THREE TYPES OF HORIZON LINE
1. ASTRONOMICAL HORIZON : In astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points that have an altitude of zero degrees.
2. VISIBLE HORIZON: Horizon, in astronomy, boundary where the sky seems to meet the ground or sea. (In astronomy it is defined as the intersection on the celestial sphere of a plane perpendicular to a plumb line.) The higher the observer, the lower and more distant is his visible horizon.
3. TRUE HORIZON : 1. The boundary of a horizontal plane passing through a point of vision.
2. In photogrammetry, the boundary of a horizontal plane passing through the perspective center of a lens system.
4. The ‘EVENT HORIZON‘ is the boundary defining the region of space around a black hole from which nothing (not even light) can escape. In other words, the escape velocity for an object within the event horizon exceeds the speed of light.
If the Earth is assumed to be a sphere with no atmosphere, then the distance to the horizon can easily be calculated. (Note: The Earth’s radius of curvature actually varies by 1% between the Equator and the Poles, so this formula isn’t absolutely exact even assuming no refraction.[