Why do you see a horizon?
You should be able to figure this out on your own, and I can't
really explain it if you lack that much spacial awareness.
Stand near the bottom of a small hill or an arched bridge. You
can't see past the peak because the land or bridge curves. The same
thing happens when you are standing on the surface of a large a
sphere such as the earth. In all directions, the earth curves away
from you. This creates the horizon. Interesting side note: The
higher up you are, the farther you can see. This is why lighthouses
were developed to be taller and taller. The taller the lighthouse,
the farther away a ship could be and still see the light.