This is part of a series of questions on the Gemara in Bechoros 8b-9a. Below is a summary of the relevant parts of the Gemara. After being challenged by Caesar, R' Yehoshua went to Athens to defeat the Athenian Elders in a battle of wits.
The Elders asked R' Yehoshua during their debate where the center of the world was. R' Yehoshua stuck a finger out (we can only hope it wasn't his middle one) and said, "It's right here."
"Prove it," the Elders demanded.
"Bring a rope, and we can measure it," R' Yehoshua replied.
Now, the Greeks knew that that the world was round; they thus didn't believe that the center of the Earth was anywhere on the surface. It's unclear what Chazal in general held - each Tanna seemed to hold something different. Nevertheless, whatever you think R' Yehoshua held, there's an issue. If R' Yehoshua held the world was round, the center couldn't be on the surface. If he held the world was flat, how could he tell with such precision that he just so happened to be in the middle of the world, and that it's not a few inches to the side?