Who was the first to measure the earth’s radius?

A. Galileo
B. Copernicus
C. Ptolemy
D. Eratosthenes
E. None of above

Check Answer

The correct answer is D) Eratosthenes.

Eratosthenes was the first to measure the Earth’s radius. He was a Greek mathematician, geographer, and astronomer who lived in the 3rd century BCE. He was the chief librarian at the Library of Alexandria, one of the most famous libraries of the ancient world.

Eratosthenes determined the Earth’s radius by measuring the angle of the sun’s rays at different locations. He knew that at noon on the summer solstice, the sun was directly overhead in the Egyptian city of Syene (now Aswan) and that the angle of the sun’s rays was exactly 7.2 degrees from the vertical in Alexandria. Using basic trigonometry, he calculated that Syene was 7.2 degrees (or 1/50) of a full circle (360 degrees) away from Alexandria on the Earth’s surface. He then used this information to calculate the Earth’s circumference and radius. His calculation for the Earth’s circumference was only about 15% off from the actual value and for the Earth radius about 25% off.

Ptolemy, Copernicus and Galileo were also famous scientists and astronomers but did not measure the Earth’s radius but their work was related to other aspects of Astronomy and Cosmology.
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