Posidonius and Eratosthenes

All his works, regardless of the subject matter, were inherently philosophical, and while they were significant investigations into their subjects, the results were not free from errors.

His attempt to measure the distance from the Earth to the Sun, for example,  delivered a result that was too small by half, though his estimate of the sun's size was more accurate than those of other contemporary astronomers.

When Posidonius tried to refine Eratosthenes’ measurement of the earth's circumference using the elevation of the star Canopus at Rhodes and Alexandria, rather than the angle of the sun at Alexandria and modern-day Aswan he made several errors.

At Rhodes, Canopus was on but never above the horizon, while at Alexandria it was much as 7½ degrees above the horizon. While his observations gave a figure of one forty-eighth of a circle, he was about fourteen per cent wide of the mark. 

He compounded the mistake by underestimating the distance between Alexandria and Rhodes as 3,750 stades, significantly less than it should have been, regardless of what length one uses for the length of a stadion

The result was a  circumference about a quarter less than it should have been. 

When Ptolemy's choice of Posidonius's miscalculation of the circumference over Eratosthenes's more accurate result, it became the accepted value for the next 1,500 years with significant implications for the voyages of Columbus and Magellan.

His writings made Posidonius a significant figure throughout the Greco-Roman world, and he was extensively cited by noted writers including Cicero, Livy, Plutarch, Strabo and Seneca the Younger. 

Today, he is recognised for his inquiring and wide-ranging mind, and a breadth of view that attempted to connect all knowledge into an overarching, unified world view. 

The circumstances of his death are unknown, but he probably died in at home in Rhodes or in Rome around 51 BC.


© Ian Hughes 2017