Posidonius (ˌpɒsI'doʊniəs; Ποσειδώνιος , "of Poseidon") "of Apameia" (ὁ Ἀπαμεύς) or "of Rhodes" (ὁ Ῥόδιος) (135-51 BC), 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.
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.