<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 centre 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>