<sect2 id="calc-apcoords">
<title>Apparent Coordinates module</title>
<indexterm><primary>Tools</primary>
<secondary>Astrocalculator</secondary>
<tertiary>Apparent Coordinates module</tertiary>
</indexterm>

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

<para>
The Apparent Coordinates module converts the <firstterm>catalog 
coordinates</firstterm> of a point in the sky to its 
<firstterm>apparent coordinates</firstterm> for any date.  The 
coordinates of objects in the sky are not fixed, because of 
<link linkend="ai-precession">precession</link>, nutation and
aberration.  This module takes these effects into account.
</para>
<para>
To use the module, first enter the desired target date and time 
in the <guilabel>Target Time &amp; Date</guilabel> section.  Then, 
enter the catalog coordinates in the <guilabel>Catalog 
Coordinates</guilabel> section.  Alternatively you can set the 
catalog coordinates by selecting an object using the <guilabel>Find Object</guilabel> 
dialog.  You can also specify the 
catalog's epoch here (usually 2000.0 for modern object 
catalogs).  Then the object's coordinates for the target date 
will be displayed in the <guilabel>Apparent Coordinates</guilabel>
section.
</para>
</sect2>