A common argument against a GE is 'the horizon doesn't curve'. But why would we expect it to curve? Where would the highest point on the curve be. The fact is that the horizon would not curve on a GE:

All points on the horizon are at the same level and the same distance from the observer in all directions, thus forming a straight line. Yes, it is that simple.

mathematically: The projection of a circle is a line, only if the observer is at the same level/same plane of the circle. If the observer is above the center of the circle, he would see part of an ellipse.

Theoretically, but this circle, the horizon, compared to the hight of the observer, is huge. So in practice it's not distinguishable if you see a line or part of an ellipse.

An example: You know a Soccer field? There's a big circle in the center of the field. If you stay in the middle of this circle, you can see, that this is a circle around you -

*All points on *~~the horizon~~ this circle are at the same level and the same distance from the observer in all directions.But the distance to the horizon is huge compared to this center circle of the soccer field with 10 yards radius.

E.g. the dip for an observers hight of 2 meters, which gives 3 nautical miles distance to horizon, is about 1 arc minute, which is max. resolution, you could see with naked eye.

And increasing hight of the observer does not help much, as the horizon will also be farther away. E.g. a dip of one degree is found for about 900 meters hight and 60 nautical miles distance to the horizon.

But this is already far beyond, what normal viewing conditions would allow you to see. The horizon would be blurred, so that no precise observation could be made.