Who first determined the Earth's circumference?

 Who first determined the Earth's circumference?

Eratosthenes, a Hellenistic geographer (273-193 B.LL) in Alexandria, was the first to calculate the Earth's circumference around 225 B.C.L; he was only off by a half of a percent. Eratosthenes knew that the Earth was a sphere, and that the sun was at least 20 times farther away than the Moon (actually, it is about 400 times farther away). Because of this, he thought that the rays of the sun should be parallel to the Earth's surface when they reached our planet.

At local noon on the summer solstice in Syene, the sun would be directly overhead (at zenith). Eratosthenes calculated that, while in Alexandria, an observer would measure the sun to be 7 degrees south of the zenith. Knowing the distance between the two cities to be 4,900 stadia (l stadium equals 0.099 miles or 0.16 kilometers), Eratosthenes calculated the Earth's circumference by reasoning the following: Divide 360 degrees (the degrees equal to the circumference of the Earth) by 7 degrees (the angular distance he found between the cities); then multiply that times the number of stadia (4,900), which equals a circumference of252,000 stadia. This is equal to about 25,054 miles (40,320 kilometers).

His results were not perfect, but he came very close to today's measurement of the Earth's circumference at 24,873 miles (40,030 kilometers). Eratosthenes' genius did not end there. He was also the first to draw a world map with latitude- and longitudelike lines-the original way to figure out where you are on the planet.