Ptolemy’s lunar theory and the determination of the Moon’s position and its distance to Earth

The purpose of this paper is to present Ptolemy’s lunar theory in the Almagest. In order to describe the complex Moon’s motion, he proposes an intricate system formed by mathematical objects common to astronomers of his time, such as epicycles, deferents and eccentrics. From his lunar theory, Ptolemy determined with good precision the ecliptic longitude and latitude of the Moon relative to the center of the Earth. He also set out to determine the distance from Earth to the Moon, having first to determine the lunar parallax. Although he was relatively successful in describing the Moon’s position, his calculations of the distances from Earth to the Moon were not correct, mainly because his lunar model was not a good representation of the Moon’s motion.

Keywords:
Ptolemy; Almagest; ecliptic latitude; ecliptic longitude; parallax; Moon