 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. He also contributed to the famous Tables of Toledo, a compilation of astronomical data of unprecedented accuracy. Arzachel was famous as well for his own Book of Tables. Many "books of tables" had been compiled, but his almanac (even our word is Arabic) contained tables which allowed one to find the days on which the Coptic, Roman, lunar, and Persian months begin, other tables which give the position of planets at any given time, and still others facilitating the prediction of solar and lunar eclipses. He also compiled valuable tables of latitude and longitude.

His work was translated into Latin by Gerard of Cremona in the 12th Century, and contributed to the rebirth of a mathematically-based astronomy in Christian Europe. Four centuries later, Copernicus mentioned his indebtedness to Al-Zarqali and quoted him, in the book that gave new meanings to 'revolution,' De Revolutionibus Orbium Coelestium.

Arzachel crater on the Moon is named after him.

• Islamic scholars
• List of Arab scientists and scholars

• A Zahoor's capsule biography of Al-Zarqali
• 'Transmission of Muslim astronomy to Europe'