At the summer solstice, when the sun was as far north as it ever gets, the sun was directly overhead in the ancient city of Syene on the Nile river. At the same time, the sun appeared 7.2 degrees (1/50 of a full circle) south of overhead in the city of Alexandria on the Mediteranean Sea.
Eratosthenes used a relay race involving Olympic runners, who knew fairly accurately how long it took to run a marthon (about 26 miles), to estimate the distance from Alexandria to Syene. They found that their task was like running 19 marathons (about 494 miles).
By adjusting the Intercity Distance slider (at the top of the screen) and the Earth's Radius slider (at the bottom of the screen) so that the angle of the sun is 7.2 degrees in Alexandria, determine the earth's radius.