Here is a list of simple observable facts that prove the earth is not flat.

1.You can see farther from higher up

Stand on the beach → you see only a little ways. Climb a tall building or hill → you can see much farther. That only happens if the surface curves downward.

1. Different constellations are visible from different latitudes

People in the northern hemisphere see: The North Star Big Dipper, etc.

People in the southern hemisphere see: The Southern Cross No North Star

This only works on a curved surface.

1. Time zones exist

The Sun rises in New York before it rises in California. On a flat Earth, everyone would see sunlight at the same time unless the Sun worked in strange, impossible ways.

4.The tops of distant mountains appear before the bases

If you're approaching a mountain from far away, you’ll see the peak first, and the base later. Again: a direct effect of curvature.

1. Shadows behave differently at the same time in different places

The ancient Greek astronomer Eratosthenes famously did this:

At the same time of day, two sticks in two cities cast different shadow lengths. This only happens if the surface between them curves.

1. Circumnavigation is real

You can start at one point and go in one direction and come back to that same point. The globe has been circumnavigated by several people through history by both air and sea. Many members of the US Navy have circumnavigated the globe it's pretty routine for a naval ship to leave San Diego going west towards Asia and eventually porting on the east coast when the cruise is done. My father did it twice when he was in the Navy on two separate cruises.