@Random832 said:
Well, for one thing, it's something that tells you that if the horizon is curved, the earth is obviously round. Because if it was flat, the horizon would be flat (if it's infinite, anyway - though, that there would be no horizon and the edge would be visible, if it were flat and finite, might be a bit beyond common-sense reasoning)
Spoken truly as by someone who knows the earth is actually a globe. Think for a moment what you would see if you were standing on an absolutely flat earth of infinite extent. You would see sky above you, and earth below you. You would see sky meet earth way, way out there. Now if you were standing in the center of a flat disk say 100 miles across, I submit that it would not look any different to the naked eye, since you couldn't make out any detail at a distance of 50 miles that would make it look different.
Stand on an earth-sized globe in a "flat" featureless area (e.g. a plain or on an ocean) and I contend that the appearance is pretty much the same (the horizon may be closer than 50 miles, but it's still hard to tell anything about what's happening at that distance with the naked eye). The horizon (the line where the sky meets the earth) is a full circle around you, but it is no more curved than in the flat-earth examples of the preceding paragraph. It is perfectly logical that someone who has not been brought up in a culture with inherent understanding of the shape of the earth would assume it to be flat, if he even thought about it. "Common sense" has nothing to do with it. The roundness of the earth is just not obvious to a single naked individual standing on the earth.
/JBL