I think what is complicating this discussion is what has been mentioned-- the need to draw a 2D picture of a 3D situation. So both Jason Thompson's figure, and hhEb09'1 's objection to it, are correct! The figure is right, because it is the best you can do in 2D. hh's objection is right if the figure is interpreted as what your eye could see without turning your head, which would always have to look like a straight line that is perpendicular to the terminator (if you don't turn your head, the curvature of the Earth is irrelevant). The problem is, in clop's illusion, you do have to turn your head, so what Grant is explaining is how your perceptions get skewed when you turn your head and reorient yourself with respect to the horizon (i.e., your mind tells you that parallel to the horizon is a straight line, and it's not.)