ERATOSTHENES'S MEASUREMENT OF THE CIRCUMFERENCE OF THE EARTH

Figure 1.  Relative location of Alexandria and Aswan.  They are close to lying on the same meridian (half of a great circle).


Figure 2.  Eratosthenes's measurement of the circumference of the Earth, based on a Theorem of Euclid.


Many of the assumptions made by Eratosthenes were not accurate; apparently, however, underfit and overfit of error balanced out to produce a good result.  For example, Syene and Alexandria are not on the same meridian; Syene is not at exactly 23.5 degrees N. Latitude, and so forth.  See the link to Astronomy Online for more discussion of historical and astronomical matters.

J. E. Diggins, The Whole Round Earth, http://www.anselm.edu/homepage/dbanach/erat.htm
Astronomy Online:  http://www.algonet.se/~sirius/eaae/aol/market/collabor/erathost/


Institute of Mathematical Geography.  Copyright, 2005, held by authors.
Spatial Synthesis:  Centrality and Hierarchy, Volume I, Book 1.
Sandra Lach Arlinghaus and William Charles Arlinghaus