The exact circumference of the Earth was measured by satellites in the middle of the 20th century and found to be 40,030 kilometers. Eratosthenes calculated nearly the same amount using a stick and his intellect more than 2,000 years ago in ancient Greece.

By comparing the lengths of the shadows cast by two sticks placed in Alexandria and Syene, two cities south of Alexandria, on the summer solstice, the Greek mathematician and head of the Alexandrian library Eratosthenes was able to determine the Earth's spherical shape and calculate its circumference.

Eratosthenes calculated the circumference of the Earth to be about 40,000 kilometers using simple proportions and hired a person to pace the distance between the two cities. This calculation is amazingly precise considering the state of technology at the time.

In 276 BC, Eratosthenes was born in the Greek colony of Cyrene in what is now modern-day Libya. Later, he relocated to Alexandria, where he held the position of chief librarian at the Library of Alexandria, one of the most significant academic institutions in antiquity.

A well-known poet, historian, and geographer, Eratosthenes also contributed to the fields of mathematics and astronomy.