The Theory of the Planets
After the Sphaera mundi, the most popular astronomical manual from the thirteenth century onward was the Theorica planetarum, whose text can be found in hundreds of manuscripts and early printed books. It was often placed after Sacrobosco’s Sphaera like in this edition published in Venice in 1478. In short, this work displayed mathematical models that summarized Ptolemy’s theory of planetary motion, starting with the Sun and the Moon, and continuing with each of the five planets. These mathematical models allowed the student to calculate the positions of a planet as it moved along the belt of the constellations of the zodiac in a longitude motion. Unfortunately, neither the author nor the date of this treatise can be determined with certainty. While most of the manuscripts are anonymous, a few of them attribute this work to a dozen scholars, including the prolific translator Gerald of Cremona (d. 1187). In fact, in many early printed editions, like the one shown here, Gerald of Cremona is explicitly acknowledged as the author of the Theorica. Nevertheless, despite the attempts to identify the author and the date of composition, the safest hypothesis is that the Theorica planetarum was written in the thirteenth century by an unidentified teacher of astronomy who wanted to improve the poor treatment of planetary theory in the fourth book of the Sphaera.
The woodcut shown here depicts the various motions of the Moon, the subject of the second chapter of the book. We can see the Moon’s epicycle, or orbit of revolution, drawn as a small circle whose center, the text explains, moves about 13 degrees per day from west to east upon the circumference of the eccentric. Whereas the eccentric circle of the Sun is immovable, the eccentric circle of the Moon moves every day about 11 degrees from east to west.
- “Anonymous: The Theory of the Planets.” Trans. Olaf Pedersen. In A Source in Medieval Science. Ed. Edward Grant, 451-465. Cambridge, Mass.: Harvard University Press, 1974.
- Pedersen, Olaf. 1981. “The Origin of the ‘Theorica Planetarum.’” Journal for the History of Astronomy 12 (2): 113-123.