Ibn Saʿīd (Sid), Isaac

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IBN SAʿĪD (Sid ), ISAAC (c. second half of the 13th century), Spanish astronomer. Isaac lived in Toledo where Isaac *Israeli called him "he-Ḥazzan" so he was probably the cantor of the synagogue at Toledo. He may well have been the otherwise unknown "Aben Çayd" or "Rabbi Zag," and King Alphonso x refers to him in official documents as "our learned Rabbi Çag." At the king's request Isaac, together with Judah ben Moses Cohen, headed a group of Jewish scholars, at the initiation and production of the "Alfonsine Tables" (1252–56), which King Alfonso of Spain commissioned to be compiled at Toledo, since discrepancies were often being found between astronomical computations and observations in his day. Consequently Isaac and his assistants not only utilized the tables of Arzarchel but also conducted their own observations of the sun, stars, and planets over some years. Their tables finally included a complicated armillary sphere showing the heavenly movements (see also *Astronomy). Alfonso also encouraged his translation from the Arabic into Spanish of several treatises concerned with astronomical instruments. Isaac ibn Saʿīd added the instructions for their correct use in observations. These were partly his own original contributions, and partly taken from other sources. His works are contained in the monumental book Libros del Saber de Astronomia del Rey Don Alfonso x de Castilla (5 vols. by Rico y Sinobas in Madrid, 1863–67). Certain records in his handwriting of his observations of lunar eclipses still in existence testify to his versatility.


Steinschneider, Uebersetzungen, 617, 975–6; Singer, in: L. Finkelstein (ed.), The Jews, their History, Culture and Religion, 2 (19603), 1394–95; G. Sarton, Introduction to the History of Science, 2 (1931), 843–4; E.R. Bevan and C. Singer, Legacy of Israel (1928), 224–5.

[Arthur Beer]

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Ibn Saʿīd (Sid), Isaac

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