Now days we all know the earth is round, but there was a time when the earth was thought to be flat.
Until the third century B.C. it was generally accepted that the world was flat, that was until a multi-talented man named Eratosthenes came across an interesting discovery.
After reading that at noon on the 21st of June, pillars in the Egyptian town of Syrene cast no shadows, Eratosthenes decided to replicate the scenario in his town of Alexandria. He was perplexed when a stick held vertically there DID cast a shadow at noon.
If the earth was indeed flat, all shadows should be the same, whether created by a pillar in Syrene or a stick in Alexandria. However, if the earth was curved, the shadows would change lengths depending on how far north or south you are, just as Eratosthenes guessed.
To visualize this, imagine a sheet of paper with two toothpicks stuck in it, several inches apart, and a lamp shining down from overhead. Neither toothpick casts a shadow. However, bend the paper at one end, and the tilted toothpick will create a shadow. The more you bend it, the longer the shadow.