In terms of horizons, you can only see up to the point where the earth begins to curve away from you, roughly how far is this. for example, if you were looking out to sea, how far can you see?
use the formula I gave you. If you are 6 ft tall, and are looking at an object at sea level you lose sight of it at 1.23xSQRT(6+0) or 1.23x 2.449 or roughly 3.013 nautical miles. If you are 6ft tall and the object is 6ft tall then the sum becomes 1.23xSQRT (6+6) or 1.23x(3.46) or 4.26 nautical miles.
The earth is a sphere, it doesnt begin to curve at a particular distance, basically you have to do the maths to get a meaningful answer.