The Earth's Flatness and the Optic Air Mass

If the earth was flat, how far could the human eye see with no obstructions? Dr. Karl: The atmosphere removes your ability to see stars on the horizon. You can see this as you watch the stars at night and you're watching stars maybe 15 degrees above the horizon. Most distant object you can see with a naked eye is the Andromeda galaxy which is two million light years away.