From observation alone people knew the world isn't flat long before much of modern science was taught. If you know any math or think at all about what happens in a lunar eclipse or about satellite e...
Exactly, the Greeks knew the earth wasn’t flat because when ships came from far away (beyond the horizont) the first thing they would see were the sails (and not the hull). So it’s just about basic observation, you don’t even need a big scientific experiment to conclude the earth must be at least, to some extent, round (aka not flat)