I think Colombus ansered that question some centuries ago, you can travel too all AROUND the earth and see for yourself...also there are instruments that show as the Earth is a shpere , sooo even if we forget about the millions of satelites orbiting the Earth ( in order to...orbit you need a sphere) there are ancient instruments to prove it.
Ancient instruments like a gyroscope?
If you get a mechanical gyro-compass and take it aboard an airplane and fly straight east or west by this gyro-compass (turn off all on-board magnetic compasses, GPS etc.) you will eventually arrive at the Antarctic ice wall.
Do you know anybody who did this? You didn't, did you? Will he/she testify in court to that effect?
I think we all know that if you fly east or west at the equator, you will ultimately wind up back where you started.
So we need a plane that can fly long distances without refueling and a pilot willing to navigate using a mechanical gyro-compass. However the plane will be flying without a flight plan. What about interference from military aircraft, homing missiles, computer controlled machine gun fire, explosive shells and EMP bursts? That's just to start with BTW, there wont be any witnesses left so who knows what kind of weapon will be deployed.
Obviously you or someone you know has done this. That's why you can so confidently tell us that we need a "plane that can fly long distances without refueling and a pilot willing to navigate using a mechanical gyro-compass."
Why do you bring up the idea of a flight plan? Seaplanes aren't required to have flight plans if they don't fly over land or within the national waters of some country that has flying laws. Any flight plan of a seaplane that doesn't fly over land can be private, for the flight crew only. So, why bring it up?
As for the military, who do you know who is willing to testify in court that he was stopped by the military when flying somewhere pertinent to our discussion? After all, the military simply warns people at first. Are your buddies so ignorant that they don't heed military warnings? and that's why you don't have any of them to testify?
