We know the Earth is definitely round but the view from your airplane seat at cruising altitude isn’t good proof of it. Using math from the 4th century B.C., the ancient Greeks were able measure the curve of Earth at sea level. Taking those same formulas, it turns out most commercial jets aren’t flying as high respective to the size of the earth as it seems.
You’ve probably seen a curved horizon at 10,000 meters during a flight but as you can see in the video above, it’s not what you think.