<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>