However, proving any information to a flat-earther is nearly impossible if not impossible. If you send one on a rocket into space and tell them to look out the window, they will say the windows are screened/faked with a display. If you put in a spacesuit into space, the glass on the spacesuit or they have been drugged.
Also, you don't understand perception, how a sextant works, and angle doesn't measure distance. Imagine the sun was twice as large, but further away.
Say this:
