Reference Work Entry

The Biographical Encyclopedia of Astronomers

pp 567-567

Ibn Sahl: Abū Saҁd al‐ҁAlā' ibn Sahl

  • Len Berggren

Flourishedlate 10th century

Ibn Sahl was a geometer who worked in the late 10th century. Although he is not mentioned in the known biobibliographical sources from the medieval period, Ibn Sahl is mentioned by Ibn al‐Haytham, whose working life spanned the late 10th and early 11th centuries. On the other hand, he commented on one of Abū Sahl al‐Kūhī's treatises, and Kūhī probably died before the end of the 10th century.

His two works most relevant to the history of astronomy are his Proof that the Vault of the Heavens Is Not Completely Transparent and his commentary on Abū Sahl al‐Kūhī's treatise on the astrolabe. In the former he gives, inspired by his study of the fifth book of Ptolemy's Optics, a proof that whatever substance one is given, such as that composing the heavenly spheres of Aristotelian cosmology, it is always possible to find a substance that refracts light less. Ibn Sahl agrees with Aristotle, however, that the heavenly spheres are indeed more transpar ...

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