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Comment by demondemidi | Hacker News Reader

How did Eratosthenese synchronize the measurements? That seems like the biggest challenge: sync'ing two clocks and transporting one to a place 500 miles away without losing time? 2000 years ago?

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The summer solstice ("the longest day of the year") is always the same date regardless of your latitude: June 20th/21st (in the northern hemisphere).

And assuming Syene is on the same longitude as Alexandria[0], Syene's noon is Alexandria's noon.

  Alexandria:       31°11′51″N    29°53′33″E
  Syene:            24°05′20″N    32°53′59″E
  Tropic of Cancer: 23°26′10.4″ N

[0] It isn't, which contributed to the measurement error, but close enough is good enough.

He timed solar noon and travelled due north / south to avoid the time zone problem.

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