Final answer:
Eratosthenes determined Earth's circumference using geometry, the sun's position in a water well, and the shadow of an obelisk. By noting the shadow angle and well illumination in two cities, he extrapolated Earth's full circumference.
Step-by-step explanation:
The individual who first determined the circumference of Earth used a combination of techniques, including geometry, observation of a water well, and the shadow of an obelisk. Eratosthenes, a Greek scholar living in Alexandria, Egypt, around 200 BCE, employed a geometric method to estimate Earth's circumference.
He observed that on the first day of summer in Syene (now Aswan), sunlight illuminated the bottom of a deep well, indicating the Sun was directly overhead. Conversely, at the same time in Alexandria, the Sun was not directly overhead and cast a shadow from a vertical column, making an angle of about 7 degrees with the vertical, which is approximately 1/50th of a full circle. By knowing the distance between Syene and Alexandria was 5000 stadia, Eratosthenes calculated the full circumference of Earth to be 50 times that distance, which he found to be 250,000 stadia.