2. what is the horizon - in other words, what does the horizon great circle represent?

Horizon is represented by a skyline that separates the earth from sky.

Explanation:

The true horizon is a theoretical line, that divides or separates the sky in two directions.  This is called the visible horizon. The horizon of the great circles is represented by the intersection of different celestial bodies sphere passing through the observe. The greater circle is defined by the intersection of the surface of the sphere by the plane that passes through the center into two halves. The Equator is considered the greater circle that circumnavigates the entire earth. The greater circle has all lines of the latitude and longitudes being the same.

The second one is the answer edg.