Alexander missed Eratosthenes by less than a century

Eratosthenes was born in Cyrene in c.275 BC, less than fifty years after the death of Alexander the Great. I can’t help wondering about the impact this mathematician, astronomer, and geographer would have had on Alexander and his campaign had both men known each other!

When Eratosthenes was about forty years old, Ptolemy III appointed him as chief librarian at the Library of Alexandria – a position he occupied until he died in c.195 BC. The Library was attached to the Museum, and together they were meant to be a university and research center. One may wonder whether the initial idea came from Alexander himself or was developed by Ptolemy I Soter. 

On the other hand, Eratosthenes was the first to write a chronology of Greek history and literature. Some people do live an amazingly well-filled life! 

The reason for bringing Alexander into the picture is double. Firstly, he was the founder of his major city of Alexandria, in 331 BC. His architect, Dinocrates of Rhodes, had roughly been planned, who complied with Alexander’s wish to create a megapolis. That is precisely what happened as the two main streets were designed to be 30 meters wide and all the other streets 15 meters. They were oriented in such a way as to profit from the sea breeze, while at the same time, they would provide shelter from the wind.  

The city walls were impressive also and ran over a distance of 15 kilometers. A mole had to be constructed to link the island of Pharos to the mainland. As a result, Alexandria would have two large harbors protected from the northerly winds. From experience in Libya, I can confirm that the strength of those winds cannot be underestimated! 

The second reason for mentioning Alexander is because of the great merit of Eratosthenes to be the first to draw a map of the world based on the imaginary lines of longitude and latitude, which enabled him to calculate the circumference of the earth.  

Pythagoras was the first to declare that the earth was a sphere, but he could not calculate its size. This is what Eratosthenes did. At noon on the summer solstice, he recorded simultaneous measurements at  Alexandria and at Syeme (close to Aswan) 5,000 stadia further south. The sun stood straight overhead in Syeme but not in Alexandria. Using a stick to measure the shadow, Eratosthenes found none in Syeme, whereas, in Alexandria, it cast a shadow with 7 degrees. Both cities lie near the same longitude line encircling the earth, 360 degrees. At this point, Eratosthenes calculated that 7 degrees are about 1/50 of 360 degrees. In other words, this means that the distance between the two cities must be 1/50 of the entire meridian. Hence, he multiplied the 5,000 stadia by 50, and he obtained an estimate of the earth’s circumference of 250,000 stadia. To simplify calculations, he rounded this figure up to 252,000, which is evenly divisible by 360. Although scholars cannot agree on the length of a stadium, their best estimate is 157 meters. As a result, Eratosthenes’ calculation of the earth’s circumference was 39,250 kilometers against 40,000 kilometers today.

 

That is an astonishing result given the lack of instruments and means Eratosthenes had at his disposal. 

Now imagine that less than a century earlier, all Alexander knew was that the earth was flat and surrounded by the Outer Ocean. How would he have looked at our planet, and how would this knowledge influence his campaigning? Had he known, his plans could have been designed in quite a different way!