The human eye has a limit just like a digital camera determined by its focal length and resolution. Telescopes improve image size, but not resolution. What happens on the horizon is like zooming in on a holiday snap, zoom in on a single pixel and trying to tell from the pixels colour what that actually is. Is it sky or water?? Add into the equation all the dust and pollution between you and the horizon and you get the picture ( or rather you get your LIMIT. No curve, just limit. The reason you think you see a curve left to right is because of the 360 degree LIMIT around you. Take your LIMIT, extend it in front of you and rotate your body on the spot 360. You just drawn a LIMIT CIRCLE around you. Your eye sees this as a slight curve from left to right. It is in fact FLAT. If you now go superman and start rising up in the air, the horizon magically rises up to meet your eye, If the ground beneath you was a ball, the horizon would fall away. Next time your in airplane, look out the window and look how high up the horizon is, I guarantee its level with your eye.

THIS EARTH IS FLAT AND INFINITE. You just happen to be lucky enough to be living in a thawed out patch of ice.