What specific solar event did Eratosthenes note occurred in Syene during his measurement?

Answer

The sun shone directly down a deep, vertical well, meaning it was at the zenith.

Eratosthenes knew that at noon on the summer solstice in Syene, the sun was directly overhead, illuminating the bottom of a deep well completely.

What specific solar event did Eratosthenes note occurred in Syene during his measurement?
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