<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>