in an old thread, someone told of a simple experiment he did:
he went to a seashore, stood near a table, and looked at the horizon while holding his eyes just above the level of the table.
result? the horizon appeared very slightly curved relative to the edge of the table!
appearances can be deceiving...
he also said that if you took a large photograph of the horizon and held a straight-edge across it, you could see a difference. it's simply too slight to see with the naked eye, except at very high altitudes.