Eratosthenes was the first person to make an accurate determination of the Earths Circumference. He achieved this back in 300BC using a very clever experiment using the ideas that sticks in the ground at different latitudes cast different shadows.
Satellite image of the two places that Eratosthenes used to help him calculate the circumference of the Earth
On June 21st , because that is the date of the summer solstice, he knew that the Sun was going to be overhead of a well in Syene on the Tropic of Cancer. Since the Sun was overhead, sticks would not generate a shadow.He then had another stick placed at Alexandria and the measured the angle of the shadow at noon that day. He recorded the angle of the shadow that was created.
Image from http://www.juliantrubin.com/bigten/eratosthenes.html
He used the equation
Angle of the sun/360° = Distance to tropic of Cancer/Earths circumference
To find,
Earths Circumference= Distance to Tropic of Cancer x 360°/Ѳ of the sun
Image from http://www.iucaa.ernet.in/~scipop/Obsetion/eratos/eratos_1.htm
To work out the circumference of the Earth, Eratosthenes put some numbers into his equation. He knew that the distance between Alexandria and Syene was 790km and he measured 7.2° as the angle from the Sun.
So 7.2°/360°=1/50 so when you flip the equation over you get 360°/7.2°=50
He then multiplied 50 by 790km and then concluded that the Earths circumference was 39500km.
The current measurement of the Earth’s circumference is 40075km, so Eratosthenes was only 1.4% out – 2400 years ago!
The distance between Alexandria and Syene is 830km. What was actually measured is the North South distance which was 800km. How much East or West shouldn't matter.
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