Finding the Radius of the World: Eratosthenes

Eratosthenes found the radius of the world over 2,000 years ago circa 230 B.C.E. (Columbus would never have attempted his journey had he known of this Greek's work)

He found a well in Syene (now Aswan on the Nile in Egypt) where the light shown straight down on a certain day. On the same day, light shown at a 7.5 degree angle in Alexandria. The distance between the two cities was known farily accurately due to geographic surveys completed by Alexander The Great. Knowing that circles have 360 degrees Eratosthenes divided 360 by 7.5 which equals 48. He then multiplied the distance between the two cities by 48 giving a circumference of the Earth of 250,000 stadia. (157.2 metres/stadium). Using the equation for the circumference of a circle circumference=2 x pi x radius, the radius is easily calculated.

The modern value for the radius of the Earth is recognized to be 638000 km. Return to the Previous Page