<sect1 id="ai-meridian"> <sect1info> <author ><firstname >Jason</firstname > <surname >Harris</surname > </author> </sect1info> <title >The Local Meridian</title> <indexterm ><primary >Local Meridian</primary> <seealso >Hour Angle</seealso > <seealso >Celestial Sphere</seealso > </indexterm> <para >The Local Meridian is an imaginary <link linkend="ai-greatcircle" >Great Circle</link > on the <link linkend="ai-csphere" >Celestial Sphere</link > that is perpendicular to the local <link linkend="ai-horizon" >Horizon</link >. It passes through the North point on the Horizon, through the <link linkend="ai-cpoles" >Celestial Pole</link >, up to the <link linkend="ai-zenith" >Zenith</link >, and through the South point on the Horizon. </para ><para >Because it is fixed to the local Horizon, stars will appear to drift past the Local Meridian as the Earth spins. You can use an object's <link linkend="equatorial" >Right Ascension</link > and the <link linkend="ai-sidereal" >Local Sidereal Time</link > to determine when it will cross your Local Meridian (see <link linkend="ai-hourangle" >Hour Angle</link >). </para> </sect1>