What was the main subject of Ptolemy's significant work in astronomy?

Ptolemy's significant work in astronomy centered around the geocentric theory, which posits that the Earth is at the center of the universe and that all celestial bodies, including the sun and stars, revolve around it. This model was foundational for many centuries and influenced not only astronomy but also the philosophical understanding of the cosmos during the medieval period. His most famous work, the "Almagest," outlined this theory in detail and provided mathematical models for predicting the movements of celestial bodies based on the geocentric perspective. This understanding shaped astronomical thought until the heliocentric model proposed by Copernicus and later developments in astronomy. The other options—prose, poetry, and medicine—are not related to Ptolemy's contributions to astronomy, as his focus was specifically on the celestial mechanics and positioning of heavenly bodies within the framework of a geocentric universe.