Eratosthenes of Cyrene: How He Calculated the Earth's Circumference

Eratosthenes of Cyrene, a prominent Greek mathematician, astronomer, and geographer, made significant contributions to astronomy (as well as science in general) in the 3rd century BCE. Born around 276 BCE in Cyrene (modern-day Libya), he was educated in Athens and later became the chief librarian at the Library of Alexandria. His work spanned various fields, and he is best known for his remarkably accurate calculation of the Earth's circumference.

(ThoughtCo)

Eratosthenes measured the Earth's circumference by knowing that at noon on the summer solstice in Syene (now Aswan, Egypt), the Sun was directly overhead and did not cast a shadow. At the same time in Alexandria, approximately 800 kilometers to the north, he measured the shadow cast by a stick and found it made an angle of about 7.2 degrees. By understanding this angle as a fraction of the full 360 degrees of a circle, he calculated the Earth's circumference to be approximately 40,000 kilometers, remarkably close to modern measurements.

(Cantor's Paradise)

Eratosthenes was also the first to calculate the tilt of the Earth's axis, an achievement that was later reported by Ptolemy (85-165 CE). His calculation of the Earth's axial tilt was incredibly accurate, owing to his understanding of the Earth's movements.

In addition to his work on the Earth's dimensions, he attempted to calculate the distances from the Earth to the Moon and the Sun, although these estimates were less accurate. Nonetheless, his efforts laid the groundwork for future astronomers to refine these measurements.

Eratosthenes also compiled an extensive catalog, which involved noting their positions and brightness to create a map of the night sky. Additionally, his work in geography led to the creation of one of the earliest maps of the known world, incorporating the knowledge gathered from Alexander the Great's expeditions and other sources.

(Medium)

Eratosthenes' innovative methods and keen observations significantly advanced the understanding of the natural world in his time. Due to his ingenuity, he was established as a foundational figure in the fields of mathematics, astronomy, and geography.