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How was the size of Earth first measured?
Earth's circumference was first accurately measured more than 2,000 years ago by the Greek astronomer Eratosthenes, who at the time lived in the Egyptian city of Alexandria. He had heard that in the nearby town of Syene midday sunlight shines straight down to the bottom of deep wells on the same day each year, indicating that the Sun was directly overhead in Syene. In Alexandria, however, sunlight on that date never reached the bottoms of wells, but instead fell upon the sides.
Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curvature of Earth's surface, and so by measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe.
On the day the Sun shone on the bottom of the wells in Syene, Eratosthenes measured the Sun's position in the sky over Alexandria. It was seven degrees away from the zenith, meaning Syene must be seven degrees away from Alexandria as measured on the circle that is Earth's circumference. Because seven degrees is about one 50th of a full circle (360 degrees), Eratosthenes simply multiplied the distance from Alexandria to Syene -- believed to have been about 515 miles (830 km) -- by 50. He calculated Earth's circumference at 26,000 miles (42,000 km), only five percent away from the modern accepted value of 24,901 miles (40,074 km).