Eratosthenes, a Greek philosopher, measured the Earth's circumference based on the angle of Sun rays at two different points. Which cities were they?

Eratosthenes, the head librarian at Alexandria, calculated the Earth's circumference around 235 BCE using geometric principles. He observed that at noon on the summer solstice, the Sun was directly overhead in Syene (modern-day Aswan), as its rays illuminated the bottom of a deep well without casting a shadow [1]. Simultaneously, in Alexandria, he measured the angle of a shadow cast by a vertical rod (gnomon) and found it to be approximately 7.2 degrees, or 1/50th of a full circle. By assuming the Sun's rays were parallel and the two cities lay on the same meridian, he deduced that the distance between Alexandria and Syene represented 1/50th of the Earth's total circumference [1]. Multiplying the known distance between these cities by 50 yielded a remarkably accurate estimate of the Earth's size for that era [1].

Sources

1. [1] https://oceanservice.noaa.gov/education/tutorial_geodesy/geo02_hist.html