# Ptolemy

**Claudius Ptolemy (AD 100 - 170)**

Very little is known about Claudius, the astronomer, mathematician, geographer, astrologer, and poet. He lived in the city of Alexandria, Egypt, in the Roman Empire, wrote in Greek, and held Roman citizenship. His first major astronomical work, *The Almagest* ('the greatest'), records Claudius' detailed observations of the night sky.

During his lifetime, Claudius' titled his great work *The Mathematical Collection*. He believed that the motions of all the astronomical bodies could be described by mathematics. He argued that the Earth is a stationary sphere in the centre of a vast celestial sphere, which revolves at a perfectly uniform rate around the Earth. The celestial sphere carries the Sun, the Moon, the stars, and the planets, causing them to rise and set each day. Throughout a year, the Sun traces out a great circle against the rotation of the celestial sphere, known as the ecliptic. The Moon and the planets also travel backwards, hence the planets were known as 'wandering stars', with respect to the 'fixed stars' of the ecliptic. The word 'planet' comes from the Greek *asteres planetai* for 'wandering stars'. This model of the solar system, in which the Earth is at the centre (geocentric), became known as the Ptolemaic system.

*The Almagest* also contains a star catalogue. It lists 48 constellations, and only those which could be seen from Europe. Claudius' *Planetary Hypotheses* was more mathematical than *The Almagest*, and presented the Universe as a set of concentric spheres. He estimated that the Sun was 1,210 Earth-radii distant, and the 'fixed stars' were 20,000 Earth-radii distant.