ABU ISHAQ IBRAHIM IBN YAHYA AL-ZARQALI (Arzachel)
(1028 - 1087 C.E.)
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.