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The Horizon
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         **Jason Harris**

         The *Horizon* is the line that separates Earth from Sky. More
         precisely, it is the line that divides all of the directions
         you can possibly look into two categories: those which
         intersect the Earth, and those which do not. At many locations,
         the Horizon is obscured by trees, buildings, mountains, etc.
         However, if you are on a ship at sea, the Horizon is strikingly
         apparent.

         The horizon is the *Fundamental Plane* of the :ref:`Horizontal
         Coordinate System <ai-skycoords-horizontal>`. In other
         words, it is the locus of points which have an *Altitude* of
         zero degrees.