Eratosthenes Calculates Earth’s Circumference ca 240 BC

Eratosthenes knew that on the summer solstice at local noon in Syene (Aswan, Egypt) the sun would appear at the zenith, directly overhead. He also knew that in Alexandria the angle of elevation of the sun was 1/50th of a circle (7°12′) south of the zenith on the solstice noon.

The first person to use the word “geography” and the and invention of a system of latitude and longitude was credited to Eratosthenes. He was the first person to calculate the circumference of the earth [24,901 miles (40,075 km)].

Summer solstice is also called Midsummer. Midsummer’s Eve was from ancient times a festival of the summer solstice.

Using his knowledge, Eratosthenes figured out the circumference of the earth with an error of less than 2%! This is an amazing feat. World class astronomer Carl Sagan explains.