Ancient calculations and observations

[scerab]

Would anyone help me understand how did the ancient Greeks managed to calculate the earth’s circumference using knowledge of the angle of elevation of the Sun. I wonder if they had any means of magnification such as lens. They did make some pretty accurate conclusions for the limited technology they used.
Another question: were the ancient Greeks the first to assume the Earth was round rather then flat?
Thanks in advance

 

[mathman]

The basis tool that was used to estimate the Earth's circumference was a stick of fixed length. The length of the shadow at noon was measured at two different locations (I don't remember the distance apart, but I believe it was over 100 miles) along a north-south line. From the shadow lengths, the angle of the sun at these locations could be calculated.

 

[Chris Hillman]

The distance between those two locations was considered "known" in terms of stadia, and here things turn a bit murky, but I believe that messengers regularly ran between the two towns and their times formed a crude estimate of distance. Thus the Greeks were able to express the diameter of the Earth in terms of a familiar unit, the stadium.

 

[russ_watters]

http://www.oceanservice.noaa.gov/education/kits/geodesy/geo02_histr.html

 

[Michael Mozina]

http://www.oceanservice.noaa.gov/education/kits/geodesy/geo02_histr.html

That was really an interesting link. Eratosthenes did a remarkable job calculating the circumference of the earth. :)