<sect1 id="ai-zenith">
<sect1info>
<author>
<firstname>Jason</firstname>
<surname>Harris</surname>
</author>
</sect1info>
<title>The Zenith</title>
<indexterm><primary>Zenith</primary>
<seealso>Horizontal Coordinates</seealso>
</indexterm>
<para>
The Zenith is the point in the sky where you are looking when you look
<quote>straight up</quote> from the ground.  More precisely, it is the point on
the sky with an <firstterm>Altitude</firstterm> of +90 Degrees; it is the Pole
of the <link linkend="horizontal">Horizontal Coordinate
System</link>.  Geometrically, it is the point on the <link
linkend="ai-csphere">Celestial Sphere</link> intersected by a line drawn from
the center of the Earth through your location on the Earth's surface.
</para><para>
The Zenith is, by definition, a point along the <link
linkend="ai-meridian">Local Meridian</link>.
</para>
<tip>
<para>Exercise:</para>
<para>
You can point to the Zenith by pressing <keycap>Z</keycap> or by selecting
<guimenuitem>Zenith</guimenuitem> from the <guimenu>Location</guimenu> menu.
</para>
</tip>
</sect1>