Question

how do we know that earth is round not flat​

Answer 1

1 Watch a ship sail off to sea

If you are able to watch a ship sail off to sea, watch its mast and flag as it fades off into the distance. You will notice that, in fact, it does not "fade off into the distance" at all; instead, you will see its mast and flag appear to slowly sink.

2. Watch a lunar eclipse

If you are able to catch a glimpse of a lunar eclipse, you can see evidence that the Earth is, indeed, round.

3 "International Space Station photos"

See photos of that are taken from space stations. you can see the shape of the Earth is round.