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How did Eratosthenes estimate the circumference of Earth in Alexandria?

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Ptolemaic sculpture — Ptolemaic dynasty Head of a queen, marble sculpture possibly made in Egypt, Ptolemaic period (305–30 bce); in the Brooklyn Museum, New York. 14 × 12 × 11.5 cm.

Eratosthenes estimated the Earth's circumference by applying a geometric method based on the Sun's position at two different locations in Egypt during the summer solstice.12

He observed that at noon in Syene, the Sun's rays fell vertically, shining directly into a well. At the same time in Alexandria, he measured the angle of the sunlight to be approximately 7.2°, or one-fiftieth of a circle, from the vertical.312

Assuming the Sun was far enough away that its rays reached Earth in parallel lines, Eratosthenes concluded that the distance between the two cities represented one-fiftieth of the Earth's total circumference. Using an estimated distance of 5,000 stadia between Alexandria and Syene, he calculated the total circumference to be 250,000 stadia. While the exact length of the stadium unit remains uncertain, his scientific approach established him as a founder of geodesy.23