Final answer:
Eratosthenes is credited with conducting the first fairly accurate measurement of Earth's circumference using observations of sunlight in wells and shadows cast by vertical objects in two different cities.
Step-by-step explanation:
Eratosthenes is credited with the first fairly accurate measurement of the Earth's circumference. Around 200 BCE, Eratosthenes, a Greek living in Alexandria, Egypt, utilized simple yet effective geometric principles to estimate the size of our planet. Observing sunlight down a well in Syene (now Aswan) on the summer solstice, he noted the Sun was directly overhead. In Alexandria, a city approximately 5000 stadia to the north, the Sun was not directly overhead, but instead cast an angle with vertical objects, calculated to be about 7° or 1/50th of a full circle. This observation suggested to Eratosthenes that the surface of the Earth must be curved, and by multiplying the distance between the two cities by 50, he estimated the Earth's total circumference. Despite uncertainties in the exact length of a stadium, Eratosthenes's calculation was remarkably close to modern measurements, being within 1% of the actual value if one interpretation of the stadium unit is assumed.