Final answer:
Eratosthenes, not Zeus, measured Earth's circumference by using the angles of shadows cast in Syene and Alexandria, and understanding the Sun's rays are parallel to the curved Earth's surface. His calculations led to an estimated circumference of 250,000 stadia, which is near the accurate measurement known today.
Step-by-step explanation:
Zeus is a mythological figure and did not perform measurements of the Earth. Instead, it was the Greek scholar Eratosthenes who, around 200 BCE, made a significant contribution to the measurement of Earth's circumference. Utilizing geometric principles and observations of the Sun, Eratosthenes noticed that at midday during the summer solstice, a well in Syene (modern-day Aswan, Egypt) experienced no shadow as the Sun was directly overhead. However, at the same time in Alexandria, there was a noticeable shadow cast by objects. He calculated the angle of the shadow to be 7.2 degrees, which is about 1/50th of a full circle. Given the known distance between Syene and Alexandria, he was able to extrapolate this to determine the Earth's full circumference with impressive accuracy for his time.
Eratosthenes further refined his calculations by understanding that the Sun's rays are parallel and that the surface of the Earth is curved. From this, knowing the angle and the distance between the two cities of Syene and Alexandria, he estimated the Earth's circumference to be around 250,000 stadia, which, depending on the exact length of a stadion used, was within 1% to 20% of the actual value known today.