Eratosthenes Shadow Experiment

This page turns one of the oldest Earth-shape measurements into a repeatable activity. The point is not that one ancient measurement settles everything; the point is that simple geometry can produce a planetary-scale prediction. 
 
 Field Version 
 Coordinate with someone north or south of you. Measure shadow length and stick height near local solar noon on the same date. Convert the shadow ratio to a solar elevation angle, compare angles, then use the distance between locations to estimate circumference. 
 Why It Belongs Here 
 This is a gateway experiment. It shows how ordinary measurements become evidence when they are coordinated, timestamped, and interpreted with clear geometry.