So let me tell you how he came up with the idea. At present, the city of Saigon, where there is no shadow well, and the city of Alexandria, which has a shady cane, are 5,000 stadia apart. “Stadia” is an ancient Greek unit of measurement, and 5000stadia is 800 km or 497 miles away. The upper right angle of the triangle, which now extends from the shadow to the rod, is 7.2 ° (7.2 °). To be a circle you need to be 360 degrees, so 7.2 degrees is one-fifth of a circle.