Summary
Highlights
The knowledge of the Earth's spherical shape dates back over 2,000 years. Pythagoras, around 500 BC, is credited with proposing the idea. Aristotle, in 350 BC, provided hard evidence by observing that lunar eclipses always cast a round shadow on the moon, indicating a spherical Earth. He also noted that visible stars change dramatically based on geographic location, like different stars seen in Egypt compared to Cyprus, proving a circular shape and a sphere of no great size.
Eratosthenes, a prominent scholar, librarian of Alexandria, and master of multiple disciplines, went a step further to not only prove the Earth was round but also measure its circumference. His ambition was to create a world map, which required knowing the Earth's size. He famously achieved this around 240 BCE using the sun and a stick.
Eratosthenes observed that at noon on the summer solstice, sunlight directly illuminated the bottom of a well in Syene (modern Aswan) without casting shadows, indicating the sun was directly overhead. In Alexandria, however, a stick placed upright at the same time cast a shadow at a 7.2-degree angle because Alexandria is north of the Tropic of Cancer. This angular difference, combined with the distance between the two cities, allowed him to calculate the Earth's circumference.
A crucial factor was the distance between Alexandria and Syene. Eratosthenes hired 'bemetists', professional walkers trained to measure distances accurately by counting steps, who determined the distance to be approximately 5,000 stadia (800 kilometers). Knowing that 7.2 degrees is 1/50th of a full circle (360 degrees), he multiplied 800 kilometers by 50 to estimate the Earth's polar circumference at roughly 40,000 kilometers.
Eratosthenes' calculation was remarkably close to the modern accepted value of 40,075 kilometers (equatorial) or 40,008 kilometers (polar). His accuracy was impressive despite making several assumptions: that the distance between cities was exactly 5,000 stadia (actual is 841 km), that Alexandria was directly north of Syene, that Syene was precisely on the Tropic of Cancer, that sun rays are parallel, and that the Earth is a perfect sphere. Modern experiments with accurate tools replicate his method to within 0.16% of the accepted polar circumference.