we know that the earth is round but how do we prove that it is... apart from photographs taken from satellites... during lunar eclipses earth's shadow falls on the moon... and if you look at the image bellow you'd notice that the shadow is round...
the oldest i guess would be lunar eclipses... the earth's shadow casted on the moon during a total lunar eclipse unique to shadows casted by objects with a circular profile...
a single lunar eclipse observation only suggests the earth is a circular disc... so several observations across all over the earth would need to suggest the earth is round...
i'm sure the greeks and egyptians figured it out... 'cos there are statements in the bible saying "It is he that sitteth upon the circle of the earth..." - isaiah 40:22...
earth is not spherically round its an oblique spheroid... that is... its equatorial diameter is greater (slightly) than than its polar diameter...
columbus' voyages are not conclusive proof to suggest that the earth is round... they only prove that the earth is not flat... his paths suggests multiple geometric possibilities... earth could have been some random quadric surface... a sphere (or a spheroid) is a special quadric surface...