<sect2 id="calc-precess">
<title>Precession module</title>
<indexterm><primary>Tools</primary>
<secondary>Astrocalculator</secondary>
<tertiary>Precession module</tertiary>
</indexterm>

<screenshot>
<screeninfo>
The Precession calculator module
</screeninfo>
<mediaobject>
  <imageobject>
    <imagedata fileref="calc-precess.png" format="PNG"/>
  </imageobject>
  <textobject>
    <phrase>Precession</phrase>
  </textobject>
</mediaobject>
</screenshot>

<para>
This module is similar to the <link linkend="calc-apcoords">Apparent 
Coordinates module</link>, but it only applies the effect of 
<link linkend="ai-precession">precession</link>, not of nutation or 
aberration.  
</para>
<para>
To use the module, first enter the input coordinates and their epoch
in the <guilabel>Original Coordinates</guilabel> section.  You must 
also fill in the target epoch in the <guilabel>Precessed 
Coordinates</guilabel> section.  Then, press the 
<guibutton>Compute</guibutton> button, and the object's 
coordinates, precessed to the target Epoch, are presented in the 
<guilabel>Precessed Coordinates</guilabel> section.
</para>
</sect2>