## Size

- Recall the ... diameter (13 000 km) of the Earth- Describe how Eratosthenes made the first accurate calculation of the circumference of the Earth

Eratosthenes was a Greek mathematician and was the librarian of Alexandria in Egypt.

He learnt that in the town of Syene, to the south, the Sun appears at the zenith (the highest point in the sky) on the summer solstice – it was well know that its reflection could be seen directly down a deep well.

By observing and measuring shadows in Alexandra, he calculated that the Sun was over 7.12 degrees from the Zenith on the same date. This is approximately 1/50th of a circle.

The distance between the cities was approx 5000 stadia (back then they measured by stadia – like football pitches are used to quickly illustrate lengths).

Eratosthenes made a calculation by multiplying the distance by the angle.

Confusingly, there were 2 different measurements for a stadia but scientists believe he used a unit which led to the circumference being within 1 degree of the modern known value.

This is all the more remarkable as the town he used as his measurement was not directly to the south and was not exactly on the Tropic of Cancer.

###### Questions

Describe how Eratosthenes made the first accurate calculation of the circumference of the Earth

###### Did you know?

Eratosthenes devised a method to calculate prime numbers.

###### Links

- The True Size Compare the size of Countries
- Erasthones Quick Summary Wired
- Carl Sagen on Erasthones YouTube (6 minutes)