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0001 <sect1 id="ai-zenith">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname>
0005 <surname>Harris</surname>
0006 </author>
0007 </sect1info>
0008 <title>The Zenith</title>
0009 <indexterm><primary>Zenith</primary>
0010 <seealso>Horizontal Coordinates</seealso>
0011 </indexterm>
0012 <para>
0013 The Zenith is the point in the sky where you are looking when you look
0014 <quote>straight up</quote> from the ground.  More precisely, it is the point on
0015 the sky with an <firstterm>Altitude</firstterm> of +90 Degrees; it is the Pole
0016 of the <link linkend="horizontal">Horizontal Coordinate
0017 System</link>.  Geometrically, it is the point on the <link
0018 linkend="ai-csphere">Celestial Sphere</link> intersected by a line drawn from
0019 the center of the Earth through your location on the Earth's surface.
0020 </para><para>
0021 The Zenith is, by definition, a point along the <link
0022 linkend="ai-meridian">Local Meridian</link>.
0023 </para>
0024 <tip>
0025 <para>Exercise:</para>
0026 <para>
0027 You can point to the Zenith by pressing <keycap>Z</keycap> or by selecting the
0028 <menuchoice><guimenu>Pointing</guimenu> <guimenuitem>Zenith</guimenuitem></menuchoice> menu item.
0029 </para>
0030 </tip>
0031 </sect1>