Posidonius

Posidonius (135-51 BCE) was a Greek Philosopher, Geographer, Astronomer and Politician. His name meant ‘of Poseidon’.
He lived in Rhodes, and was twice the Ambassador to Rome between 97-82 BCE. He travelled extensively to research his scientific studies and material for his books.

All his works are lost but are preserved by quotations from other authors such as Diodorus Siculus, Livy and Plutarch:
- The Art of War
- The Histories, in 52 Books.
- A Work on Euclidean Geometry.
- A Geography of the Celtic lands.

Between 87-86 BCE he was Ambassador to Rome during the Civil Wars between Marius and Sulla (88-87 and 83-82 BCE).
He met Pompey during his stay in Rome and again during Pompey's War against the Pirates in 67 BCE.
Posidonius travelled widely to make scientific studies.

A school developed around Posidonius in Rhodes, and was continued by his grandson.
Posidonius promoted the Philosophy of Stoicism.
Students from Greece and Rome came to read his extensive writings and hear his lectures.
Cicero attended his Lectures.

Whilst in Cadiz, Posidonius recorded the great height variations of the Atlantic Tides and observed that the height varied according to the cycle of the Moon.
He calculated that the relative positions of the Sun, Moon and Earth would affect the Tides, with the Equinoxes and the Solstices either enhancing or cancelling the effect of the Moon.

Poseidonius wrote that the sea captain Eudoxus of Cyzicus was sent to India by Ptolemy VIII.
He made two voyages using an Indian pilot in 118 BCE and sailing unguided in 116 BCE. Eudoxus returned with a cargo of spices and precious stones.

Posidonius travelled in Gaul and studied the Celts, noting that they were ruled by the Druids. He wrote a Geography of the Celtic lands, now lost, which was referred to by Julius Caesar.

Posidonius attempted to calculate size and the distance of the Sun from the Earth, and he calculated the size of the Moon and its distance from the Earth.
Cicero wrote that Posidonius constructed an Orrery similar to the Antikythera Mechanism, showing the positions of the Sun, Moon and the five planets.
Posidonius calculated the Earth’s Circumference using the elevation of the star Canopus.
He reached a slightly more accurate figure than Eratosthenes who, a century earlier, had used the elevation of the Sun.

Posidonius wrote ‘The Art of War’, a Book on Tactics, described by Arrian as written for experts.

Euclidean Geometry: Posidonius set out to prove Euclid’s Fifth Postulate of Geometry.

Posidonius wrote the ‘Histories’ in 52 Books, describing the Rise of Rome between 146-88 BCE.

Posidonius was quoted by Strabo, Diodorus Siculus, Plutarch, Livy, Timagenes, Julius Caesar, Cicero and Seneca the Younger.
Pompey asked Posidonius to write his Biography.
Ptolemy quoted Posidonius’ figure for the Circumference of the Earth, in his Geographia.