Why do I know the earth is a sphere? Because the human race has known it to be true for millennia. It was known to the Greeks as early as 500 BC, but was simply philosophy until Aristotle proved the theory, with multiple arguments. Chief among these were 1. that the shadow the earth casts on the moon during a lunar eclipse is round, 2. that the constellations you see on one side of the equator are different than than those on the other, and 3. the further you went south from the equator, the higher southern constellations rose above the horizon.