Part 4/10:
By measuring the angle of the Sun's rays in Alexandria, he found it to be about 7 degrees off vertical, while the Sun appeared directly overhead in Syene. This angle provided a crucial ratio—7 degrees of a complete 360-degree circle—that allowed Eratosthenes to compute the Earth’s circumference with remarkable accuracy for his time, estimating it to be around 25,000 miles, a figure quite close to modern measurements.