I used a microscope on my football, MY GOD, this revealed a flat edge to the ball, I have been lied to, by football is really a flat disc.
It is called perspective, you are seeing a small section of the huge earth and this appears flat.
Go up further into space and whatever angle,top, bottom, east or west, USA or Australia, from whatever side you take a photo or video, the earth looks round, so if it is round from all angles then it must be round.
Desperate! Forget infrared, they need an MRI for that thing above their shoulders to see if it is hollow.