A plane specific to a given location on Earth, which divides the visible hemisphere from the invisible hemisphere. The Apparent Horizon is a plane tangent to earth from a specific location - what we actually see as horizon dividing earth and sky and depending on elevation of the location above seas level. The is a Great Circle, defined by an imaginary plane passing through earth's Geocentric Horizon core, which is parallel to the Apparent horizon. The rational or true horizon is a great circle parallel to the visible horizon whose plane goes through the center of the Earth. The celestial horizon is the rational horizon extended infinitely into space to intersect the celestial sphere.