Ptolemy (Claudius Ptolemaeus, 87–150 AD) was an astronomer, geographer, and astrologer. He was a late representative of the Hellenistic civilization that had its capital in Alexandria and blossomed during the last three centuries BC, fading away during the first two centuries AD. Ptolemy's main work is the Almagest, in which is compiled all the then existing astronomical knowledge. In Ptolemy's cosmology the Earth is the center of a spherically shaped universe, and the Sun, Moon and the five planets known at the time (Mercury, Venus, Mars, Jupiter, and Saturn) orbit the Earth.
The Babylonians had already observed as early as 700 BC that the planets show retrograde motions. For instance Mars, as observed from Earth, seems to move almost always to the east, but every two years (more precisely every 25.7 months) it seems to move to the west. This retrograde motion of Mars lasts for more than a month. Because Ptolemy was obviously aware of this, he could not use a model in which all heavenly bodies orbit the Earth uniformly. Following Hipparchus (ca. 190–120 BC) he assumed that the bodies made small circular motions (epicycles) and that the centers of the epicycle move in a circular orbit (the deferent) around the Earth. With this model he was able to predict the positions of the planets in reasonable agreement with observations. His predictions were only improved, almost 15 centuries later, by Johannes Kepler (ca. 1610 AD), who used the heliocentric model of Nicolaus Copernicus together with elliptical orbits.
As a geographer Ptolemy is mainly known for his book Geographia, in which he described the world and its inhabitants as far as they were known in his time. Ptolemy knew that the Earth was round and for his maps he used a sphere projected on to a plane surface. Following Eratosthenes (ca. 284–202 BC) and Posidonius (135–51 BC) he estimated the circumference of the Earth. In hindsight we know that Eratosthenes was close to the modern value and that Posidonius' estimate was about 30% lower. Ptolemy took the latter value to be the more reliable one and because of Ptolemy's enormous prestige, Christopher Columbus, who used it as his primary mapping source in pursuit of discovering an Atlantic route to Asia, also believed it. It can be speculated that Columbus would not have dared to explore the western route to China and India had he believed the larger Eratosthenes value.
The Almagest and the Geographia had an enormous influence up until the Renaissance. After Copernicus and the great discoveries of the fifteenth and sixteenth centuries the influence of these books waned. Now they are only of historical significance. This is not true of Ptolemy's third book, the Tetrabiblos, on astrology, which is still of relevance today, provided we correct for the fact that Ptolemy lived in the age of Aries, while we are now in the age of Pisces.