<sect1 id="ai-horizon"> <sect1info> <author ><firstname >Jason</firstname > <surname >Harris</surname > </author> </sect1info> <title >The Horizon</title> <indexterm ><primary >Horizon</primary> <seealso >Horizontal Coordinates</seealso> </indexterm> <para >The <firstterm >Horizon</firstterm > 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. </para ><para >The horizon is the <firstterm >Fundamental Plane</firstterm > of the <link linkend="horizontal" >Horizontal Coordinate System</link >. In other words, it is the locus of points which have an <firstterm >Altitude</firstterm > of zero degrees. </para> </sect1>