menu 5 Solar System
Erasmus+: Key action 2:Strategic Partnerships in School Education

Practical Exercise 11: GREEK, HOW BIG IS THE EARTH?

ahe first measurement of the Earth’s diameter was made by the ancient Greeks around the year 250 BC. ahe Greek scholar Eratosthenes of Cyrene needed only two cities with a suitable location for this. Eratosthenes learned from travellers that on the day of the summer solstice (June 21), at noon, the reflection of the sun could be observed even in the deepest wells of the city of Syene. ahe sunbeams thus fall square to the Earth’s surface and cast no shadow. On the same meridian, 800 km far from Syene, there was the Egyptian city of Alexandria, where the sunbeams cast at noon a shadow at an angle of 7,2°.

Your task:

a) draw a picture describing the situation,

b) find the radius of the Earth.


a) The most important part of the sketch is the correct drawing of identical angles.

b) From the sketch we can simply calculate the radius of the Earth:

, i. e. , i. e. , where , where , i. e. , where .