In University study from the 13th through 16th centuries, the most common introduction to the geocentric cosmos was the medieval work, On the Sphere, by Sacrobosco. In one illustration, the Sun revolves around the Earth following an off-center or “eccentric” path within its solid celestial sphere. This printed edition also includes the New Theory of the Planets by Peurbach, which became the standard introduction to Ptolemaic planetary astronomy in the 15th century.