<p style='text-align: justify;'>Although <i>Theorica planetarum</i> is a great classic in the history of astronomy, its author and date of composition are still controversial. The attribution to the twelfth-century translator and scholar Gerardus Cremonensis (c. 1114-1187) was challenged at the end of the nineteenth century by B. Boncampagni, who proposed as its author the thirteenth-century astrologer Gerardus of Sabbioneta. More recently, R. Lemay and G. Federici Vescovini have defended the old attribution to Gerardus Cremonensis, whereas O. Pedersen ascribed the work to an unknown early thirteenth-century author. <i>Theorica planetarum</i> was the standard elementary introduction to theoretical astronomy in the late Middle Ages. In several manuscripts as well as printed editions it was accompanied by other texts, which varied according to the location, forming a corpus of treatises used for university teaching for many centuries. The edition from which this image is taken was bound with <i>Johannis de Sacrobusto anglici viri clarissimi spera mundi</i>. The first edition of <i>Theorica planetarum</i> appeared in 1472 at Ferrara and was printed by Piero Buono Avogarium Ferrariensis. Notice that the stationary points of the planets in this diagram are, incorrectly, determined by drawing tangents from the epicycle to the centre of the World. For a diagram that correctly explains retrograde motion and the stationary points of planets, see Erhard Ratdolt's 1482 edition.</p>
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