Claudius Ptolemy was a Roman astronomer who lived in Alexandria, Egypt. During this period in history Alexandria was the center of the widespread Greek culture. He is most well known for the book he wrote explaining the motions of the planets and stars, entitled the "Almagest". Ptolemy was very influenced by the ancient Greeks and drew on many of their ideas about the universe to develop his own theory to explain the strange movements of the heavens.
Ptolemaic System of the Universe
Ptolemy embraced the generally accepted theory that the Earth did not move and was located at the center of the universe. Because the Greek culture believed in the perfection of nature Ptolemy wrote that the planets and stars moved continuously in perfectly circular orbits. He then elaborated on these traditional views in an attempt of explain the astronomical puzzles this presented, like the apparent backwards motion of the planets and the variations in the size and brightness of the moon and the planets. Ptolemy proposed that the planets, Sun, and Moon moved is small circles while traveling in their much larger orbits around the fixed Earth. These small circles were called epicycles. In order to make his system fit most of the observations that astronomers had recorded he departed from traditional mathematics and explained that these bodies had varied speed and the epicycles had different diameters.
The Ptolemaic System of the universe thus gave an acceptable description of planetary motion. Ptolemy was the last of the great ancient astronomers. His description of the universe lasted more than 1000 years until Nicolaus Copernicus rejected this idea and developed his own heliocentric system. Even though the Copernican System of the planets correctly stated that the Sun was located at the center of the solar system it still retained an elaborate system of epicycles.
Besides this very influential work, Ptolemy also cataloged 1028 stars and described all the known variances of the constellations and the legends that went along with them. He gave these formations their traditional Greek and Roman names which are still used today. Besides being an astronomer Ptolemy was a renowned cartographer. His book the "Geographia" contained detailed maps of the world, that served travelers for hundreds of years. Even Christopher Columbus used his maps in 1492 when he discovered the Americas.