ASTRONOMER, GEOGRAPHER

Life
Celebrated astronomer, geographer and mathematician. According to Theodorus Meliteniotos of Byzantium, he was born in the Hellenistic city of Ptolemais Hermii on the Nile in Upper Egypt in the second century AD. His astronomical observations were made chiefly in Alexandria, where there was an observatory. He continued the work of Hipparchus, and was the first to systematise Greek astronomy. He travelled widely for scientific reasons, and purchased many astronomical instruments. He is cited by Suidas and Stobaeus. One of the craters on the moon has been named "Ptolemy" in his honour.


Work
Ptolemy was a prolific writer, and several of his works have survived. These include: "Optics", "Guide to Geography" and the great "Mathematical Collection". This last is an encyclopaedic work in 13 books better known as "Almagest". It is still considered a fundamental text on astronomy, and nothing remotely similar was written between then and the 16th century. Other works of his include: the "Canon of Kings", a chronological record of the dynasties of Egypt; "Spreading of the surface", describing a theory of cartography; "Spherical trigonometry", with which Ptolemy solved problems in spherical astronomy; "Analemma"; "Harmonics" in three books; and "Phases of fixed stars and meeting of points". His system for the interpretation of the universe was called the "Ptolemaic" or "geocentric" system, and was wholly opposed to the heliocentric system of Aristarchus.

In Ptolemy's system, the Earth is the centre of the world; and the Sun, the planets, the stars and the universe in general revolve around it. The Earth is round, and relatively small compared to the boundless Heaven, but it is still the centre of the world. The Moon is the closest planet to the Earth, and moves around it in an exceptionally complicated manner. The celestial bodies are arranged in the following order: Earth (centre), Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn.

The geocentric system was universally accepted until the 16th century, when Polish astronomer Nicolaus Copernicus discovered the work of Aristarchus and re-introduced a heliocentric system, which thus finally supplanted the Ptolemaic system.

Other works:

"Planisphaerium": Survives in Latin translation.

"On the elements": Cited by Simplicius. Lost.

"Tetrabiblos": Four books on forecasting changes in the weather.

"Planetary hypothesis": 2 books describing the Ptolemaic system. Both books survive, the second only in Arabic translation.