Posidonius of Apamea · Philosophy
Posidonius turned his cosmic piety into hard measurement: he estimated the size and distance of the sun, took the measure of the earth’s circumference, mapped its zones and peoples, and traced the tides. One of his figures for the earth, too small, would echo down to Columbus.
For Posidonius, revering the ordered cosmos meant measuring it. He was not content to praise the harmony of the world in general terms; he wanted numbers - the size of the sun, its distance, the girth of the earth, the height of the atmosphere, the rhythm of the tides. In this he joined the great tradition of Greek mathematical astronomy, applying geometry and careful observation to questions that seem impossibly beyond human reach. How large is the sun? How far away? How big is the earth beneath our feet? That a person standing on a small island could hope to answer such questions at all is one of the astonishing achievements of Greek science, and Posidonius pressed it as far as his age allowed. His measurements were often wrong, sometimes wildly so; but the audacity of the attempt, and the method behind it, mark him as a scientist in the fullest sense - one who believed the cosmos could be not only admired but weighed.
Here the story grows tangled, and honesty demands we tell it carefully. Our sources disagree about the figure Posidonius reached. The astronomer Cleomedes, describing the Canopus method, reports that Posidonius got a circumference of 240,000 stades - a value that happens to be reasonably close to the truth. But the geographer Strabo reports Posidonius’s figure as 180,000 stades, a good deal too small, apparently because a shorter estimate of the Rhodes-to-Alexandria distance was used. Which figure was Posidonius’s own final view is genuinely uncertain; the ancient testimony is divided. What is certain is the fate of the smaller number. The influential geographer Ptolemy adopted the 180,000-stade circumference, and through Ptolemy’s authority it became the standard medieval value - shrinking the globe by roughly a quarter and setting a trap for the future.
The earth was only part of Posidonius’s measuring. He also attacked the hardest target in ancient astronomy: the size and distance of the sun. He concluded, correctly in spirit, that the sun is enormously larger than the earth and immensely far away - reaching, in fact, a figure for its size that was larger and closer to the truth than his predecessors had managed, though his distances remained far too small. And he mapped the earth as a whole, dividing it into climatic zones by latitude - frozen, temperate, and torrid - and arguing that the character and even the temperament of peoples are shaped by the zone they inhabit. His geography ran from the heavens down to human custom: he described the peoples at the edges of the known world, the fierce Celts of the north with their feasts and their headhunting, the silver mines of Spain and the men who laboured in them. To measure the world, for Posidonius, was to measure everything in it, from the diameter of the sun to the ways of distant tribes.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that Posidonius measured the world - estimating the earth’s circumference from the star Canopus (our sources report both 240,000 and 180,000 stades), gauging the sun’s size and distance, and mapping the earth’s zones and peoples - and that his smaller circumference, adopted by Ptolemy, later encouraged Col…
Leads to Strabo.
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