Eratosthenes measured the circumference of Earth to within 4% of the correct value A. Looking at stick shadows on the equinox.
B. Noting how much more of the celestial sphere came into view when walking south.
C. Sighting the stars from within the deepest well in Egypt on the day of the solstice.
D. Using the detailed reports of far-sailing mariners

A. Looking at stick shadows on the equinox.

Explanation:

He measures the circumference by placing a pole when the sun is overhead on the days of solstice. He determined the circumference by measuring the difference between  Alexandria to Syene and the angle of the pole's shadow.

The equation used for the calculation of circumference is as follow

Please refer to the following link for more details

https://www.khanacademy.org/humanities/big-history-project/solar-system-and-earth/knowing-solar-system-earth/a/eratosthenes-of-cyrene

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