Claudius Ptolemaeus

Born: 85 AD
Died: 165 AD, at 80 years of age.

Nationality: Roman
Categories: Astronomers


Claudius Ptolemy was a Greek mathematician and astronomer who lived in Alexandria, Egypt during the second century CE. While little is known about the life of Ptolemy, his nomen, or clan name, Claudius, indicates that he was a Roman citizen while Ptolemy is a Homeric name appearing in Greek mythology. Ptolemy was born around the year 90 CE, likely in the town of Ptolemais Hermiou, and died around the year 170 CE in Alexandria, Egypt. During his lifetime, Ptolemy wrote on a wide variety of topics including astronomy, mathematics, geography, and astrology. Though little is known about his personal life, the works of Ptolemy have survived for nearly two millennia.


In his most famous work, the Almagest, Ptolemy lays out, in 13 books, the motion of the several known planetary bodies at the time. Building on the geocentric premise described by Aristotle, Ptolemy goes on to provide a theory to explain the motions of the five known planets, the moon, and the sun. In the Almagest, Ptolemy laid out a mathematical model for the motion of each individual planetary body. Ptolemy’s theory introduced discrete motion for each individual planet and eccentricities in the circular motion of the planetary bodies. Discrete motion explained the appearance of planetary bodies at different points in the sky and eccentricities explained why planetary bodies didn’t always appear to be at the same distance from Earth. The theories introduced by Ptolemy were widely accepted and remained relatively unchanged for nearly 14 centuries.


Ptolemy also wrote an important eight-book work on geography in which he tried to map the known world using a system of latitude and longitude coordinates. The system that Ptolemy devised suffered from numerous inaccuracies in scale and measurement. Using inaccurate measurements, and relying on flawed data about the world outside of the Roman Empire, Ptolemy’s Geographia was highly inaccurate.


Ptolemy also produced a four-book work on astrology in which he used natural philosophy to attempt to establish a system of astrology built on sound science and observable phenomena.


Some of Ptolemy’s lesser-known works include a treatise on optics that no longer survives in its complete form and a work on music theory entitled Harmonics that would enjoy a brief period of fame during the Renaissance.


Though Ptolemy lived nearly18 centuries ago, his influence is still felt in modern society. Craters on the Moon and Mars bear his name as does the asteroid, 4001 Ptolemaeus.








About 85 AD - Born in Egypt (Ptolemais Hermiou).


AD 127-41

- made his astronomical observations from Egypt

- On March 26, 127, Ptolemy made his first observation and his last on February 141.


Claudius Ptolemy emerged as an influential astronomer and geographer.


The Greek scholar's most famous contribution was Almagest where he developed geometrical evidences explaining the motion of the sun along with the moon and the planets.


For 1,400 years, his contribution of geocentric theory was widely used as resource for astronomy until it was found out that he had a mistake in identifying the Earth as the center of the solar system instead of the sun.


Ptolemy also attempted to make a world map and even if it was more of inaccurate it still laid down the foundation of geography. He was certain that the earth had a spherical shape.


About 165 - Died in Alexandria, Egypt.

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