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+<sect2 id="calc-julian">
+<title>Julian Day module</title>
+<indexterm><primary>Tools</primary>
+<secondary>Astrocalculator</secondary>
+<tertiary>Julian Day module</tertiary>
+</indexterm>
+
+<screenshot>
+<screeninfo>
+The Julian Day calculator module
+</screeninfo>
+<mediaobject>
+  <imageobject>
+    <imagedata fileref="calc-julian.png" format="PNG"/>
+  </imageobject>
+  <textobject>
+    <phrase>Julian Day</phrase>
+  </textobject>
+</mediaobject>
+</screenshot>
+
+<para>
+This module converts between the calendar date, the <link 
+linkend="ai-julianday">Julian Day</link>, and the 
+<firstterm>Modified Julian Day</firstterm>.  The Modified Julian Day
+is simply equal to the Julian Day - 2,400,000.5.
+</para><para>
+To use the module, select which of the three dates will be the input, 
+and then fill in its value.  Then press the 
+<guibutton>Compute</guibutton> button, and the corresponding values for 
+the other two date systems will be displayed.
+</para>
+
+<tip>
+<para>Exercise:</para>
+<para>
+What calendar date does MJD = 0.0 correspond to?
+</para>
+</tip>
+
+</sect2>