A question came up with some friends recently that googling hasn't turned up a satisfying answer to. Simply put: If the world was flat and infinite (ignoring all the stupidity of this) how far could you see from sea level before the atmosphere itself is preventing any light reaching you? Obviously if Earth was flat then someone standing on the west coast of Portugal isn't going to see someone on the eastern seaboard with the naked eye, but assuming they had a telescope would they be able to see through the thousands of kilometres of atmosphere? Is there a simple way of figuring out an answer to this question? A lot of people have asked this question online but there's a lot of guesstimates and unexplained answers which aren't helpful.