Hi Wong,
You might want to check out these three websites dealing with Eratosthenese who measured the circumference of the earth in approx 300bc. One of my physics students did an experiment testing the accuracy of ancient measuring techniques and this is one he did.
http://en.wikipedia.org/wiki/Eratosthenes
http://www.juliantrubin.com/bigten/eratosthenes.html
http://www.physics2005.org/projects/...chersGuide.pdf
Good luck and have fun.
Mark