Al-Zarqali

Al-Zarqali

Zerkali, or by his full name Ebu İshak İbrahim el-Zerkali (in Latin, Arzachel) (1029 - 1087), was an astronomer of Arab origin from al-Andalus.

He had an observatory built in Toledo and collected the work he carried out there between 1061 and 1087 in a book. This book would later lead the way to the production of the works known as the Alfonsine Tables.

Al-Zarqali was one of the famous astronomy scholars trained in al-Andalus. His name was İbrahim bin Yahya et-Tecibi en-Nekkaş, and his kunya was Ebu İshak. He became famous as Zerkali. He was born in 1029 in the city of Toledo. He began to acquire knowledge at a young age. Learning the religious and natural sciences in a short time, al-Zarqali became an authority in the science of astronomy. He carried out most of his astronomical studies and observations in Toledo. Toward the end of his life he settled in Córdoba and died there in 1087.

Al-Zarqali was the first, contrary to Ptolemy, to discover and formulate into a law the true orbital-point motion of the earth and that the equant center of the sun depends on a secular change. Ptolemy, however, had accepted the orbital point of the solar system as fixed and the equant center as unchanging. By giving the orbital point of the sun a correct direction of about 12 seconds, that is, a change from east to west, he put forward a new theory for the sun and also eliminated the irregularity at the equant center.

In Ptolemaic theory, the apogee, the sun's farthest position from the earth, was accepted as stationary, and the differing data reported by observations were explained by observational errors. In the Islamic world, Sâbit bin Kurre (Thabit ibn Qurra) doubted this view but could not put in its place another, more satisfactory, view. Al-Zarqali, however, argued that the apogee point is not stationary and that it shifts from West to East at an angle of 12 seconds per year; according to him, it is possible to explain this displacement by assuming that the center of the sun's orbit revolves along a circle. Thus al-Zarqali caused the doubts about the correctness of Ptolemaic theory to grow stronger.

Al-Zarqali compiled the first astronomical tables, famous under the name Toledo. These tables, concerning the movements of the sun, the planets, and the other stars, soon came to be used throughout Europe. Making his examinations with patience and care, al-Zarqali calculated the distance of the earth from the sun. In addition, in order to reconcile this distance with the solstices based on the solar orbit and the precession of the equinoxes, he made more than 402 observations in Toledo, and likewise calculated the magnitude of the precession correctly in the same manner.

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