Figure 3-14: Eratosthenes’s Method of Determining the Diameter of Earth Around 200 b.c.e., Eratosthenes used observations of the Sun’s position at noon on the summer solstice to show that Alexandria and Syene were about 7° apart on the surface of Earth. This angle is about one-fiftieth of a circle, so the distance between Alexandria and Syene must be about one-fiftieth of Earth’s circumference.