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Size of the Earth

Size of the Earth. How a scientific proof works. How big is Earth?. Having established its shape, what is Earth’s size? How do you measure something really big? Eratosthenes devised a way to determine the size of Earth around 200BC. Eratosthenes.

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Size of the Earth

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  1. Size of the Earth How a scientific proof works.

  2. How big is Earth? • Having established its shape, what is Earth’s size? • How do you measure something really big? • Eratosthenes devised a way to determine the size of Earth around 200BC.

  3. Eratosthenes • Based his calculation on observations of the sun on June 20 in Alexandria and Syene – two cities 5000 stadia apart (500 miles) in Egypt. • On this date, the sun was straight overhead at noon in Syene, but 7.5o south of overhead at Alexandria

  4. Syene Alexandria

  5. Eratosthenes • Made assumptions that… • the Earth is spherical, and • the sun’s rays are parallel • Using simple geometry, he set up an equation to determined the circumference of Earth.

  6. Eratosthenes’ Result • Earth’s circumference is 240,000 stadia (about 39,250 km or 23,900 miles) • This is quite close to modern equatorial measurements (24,900 miles). • From his knowledge of geometry, he was able to calculate the radius of Earth, r. • Circumference = 2r

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