Eratosthenes and the Circumference of the Earth

While an impressive scientist and philosopher in his own right, as well as the Chief Librarian of the famed Library of Alexandria, Eratosthenes is most well known for his work in the calculation of the circumference of the Earth in ancient times using clever deduction and a shockingly exact method, giving him a margin of error of less than one percent.

Graphical Depiction of Eratosthenes's deduction

To make this exact measurement, Eratosthenes observed that the rays of the sun could reach the bottom of a well in Syene (modern-day Aswan, Egypt). On the same date and time in Alexandria, a city due north of Syene, a vertical stick would cast a shadow at around 7.2 degrees, or 1/50 of a circle. Because the sun's rays are almost exactly parallel, this dissonance means the distance between Syene and Alexandria is about 50 times less than the diameter of the Earth.

Eratosthenes then paid a man to walk the distance between the cities and count his paces, providing him with an approximate distance of 5,000 stadia (approximately 800km). Multiplying these two figures, Erasthostenes found that the circumference of the earth should be 250,000 stadia, or about 40,000 km. This measurement was incredibly accurate for the time and found the circumference within 1% of the figure we found today.

In an age without modern satellites, compasses, or even longitude, Erasthostenes made these exact measurements with little more than sunlight, shadows, and reason, standing as a testament to the human mind and one of the most extraordinary feats of the ancient world.