with an origin at the center of Earth and measured the relative orientation of sunlight as it struck Earth. The key to his spatial analysis was the measurement of angles relative to a plane perpendicular to a radius to the center of Earth, an absolute measure of direction (Figure 3.2).
The debates over his data (e.g., the interpretation of the distance of a stade and the accuracy of the distance measurement) and the fact that Alexandria and Syene do not lie on the same meridian are incidental to the magnitude of his achievement. It is the integration of geography and geometry, the use of observation and empirical data, the use of a superb sense of spatial intuition, and the marriage of deduction and calculation that make Eratosthenes’ feat quite remarkable. From a few pieces of data and some beliefs, he used the power of spatial thinking to capture the size of Earth.