Introduction to Earth Sciences I
1.1 The Earth's Size and Shape, continued
If the Earth were flat the "sun angle" would be the same everywhere. The sun is so far from the earth that the light "rays" from the sun are more or less parallel
If the Earth is round then the "sun angle" would change and he reasoned that the amount of sun angle change had to have something to do with the Earth's total size. Apparently the idea that the Earth is round (spherical) was know at this time, well before Europeans made the same deduction. If it is round it has a radius and he knew how to find its radius because he knew geometry from Phythagoras.