Claudius Ptolemy

Johannes Kepler
Credit: Portolanero

Occupation: Mathematician, Astronomer, Geographer

Year born: 100 AD

Research Areas: Movements of planets and stars, cartography


Early Life

Claudius was born in Alexandria, Egypt, which was part of the Roman Empire at the time. Very little is known about his life. The information we do have, has been taken from the books he wrote. 

Career Highlights

Claudius’ finished his first major work on astronomy was he was 50 years old. It was based on the observations he had made over the previous 20 to 30 years. He called his book, The Almagest, which meant ‘The Greatest’. In his work, Claudius described the movements of objects in the night sky using maths. He believed the paths of the stars and planets were not random. He explained them by combining several circular movements. 

In Claudius' model of the Solar System, the Earth was stationary. It was surrounded by a great sphere which carried the stars, planets, Sun and Moon around the Earth. This idea of a geocentric (Earth centred) Solar System became known as the Ptolemaic system.

The Almagest also contains a star catalogue. It listed 48 constellations which could be seen from the Northern Hemisphere. Claudius' later work, ‘Planetary Hypotheses’ was more mathematical than The Almagest. It set out the Universe as a series of spheres and estimated the distances to the Sun and the 'fixed stars'.


Claudius' work was used by astronomers for over 1000 years. A crater on the Moon, a crater on Mars, and an asteroid are all named after him. 

Other Interests

Claudius was also a famous geographer. He made a map of the world which included longitudes and latitudes for about 8,000 places. This gives us a detailed account of the world as it was about 2000 years ago.