Who was the first person to measure the Earth’s circumference?

Eratosthenes of Cyrene, a Greek mathematician, geographer, and astronomer, is credited as the first person to accurately estimate the circumference of the Earth around 240 BCE. Working as the chief librarian at the Library of Alexandria, Eratosthenes combined observations, mathematics, and geometry to measure the planet’s size with remarkable accuracy for his time.

Eratosthenes knew that in the city of Syene (modern-day Aswan, Egypt), the Sun was directly overhead at noon during the summer solstice, as vertical objects cast no shadow. In Alexandria, located north of Syene, he measured the angle of the shadow cast by a vertical stick at the same time. He found the angle to be approximately 7.2 degrees, which is 1/50th of a full circle (360 degrees).

Using the distance between Syene and Alexandria, which was about 5,000 stadia (ancient measurement units), Eratosthenes applied a simple proportion:

$$\text{Angle / 360°}=\text{Distance / Circumference of Earth}$$

By solving this proportion, he calculated the Earth’s circumference to be around 39,375 km, which is surprisingly close to the modern value of approximately 40,075 km. His method demonstrated not only the spherical nature of the Earth but also the power of observation and mathematical reasoning.

Eratosthenes’ measurement is historically significant because it combined astronomy, geometry, and geography in a practical application. This achievement laid the foundation for future cartography, navigation, and understanding of the Earth as a measurable sphere. It remains a classic example of scientific thinking in ancient times.

Therefore, the correct answer is Eratosthenes, highlighting his ingenuity in using simple observations and geometry to measure the size of our planet.