Also:Seeing ships sink below the horizon in the distance.
Sailing towards a tall mountain and seeing its top visible before its bottom.
Seeing elevated objects sooner means that you can see them at a greater distance away, which is why lighthouses are tall.
Seeing different patterns in the way the sun and planets behave, and theorizing why that might be, and then confirming it with math via Kepler and Newton's equations.
Seeing how the constellations are different in the northern and southern hemispheres.
Knowing how "line of sight" works.
Seeing how the sun's noon rays make different angles at different parts of the Earth.
Inventing telescopes and seeing how everything else in space is spherical in nature so why the hell wouldn't we be the same too.
Seeing how in nature spheres form quite often as a sphere has the least surface area per volume (e.g. bubbles), and discovering how gravity works and applying that knowledge to the formation of our own planet.
Seeing how time zones work and how we had to develop that concept as humans get the capability to travel across the Earth at greater and greater speeds and that when one side of the Earth is hit by the sun (day_, the other side is dark (night)
Predicting solar and lunar eclipses using gravitational equations.
Seeing how the shadow of the Earth in a lunar eclipse is and has always been a shadow that is cast by a spherical object.
Pick one.
- Exactly half of the earth being lit by the sun
- Different sky on northern and southern hemisphere
- Sky "moving" in oposite direction on northern and southern hemisphere
- Polar day on antarctica
- Heck....antarctica itself ...on flat earth its coast would be about 4 times longer then it actually is.
- Sunset and sunrise
- ISS being clearly visible with good camera
The list goes on and on and on....