This edition of Sacrobosco’s De sphaera and De computo, published in Wittenberg in 1549, contains numerous volvelles. Written or printed on parchment or paper, volvelles consisted of rotating parts stacked on a central pivot, so that they could be manipulated by readers according to instructions in the accompanying text. Volvelles were especially popular in medical and astronomical books, allowing readers to perform various hand-on exercises such as pedagogical demonstrations or even complex calculations. The earliest surviving examples come from manuscripts carrying the text of Ars Magna by the Spanish scholar Ramon Llull, who introduced the concept of the volvelle in Europe in 1305. The volvelle shown here illustrates the lunar eclipse. This copy was heavily annotated by a contemporary reader.