the earth is round, that's obvious if you just go to the beach, but the moon is a flat disk. the back side of the disk is home to a dinosaur utopia that nasa doesn't want us to know about.
The moon wobbles slightly so over time you'll see the "edges" change as the moon wobbles back and forth while still being relatively one-sided. If you'd be persuaded by looking out from a beach you'll probably also take the above as evidence that the moon is round.