Posidonius (; , "of Poseidon") "of Apameia" (ὁ Ἀπαμεύς) or "of Rhodes" (ὁ Ῥόδιος) (), was a Greek politician, astronomer, astrologer, geographer, historian, mathematician, and teacher native to Apamea, Syria. He was considered the most learned man of his time and, possibly, of the entire Stoic school. After a period learning Stoic philosophy from Panaetius in Athens, he spent many years in travel and scientific researches in Spain, Africa, Italy, Gaul, Liguria, Sicily and on the eastern shores of the Adriatic. He settled as a teacher at Rhodes where his fame attracted numerous scholars. Next to Panaetius he did most, by writings and personal lectures, to spread Stoicism to the Roman world, and he became well known to many leading men, including Pompey and Cicero.
His works are now lost, but they proved a mine of information to later writers. The titles and subjects of more than twenty of them are known. In common with other Stoics of the middle period, he displayed syncretic tendencies, following not just the earlier Stoics, but making use of the works of Plato and Aristotle. A polymath as well as a philosopher, he took genuine interest in natural science, geography, natural history, mathematics and astronomy. He sought to determine the distance and magnitude of the Sun, to calculate the diameter of the Earth and the influence of the Moon on the tides.
Life
Early life and education
Posidonius, nicknamed "the Athlete" (Ἀθλητής), was born around 135 BC. He was born into a Greek family in Apamea, a Hellenistic city on the river Orontes in northern Syria. As historian Philip Freeman puts it: "Posidonius was Greek to the core". Posidonius expressed no love for his native city, Apamea, in his writings and he mocked its inhabitants.
As a young man he moved to Athens and studied under Panaetius, the leading Stoic philosopher of the age, and the last undisputed head (scholarch) of the Stoic school in Athens. When Panaetius died in 110 BC, Posidonius would have been around 25 years old. Rather than remain in Athens, he instead settled in Rhodes, and gained citizenship. In Rhodes, Posidonius maintained his own school which would become the leading institution of the time.
Travels
Around the 90s BC Posidonius embarked on a series of voyages around the Mediterranean gathering scientific data and observing the customs and people of the places he visited. He traveled in Greece, Hispania, Italy, Sicily, Dalmatia, Gaul, Liguria, North Africa, and on the eastern shores of the Adriatic.
In Hispania, on the Atlantic coast at Gades (the modern Cadiz), Posidonius could observe tides much higher than in his native Mediterranean. He wrote that daily tides are related to the Moon's orbit, while tidal heights vary with the cycles of the Moon, and he hypothesized about yearly tidal cycles synchronized with the equinoxes and solstices.
In Gaul, he studied the Celts. He left vivid descriptions of things he saw with his own eyes while among them: men who were paid to allow their throats to be slit for public amusement and the nailing of skulls as trophies to the doorways. But he noted that the Celts honored the Druids, whom Posidonius saw as philosophers, and concluded that, even among the barbaric, "pride and passion give way to wisdom, and Ares stands in awe of the Muses." Posidonius wrote a geographic treatise on the lands of the Celts which has since been lost, but which is referred to extensively (both directly and otherwise) in the works of Diodorus of Sicily, Strabo, Caesar and Tacitus' Germania.
Political offices
In Rhodes, Posidonius actively took part in political life, and he attained high office when he was appointed as one of the Prytaneis. This was the most important political office in Rhodes, combining presidential and executive functions, of which there were five (or possibly six) men holding the office for a six-month period.
He was chosen for at least one embassy to Rome in 87/86, during the Marian and Sullan era. Although the purpose of the embassy is unknown, this was at the time of the First Mithridatic War when Roman rule over the Greek cities was being challenged by Mithridates VI of Pontus and the political situation was delicate.