# 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.

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## 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.