Measure the Circumference of the Earth

Principle: Use Eratosthenes' method ( around 230 BC ) to measure the circumference of the Earth.

               Eratosthenes' method was: at the summer solstice, the sun was shining directly into a well at the city Syene at noon, in the meanwhile, the angle of inclination of the ray of the sun can be measured at the city Alexandria which was 787 km due north of Syene.  Once the inclination angle of sun ray could be found out in Alexadria, the diameter of the Earth can be calculated.

               Use Eratosthenes' method to find out the circumference of the Earth. And find out the reasons of the errors with regard to this experiment ( the earth's radius is around 6380 km ).