Rasā’il Ikhwān al-Ṣafā’ (The Epistles of the Brethren of Purity). Composed circa 980, it contains 51 epistles (a fifty-second considered spurious) and is grouped into Four Sections: (I) mathematical sciences; (II) physical sciences; (III) psychological and intellectual sciences; and (IV) metaphysical and legal sciences. Astronomy is the subject of the third, sixteenth, and thirty-sixth epistles. The sixteenth epistle (the second of Section II), treats “The Sky and the World” (al-samā’ wa-’l-`ālam/de caelo et de mundo). It is mainly based upon the Ptolemaic system and Aristotelian physics with eleven spheres/orbs including the Earth and the atmosphere. In addition, the diameters of the planets are given. [Živa Vesel, BEA]