Levi ben Gerson's Preliminary Remarks for a Theory of Planetary Latitudes

Levi ben Gerson (Gersonides, d. 1344) was an outstanding medieval astronomer who sought to improve Ptolemy's account of planetary motion both in longitude and latitude. In this article we consider the models he proposed for planetary latitudes. These geometric models are difficult to interpret and the focus here is on the non-technical discussions that surround the descriptions of them, including text, translation, and commentary on chapters 123, 126, and 127 of Levi's Astronomy. One of his themes is that Ptolemy's models are faulty although the data on which they were based are satisfactory. Levi drew on passages in Ptolemy's Almagest and book ii of his Planetary Hypotheses, and commented on them. He noted that, in contrast to Ptolemy's practice in the Almagest of citing dated observations for establishing his theories of planetary longitudes, Ptolemy reported no such dated observations for planetary latitudes. Levi, however, made many dated observations of planetary latitude and they are recorded in chapter 122; but, surprisingly, they are not used at all in the subsequent chapters. Unfortunately, no tables corresponding to Levi's theory of planetary latitudes are extant.