Ptolemy was the last great astronomer of the Alexandrian school in Egypt. He also was a prolific writer in science, best known for his book, the Almagest. He wrote the Almagest, originally named Syntaxis mathematica (“mathematical collection”), during the second century CE. The Arabs were so taken with this book that they began to call it Al magiste (“the greatest”), later corrupted to Almagest.

The book is a full description of all that was known in astronomy around Ptolemy’s time and is a synthesis of Ptolemy’s ideas and of those of other Greek scholars, such as Aristotle, Pythagoras, Apollonius of Perga, and Hipparchus. These scholars believed that Earth is the center of the universe. Ptolemy also included a catalog of stars compiled by Hipparchus in 130 BCE.

The most important part of the Almagest is its description of the Ptolemaic system, a model of planetary motion in which Earth is the center of the universe and the Sun and Moon move around Earth in perfect circles. Because the planets seem to move backward some of the time, however, their observed motion cannot be explained by single circles. Ptolemy adopted a solution to this problem that he attributes to Apollonius (although earlier Greek writers, such as Hipparchus, also used this concept): Each planet moves on a small circle, called an epicycle. The epicycle has as its center a point called the deferent, and the deferent itself moves on a perfect circle around Earth.

As seen from Earth, the planet moves faster when its path about the deferent is advancing and, because the orbit about the deferent takes less time than the orbit about Earth, the planet first slows and then moves backward on the other half of the trip around the deferent. Although these complex motions seem strange to those familiar with modern astronomy, they succeed in accounting for observed motions.