ὁρίζων (κύκλος)

The horizon is where the observer's view ends. In case of black holes for instance, it is, therefore, the sphere of no return but the classical phenomenological meaning of the term characterises the line where the sky meets the earth. Standing atop a mountain, the observer's sight will be limited by the ground but standing in a valley, the mountain will cover parts of this (theoretical) ground line. Thus, astronomy distinguishes between the "natural horizon" (with mountains, trees or houses covering parts of the sky) and the "mathematical horizon" until which the observer would be able to see if there were no obstacles.