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 assum

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.