Let's see, ships DO disappear behind the horizon because they go downwards, so to say
Why does the earth look like a 2D disc when looked at? Does a ball look like a 2D disc when looked at, yes, they both do, is the ball a disc then? No, it isn't.
I remember there was a mathematic formula that showed how big an area is that you can see on a round object depending on haw far away from it you are, this works pretty well on earth too, the moon, and the other planets that are in our solar system. The only thing you need is their diameter, pii and you distance from the round object, can't remember how the formula was right now.