How radius of earth was calculated?

Answer 1

Greek scholar and philosopher Eratosthenes (276 BC - 195 BC) figured out that the length of the shadow at exactly noon, drawn from a stick of a certain length, stuck in the ground in Aswan, Egypt, was of a different length than the shadow at exactly noon, drawn from a similar stick in Alexandria, Egypt. From this, the circumference of the Earth was calculated, and the diameter and radius soon followed.

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Answer 2

The radius of the Earth was first calculated by the ancient Greek mathematician Eratosthenes in the 3rd century BCE. He did this by comparing the angles of shadows cast by a vertical stick at two different locations and using simple trigonometry to determine the Earth's curvature and radius. His method was surprisingly accurate for its time.