<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>
