I have flown many many time over the pond. My eyeball tell me when I can look out the window and see the curvature of the earth, it isnt flat.

Somehow the shortest distance between Denver and Frankfurt/Munic is flying north over greenland / iceland. Basically 9 hours at around 600mph, or 5000 miles. If the earth was flat and a straight line is the shortest distance, the plane would fly over the east coast.

A few flights as a passenger and people can understand it by the route.