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Eratosthenes Lesson - Video

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Parts (2) Lesson

I asked the OpenAI ChatGPT: “How did Eratosthenes calculate the size of the Earth and how accurate was he?”  I put what it said in a video which you will find in this lesson. Watch the video and then you should know enough about Eratosthenes and how he measured the world over two thousand years ago to answer the self-test exercises and practice material for the vocabulary in the video and for the IPA symbols.

History Course

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How did Eratosthenes calculate the size of the Earth and how accurate was he?

Eratosthenes was a Greek mathematician, geographer, and astronomer who lived in the 3rd century BCE. He is best known for his calculation of the size of the Earth, which he made using a simple and clever method that relied on the fact that at noon on the summer solstice, the Sun is directly overhead at the Egyptian city of Syene (now Aswan) but is at an angle of about 7.2 degrees from the vertical at Alexandria, a city located about 800 kilometers (500 miles) to the north.

Eratosthenes reasoned that if the Earth were a perfect sphere, the angle between the Sun and the vertical at Alexandria would be the same as the angle between the vertical and a line drawn from the center of the Earth to Syene. By measuring the angle between the vertical and the Sun at Alexandria on the summer solstice, he was able to calculate the circumference of the Earth.

Eratosthenes' calculation of the Earth's circumference was remarkably accurate. He estimated the circumference to be about 39,375 kilometers (24,662 miles), which is only about 16% larger than the true value of about 34,000 kilometers (21,000 miles). This error can be attributed to the fact that the Earth is not a perfect sphere, but rather is an oblate spheroid that is slightly flattened at the poles and bulges at the equator.

Thank you. That was very informative.

You’re very welcome.

By the way! You might like to know that you can test your understanding of this topic in the Britlish Library at You can find a link below.

Bye bye for now!

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