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Eratosthenes, a Greek philosopher measured the Earth’s circumference b

June 1, 2025 by rawan239

Eratosthenes, a Greek philosopher measured the Earth’s circumference based on the angle of Sun rays at two different points. Which cities were they ?

Alexandria and Syene

Syene and Troy

Alexandria and Troy

Alexandria and Thebes

This question was previously asked in

UPSC CDS-2 – 2024

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The Greek mathematician, geographer, and astronomer Eratosthenes is famous for calculating the circumference of the Earth around 240 BC. He did this by observing the angle of the sun’s rays at noon on the summer solstice in two different Egyptian cities, Syene (modern Aswan), where the sun was directly overhead (no shadow in a well), and Alexandria, where the sun’s rays cast a shadow at a measurable angle. He then used the distance between the two cities and basic geometry to estimate the Earth’s circumference.

Eratosthenes was one of the first to accurately estimate the size of the Earth using geometrical principles and astronomical observations.

Syene is located close to the Tropic of Cancer. On the summer solstice, the sun is directly overhead at noon along the Tropic of Cancer. Alexandria is located north of the Tropic of Cancer, so the sun would not be directly overhead there. The difference in the angle of the sun’s rays at the two locations allowed Eratosthenes to calculate the curvature of the Earth and subsequently its circumference.

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