Simple explanation of Ancient Greek Eratosthenes Earth circumference calculation

The short video above is a really simple  description of how the ancient Greek Eratosthenes from Cyrene calculated the circumference of the planet Earth around 240 BCE.  (This clip is taken from a longer segment below.)

Quote

 

Eratosthenes had heard from travelers about a well in Syene (now Aswan, Egypt) with an interesting property: at noon on the summer solstice, which occurs about June 21 every year, the sun illuminated the entire bottom of this well, without casting any shadows, indicating that the sun was directly overhead. Eratosthenes then measured the angle of a shadow cast by a stick at noon on the summer solstice in Alexandria, and found it made an angle of about 7.2 degrees, or about 1/50 of a complete circle.

He realized that if he knew the distance from Alexandria to Syene, he could easily calculate the circumference of Earth. But in those days it was extremely difficult to determine distance with any accuracy. Some distances between cities were measured by the time it took a camel caravan to travel from one city to the other. But camels have a tendency to wander and to walk at varying speeds. So Eratosthenes hired bematists, professional surveyors trained to walk with equal length steps. They found that Syene lies about 5000 stadia from Alexandria.

Eratosthenes then used this to calculate the circumference of the Earth to be about 250,000 stadia. Modern scholars disagree about the length of the stadium used by Eratosthenes. Values between 500 and about 600 feet have been suggested, putting Eratosthenes’ calculated circumference between about 24,000 miles and about 29,000 miles. The Earth is now known to measure about 24,900 miles around the equator, slightly less around the poles.

 

interesting to note that Eratosthenes was from Cyrene, noted for its cultivation of the near-mythical plant Silphium (a frequent topic in the past found in the post below).

https://www.aps.org/publications/apsnews/200606/history.cfm

What!? A Carl Sagan bit with no "Billions and billions and billions...." Must be an AI forgery.

One of the two or three most amazing things in science history to me  is that those genius Greeks figured out all that geometry without the use of paper to draw on (not to mention the concept of "zero."). They scribbled in the sand with sticks.

Pliny the Elder, author of the encyclopedic Natural History in the first centry AD https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0137, knew the world was round.

As long as we're on the subject shadows, time and such-- The ancient Sumerians used a base 12 & 60 system in arithmentic, still in use on our time mesurements-- They came to use this, no doubt, because they calculated on their fingers as a form of an abacus-- excepting the thumb, each of our four other fingers have three phalangeal bones. 12 in all.