<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>Pointing</guimenu> menu. </para> </tip> </sect1>