Posidonius measured tides, the Sun and the Earth, and told pain it was no evil
Called the most learned man of his age, the Stoic Posidonius tried to calculate the size of the Earth, the distance of the Sun and how the Moon drives the tides. Every one of his books is lost, yet later writers from Strabo to Tacitus quarried them for information.
Born around 135 BC to Greek parents in Apamea, beside the Orontes river in northern Syria, he showed little affection for his hometown and mocked its people. His nickname was the Athlete. As a young man he studied in Athens under Panaetius, the last undisputed head of the Stoic school, but after Panaetius died in 110 BC he chose Rhodes, became a citizen and opened a school that grew into the leading institution of its day. Under him Rhodes overtook Athens as the centre of Stoicism.
Around the 90s BC he set off on long research journeys through Spain, Gaul, Italy, Sicily, North Africa and the Adriatic coast. At Gades, modern Cádiz, he watched Atlantic tides far higher than anything in the Mediterranean and concluded that the daily rhythm follows the Moon's orbit, that tidal heights change with its phases, and that there might be yearly cycles tied to equinoxes and solstices. In Gaul he studied the Celts, noting that they honoured the Druids, whom he regarded as philosophers. His lost treatise on Celtic lands survives in echoes through Diodorus, Strabo, Caesar and Tacitus.
Like other Stoics of the middle period he blended traditions, drawing on Plato and Aristotle as well as earlier Stoics. More than twenty of his titles are known. He also held real power: Rhodes made him one of its Prytaneis, the handful of officials who served six-month terms at the top of government, and he went on an embassy to Rome in 87/86 BC during the tense years of the First Mithridatic War.
Next to Panaetius, he did most to carry Stoicism into Roman life, and he knew Cicero and Pompey. Pompey attended his lectures in 66 and again in 62 BC. On the second visit the elderly philosopher, suffering from gout, was speaking on the idea that moral good is the only good. He gestured at his aching leg and told the pain that, however troublesome it might be, it would never convince him it was an evil. He died around 51 BC.
Source: Posidonius
