The correct relations in the length of degrees of latitude and longitude are maintained in the first case along the latitude of Thule and the equator, in the second along the parallel of Agisymba, the equator and the parallels of Meroe, Syene and Thule.
Being informed that at Syene (Assuan), on the day of the summer solstice at noon, a well was lit up through all its depth, so that Syene lay on the tropic, he measured, at the same hour,the zenith distance of the sun at Alexandria.
It was upon a map based upon such a source that Eratosthenes (276-196 B.C.) measured the distance between Syene and Alexandria which he required for his determination of the length of a degree.
This city was called Swan, the Mart, whence came the Greek Syene and Arabic A swan.