<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 >catalogue 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/Date</guilabel > section. Then, enter the catalogue coordinates in the <guilabel >Catalog Coordinates</guilabel > section. You can also specify the catalogue's epoch here (usually 2000.0 for modern object catalogues). Finally, press the <guibutton >Compute</guibutton > button, and the object's coordinates for the target date will be displayed in the <guilabel >Apparent Coordinates</guilabel > section. </para> </sect2>