When you triangulate the coordinates that are as far out, yet not misted, as possible, preferably in the ocean as you get the biggest results, you get 3 60 degree angles. 3 60 degree angles adds up to 180, which is required for a triangle. Thus, the earth is a equilateral triangle.

The curve is merely a construct of your eye. Since it is a circle type shape, the retina will see things in a circle type way.

When you turn, the curve changes, proving another point, that it is just your eye messing around