The name "Almagest" is an English spelling of the Arabic, al-majisti, which was, a rough translation of the original Greek title, He Megale Syntaxis (The Great Treatise).
Ptolemy's most famous work contained thirteen sections, called "books." These contained an impressive quantity of data including a catalog of more than a thousand stars, their positions and magnitudes (a system still used in modified form by today's astronomers), the motions of the planets, and even Hipparchus' legendary discovery of precession of the equinoxes.
Based on the philosophy and writings of earlier Greeks, the Ptolemy's ideas held that Earth was at the center of everything and that all other celestial objects revolved around it. All celestial motions were thought to be those of perfect circles. The ancient Greeks were heavily influenced by shapes in their most perfect or simplest form.
For over a thousand years, the Ptolemaic ideas were the basis of all astronomical science. The highly influential Catholic Church, during the Middle Ages and early Renaissance, supported Ptolemy's work, because it seemed only natural that God would place the home of his children in the center of everything. The Ptolemaic universe supported that belief.