He settled around 95 BCE in Rhodes, a maritime state which had a reputation for scientific research, and became a citizen.
Along with other Greek intellectuals, Posidonius favored Rome as the stabilizing power in a turbulent world. His connections to the Roman ruling class was for him not only politically important and sensible but was also important to his scientific researches. His entry into the highest government circles enabled Posidonius to undertake his travels into the west beyond the borders of Roman control, which, for a Greek traveler, would have been impossible without such Roman support.
In Hispania, on the Atlantic coast at Gades (the modern Cadiz), Posidonius studied the tides. He observed that the daily tides were connected with the orbit and the monthly tides with the cycles of the Moon, and he hypothesized about the connections of the yearly cycles of the tides 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 has been assumed to be one of the sources for Tacitus' Germania.
Posidonius wrote on physics (including meteorology and physical geography), astronomy, astrology and divination, seismology, geology and mineralogy, hydrology, botany, ethics, logic, mathematics, history, natural history, anthropology, and tactics. His studies were major investigations into their subjects, although not without errors.
He accepted the Stoic categorization of philosophy into physics (natural philosophy, including metaphysics and theology), logic (including dialectic), and ethics. These three categories for him were, in Stoic fashion, inseparable and interdependent parts of an organic, natural whole. He compared them to a living being, with physics the meat and blood, logic the bones and tendons which held the organism together, and ethics – the most important part – the soul. His philosophical grand vision was that the universe itself was similarly interconnected, as if an organism, through cosmic "sympathy", in all respects from the development of the physical world to the history of humanity.
Although a firm Stoic, Posidonius was, like Panaetius and other Stoics of the middle period, eclectic. He followed not only the older Stoics, but Plato and Aristotle. Although it is not certain, Posidonius may have written a commentary on Plato's Timaeus.
He was the first Stoic to depart from the orthodox doctrine that passions were faulty judgments and posit that Plato's view of the soul had been correct, namely that passions were inherent in human nature. In addition to the rational faculties, Posidonius taught that the human soul had faculties that were spirited (anger, desires for power, possessions, etc.) and desiderative (desires for sex and food). Ethics was the problem of how to deal with these passions and restore reason as the dominant faculty.
Posidonius advanced the theory that the Sun emanated a vital force which permeated the world.
He attempted to measure the distance and size of the Sun. In about 90 BCE Posidonius estimated the astronomical unit to be a0/rE = 9893, which was still too small by half. In measuring the size of the Sun, however, he reached a figure larger and more accurate than those proposed by other Greek astronomers and Aristarchus of Samos.
Posidonius also calculated the size and distance of the Moon.
Posidonius measured the Earth's circumference by reference to the position of the star Canopus. As explained by Cleomedes, Posidonius observed Canopus on but never above the horizon at Rhodes, while at Alexandria he saw it ascend as far as 7 1/2 degrees above the horizon (the arc between the latitude of the two locales is actually 5 degrees 14 minutes). Since he thought Rhodes was 5,000 stadia due north of Alexandria, and the difference in the star's elevation indicated the distance between the two locales was 1/48th of the circle, he multiplied 5,000 by 48 to arrive at a figure of 240,000 stadia for the circumference of the earth. While translating stadia into modern units of distance is problematic, as an ancient stadium could measure anywhere from about 157 to around 211 meters, it is generally thought that the stadium used by Posidonius was almost exactly 1/10 of a modern statute mile, near the middle of the ancient range. Thus Posidonius' measure of 240,000 stadia translates to 24,000 miles, not much short of the actual circumference of 24,901 miles.
Posidonius was informed in his approach to finding the earth's circumference by Eratosthenes, who a century earlier used the elevation of the sun at different latitudes to arrive at a figure of 250,000 stadia (which he rounded to 252,000 so that it would be divisible by 60). As with Posidonius, Eratosthenes' stadium is thought to have equated to 1/10th of a mile, so that his measure translates to 25,000 (or 25,200) miles. Both men's figures for the earth's circumference were uncannily accurate, aided in part in each case by mutually compensating errors in measurement. However, Posidonius later revised his original calculation by correcting the distance between Rhodes and Alexandria to 3,750 stadia, resulting in a circumference of 180,000 stadia, or 18,000 miles. Ptolemy discussed and favored this revised figure of Posidonius over Eratosthenes in his Geographia, and during the Middle Ages scholars divided into two camps regarding the circumference of the earth, identified with Eratosthenes' calculation on the one hand and Posidonius' 180,000-stadium measure on the other.
Like Pytheas, Posidonius believed the tide is caused by the Moon. Posidonius was, however, wrong about the cause. Thinking that the Moon was a mixture of air and fire, he attributed the cause of the tides to the heat of the Moon, hot enough to cause the water to swell but not hot enough to evaporate it.
Of Posidonius's work on tactics, The Art of War, the Roman historian Arrian complained that it was written 'for experts', which suggests that Posidonius may have had first hand experience of military leadership or, perhaps, utilized knowledge he gained from his acquaintance with Pompey.
Posidonius appears to have moved with ease among the upper echelons of Roman society as an ambassador from Rhodes. He associated with some of the leading figures of late republican Rome, including Cicero and Pompey, both of whom visited him in Rhodes. In his twenties, Cicero attended his lectures (77 BCE) and they continued to correspond. Cicero in his De Finibus closely followed Posidonius's presentation of Panaetius's ethical teachings. Posidonius met Pompey when he was Rhodes's ambassador in Rome and Pompey visited him in Rhodes twice, once in 66 BCE during his campaign against the pirates and again in 62 BCE during his eastern campaigns, and asked Posidonius to write his biography. As a gesture of respect and great honor, Pompey lowered his fasces before Posidonius's door. Other Romans who visited Posidonius in Rhodes were Velleius, Cotta, and Lucilius.
Ptolemy was impressed by the sophistication of Posidonius's methods, which included correcting for the refraction of light passing through denser air near the horizon. Ptolemy's approval of Posidonius's result, rather than Eratosthenes's earlier and more correct figure, caused it to become the accepted value for the Earth's circumference for the next 1,500 years.
Posidonius fortified the Stoicism of the middle period with contemporary learning. Next to his teacher Panaetius, he did most, by writings and personal contacts, to spread Stoicism in the Roman world. A century later, Seneca referred to Posidonius as one of those who had made the largest contribution to philosophy.
At one time, scholars perceived Posidonius's influence in almost every subsequent writer, whether warranted or not. Today, Posidonius seems to be recognized as having had an inquiring and wide-ranging mind, not entirely original, but with a breadth of view that connected, in accordance with his underlying Stoic philosophy, all things and their causes and all knowledge into an overarching, unified world view.
Freeman,Phillip,The Philosopher and the Druids: A Journey Among The Ancient Celts, Simon and Schuster,2006.