Archimedes did all sorts of amazing things and was friends with another famous mathematician: Eratosthenes. Eratosthenes had several accomplishments, including the calculation of a pretty close measure for the circumference of the earth. Just think, back then, they did not have planes, trains, and automobiles. They did not have space craft or satellites, either. So, Eratosthenes had to find another way to figure out this number. At noon on the summer solstice, he measured the shadows cast with wells in Alexandria and Syene. He knew the distance between the two cities and was able to use the angles with those shadows plus the distance to get an approximation of the distance around the entire earth. He ended up with a measure somewhere between 39,690 km and 46,620 km. Considering that we have it as 40,008 km today, he did a good job!