Ptolemy

Ptolemy (Claudius Ptolemaeus, 87-150 AD) was an astronomer, geographer, and astrologist. He was a late representant of the Hellenistic civilization that had its capital in Alexandria and blossomed during the last three centuries BC and faded away during the first two centuries AD. Ptolemy's main work is the Almagest in which all the then existing astronomical knowledge is treated. In the Ptolemean 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, the planet Mars, observed from Earth, seems to move almost always to the east, but every two years (more precisely every 25.7 month) Mars 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 reasonably 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 elliptic orbits.

As geographer Ptolemy is mainly known by 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 is a sphere and he used for his maps a projection of a spherical onto a planar 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% low. Ptolemy took the latter value to be the more reliable one and because of his enormous prestige Christopher Columbus believed this value. It can only 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 have had an enormous influence 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 different for Ptolemy's third book, the Tetrabiblos. This book on astrology is still of relevance today, provided we correct for the fact that Ptolemy lived in the age of Aries, while we now live in the age of the Pisces.